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• Published: 01 August 2020

Kiwifruit Genome Database (KGD): a comprehensive resource for kiwifruit genomics

• Junyang Yue   ORCID: orcid.org/0000-0002-5299-0160 1 , 2   na1 ,
• Jiacheng Liu 3   na1 ,
• Wei Tang 1 ,
• Ya Qing Wu 3 ,
• Xiaofeng Tang 2 ,
• Ying Yang 1 ,
• Lihuan Wang 1 ,
• Shengxiong Huang 2 ,
• Congbing Fang 1 ,
• Kun Zhao 3 ,
• Zhangjun Fei   ORCID: orcid.org/0000-0001-9684-1450 3 , 4 ,
• Yongsheng Liu 1 , 2 , 5 &
• Yi Zheng   ORCID: orcid.org/0000-0002-8042-7770 3 , 6 , 7  

Horticulture Research volume  7 , Article number:  117 ( 2020 ) Cite this article

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• Bioinformatics
• Gene expression profiling
• Plant breeding

Kiwifruit ( Actinidia spp.) plants produce economically important fruits containing abundant, balanced phytonutrients with extraordinarily high vitamin C contents. Since the release of the first kiwifruit reference genome sequence in 2013, large volumes of genome and transcriptome data have been rapidly accumulated for a handful of kiwifruit species. To efficiently store, analyze, integrate, and disseminate these large-scale datasets to the research community, we constructed the Kiwifruit Genome Database (KGD; http://kiwifruitgenome.org/ ). The database currently contains all publicly available genome and gene sequences, gene annotations, biochemical pathways, transcriptome profiles derived from public RNA-Seq datasets, and comparative genomic analysis results such as syntenic blocks and homologous gene pairs between different kiwifruit genome assemblies. A set of user-friendly query interfaces, analysis tools and visualization modules have been implemented in KGD to facilitate translational and applied research in kiwifruit, which include JBrowse, a popular genome browser, and the NCBI BLAST sequence search tool. Other notable tools developed within KGD include a genome synteny viewer and tools for differential gene expression analysis as well as gene ontology (GO) term and pathway enrichment analysis.

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Introduction.

Kiwifruit, belonging to the basal genus Actinidia within the family Actinidiaceae, consists of ~54 species and 75 taxa 1 . All species in this genus are perennial, deciduous and dioecious plants with a climbing or scrambling growth habit. They are native to southwestern China, although they are prevalent in New Zealand after being introduced in the early 20th century 2 . Despite the availability of diverse, rich germplasm resources for kiwifruit, only several economically important horticultural species have been successfully domesticated and widely cultivated, including A. chinensis Planchon, A. deliciosa ( A. chinensis var. deliciosa A. Chevalier), A. arguta (Siebold and Zuccarini) Planchon ex Miquel and A. eriantha Bentham 3 .

Despite its relatively short history of domestication, kiwifruit has become an important fresh fruit worldwide. Owing to its remarkably high contents of vitamin C and minerals, kiwifruit is commonly celebrated as ‘the king of vitamin C’ and ‘the king of fruits’. In addition to being primarily consumed as fresh fruits, kiwifruit can be used for medicinal purposes (as observed for the fruits and roots) and for its ornamental value (as observed for the flowers) 4 . Furthermore, kiwifruit provides a distinct model system for studies of several fundamental biological processes, such as ascorbic acid biosynthesis 5 and sex determination 6 , 7 .

To facilitate genetic research and molecular breeding in kiwifruit, in 2013, the International Kiwifruit Genome Consortium assembled and published the first reference kiwifruit genome for the Chinese cultivar A. chinensis ‘Hongyang’ (2 n  = 2× = 58) 8 . This genome sequence, along with its structural and functional annotations, was subsequently released online at the Kiwifruit Information Resource (KIR; http://kir.atcgn.com/ ) 9 . KIR has played a vital role by providing the scientific community with access to the ‘Hongyang’ genome sequence and associated annotation data. However, KIR also meets new requirements for broader utility as large volumes of genome and transcriptome data have been generated at an increasingly rate during the past few years, especially in the form of two recently published kiwifruit genome assemblies from A. chinensis ‘Red5’ 10 and A. eriantha ‘White’ 11 and an improved assembly of ‘Hongyang’ 12 . Moreover, a number of transcriptome studies have been recently reported in various kiwifruit species, including A. arguta, A. chinensis, A. deliciosa , and A. eriantha 13 , 14 , 15 . Therefore, there is an urgent need for a central and integrated database to store, analyze, mine, manage and disseminate these large-scale datasets for the kiwifruit research and breeding community.

For this purpose, we rebuilt and describe herein an updated kiwifruit genome database (KGD; http://kiwifruitgenome.org/ ), which currently integrates rich genome and transcriptome resources of kiwifruit, including assembled genomes and predicted gene models. At the same time, we performed comprehensive functional annotations for these gene models, identified conserved syntenic genome blocks between different kiwifruit species, and incorporated gene expression profiles based on publicly available RNA-Seq datasets. KGD was constructed using the Tripal system 16 , a specific toolkit for the construction of online community genomic databases, by integrating the GMOD Chado database schema 17 and the Drupal open source platform ( https://www.drupal.org/ ). Furthermore, a set of modules and user-friendly interfaces have been implemented in KGD to analyze and visualize comparative genomic and transcriptome profiling datasets for different kiwifruit species.

Database content

Genome sequences.

High-quality genome sequences of three kiwifruit cultivars have been assembled to date, including two from A. chinensis (‘Hongyang’ and ‘Red5’) 8 , 10 , 12 and one from A. eriantha (‘White’) 11 . For the ‘Hongyang’ cultivar, there are two versions (2.0 and 3.0) of the genome assembly, among which version 3.0 was assembled using PacBio long reads 12 and thus shows much higher genome contiguity and sequence quality than version 2.0, which was assembled purely from Illumina short reads 8 . The genome, transcript and protein sequences of the predicted protein-coding genes and the gene annotation files in GFF3 format were downloaded from the Kiwifruit Information Resource 9 (ftp://www.atcgn.com/kiwifruit/) for ‘Hongyang’ version 2.0 and ‘White’, from https://doi.org/10.6084/m9.figshare.10046558 for ‘Hongyang’ version 3.0, and from the Ensemble database ( https://plants.ensembl.org ) for ‘Red5’.

Functional annotation of protein-coding genes

A total of 156,257 protein-coding genes were predicted from these four genome assemblies, including 39,761 from ‘Hongyang’ version 2.0, 40,464 from ‘Hongyang’ version 3.0, 33,044 from ‘Red5’, and 42,988 from ‘White’. A standard, unified procedure was used to comprehensively annotate the predicted protein-coding genes. First, the protein sequences of the predicted genes were aligned against the NCBI non-redundant (nr), UniProt (Swiss-Prot and TrEMBL), and Arabidopsis protein (TAIR) databases using the BLASTP command of DIAMOND 18 with an E-value cutoff of 1e-5. All of these protein sequences were further compared against the InterPro database using InterProScan 19 to identify functional domains. The BLASTP results derived from the nr database and the identified InterPro domains were fed into the Blast2GO pipeline 20 to assign gene ontology (GO) terms to each protein-coding gene. The BLASTP results against the UniProt and TAIR databases were fed to the AHRD program ( https://github.com/groupschoof/AHRD ) to obtain concise, precise and informative gene function descriptions. We also used the PathwayTools program 21 to predict biochemical pathways encoded by each of the kiwifruit genomes. For each genome, the gene function descriptions derived from the AHRD analysis, the GO terms assigned by the Blast2GO tool, and the enzyme commission (EC) numbers collected from the UniProt database were integrated into a single file in PathoLogic format, which was directly used by PathwayTools for pathway prediction. In total, 342 to 405 predicted biochemical pathways were obtained from each of these four kiwifruit genomes.

Comparative genomic analysis

We identified syntenic blocks and homologous gene pairs within syntenic blocks in the four kiwifruit genome sequences, including comparisons both within each genome and between any two genomes. The protein sequences were first aligned against themselves (within each genome) as well as between each other (pairwise comparisons) using BLASTP 22 with an E-value cutoff of 1e-5 and a maximum of five alignments. Based on the BLASTP results and gene positions, syntenic blocks were determined using MCScanX 23 with default parameters. A total of 14,125 syntenic blocks and 335,140 homologous gene pairs were identified, among which approximately 800–1100 syntenic blocks and 15,000–20,000 homologous gene pairs were identified within each genome, and 1500–2700 and 48,000–55,000 were identified between any two of the four genomes.

Gene expression profiles

We collected all publicly available kiwifruit RNA-Seq datasets from the NCBI Sequence Read Archive (SRA) database, including data from nine projects and 80 samples. Most of these samples were derived from fruits (35 samples), dormant buds (17 samples), and phloem tissues (10 samples), and others were derived from leaves, seedlings, roots and stems. A unified pipeline was applied to process and analyze these RNA-Seq datasets. Briefly, raw RNA-Seq reads were processed to remove adaptor and low-quality sequences using Trimmomatic 24 . Trimmed reads shorter than 80% of their original length were discarded. The remaining cleaned reads were then aligned against the SILVA rRNA database 25 using the Bowtie program (version 1.1.2) 26 allowing up to three mismatches, and the mapped reads were removed. The resulting high-quality reads were aligned to the kiwifruit genomes using the STAR program 27 with a maximum of two mismatches. Finally, based on the alignments, the read counts of each gene were calculated and normalized to FPKM (fragments per kilobase of transcripts per million mapped fragments) values. The mean and standard error of the FPKM values of the biological replicates were then derived.

Transcription factors and transcriptional regulators

We used the iTAK program 28 to identify transcription factors (TFs) and transcriptional regulators (TRs) from the four kiwifruit genomes and classified them into different families. The protein sequences of the predicted protein-coding genes were fed into iTAK for TF and TR identification and classification with the default parameters. A total of 9906 TFs (2323–2718 from each genome) belonging to 54 different families and 2211 TRs (533–563 from each genome) belonging to 25 different families were identified.

Database implementation

The Tripal system 16 was employed to facilitate the construction of KGD. Tripal provides dozens of extension models for building online genomic databases. The genome sequences, predicted gene models, mRNA and protein sequences were loaded into the database using the ‘Data Loaders’ function of Tripal. For gene functional annotations, the top BLASTP hits as well as the GO terms and InterPro domains assigned to each gene were imported into KGD through Tripal Analysis Extension Modules. The functional descriptions generated by the AHRD program were loaded into KGD using an in-house Perl script. Additionally, TFs and TRs were imported into KGD using the gene family extension module that we developed previously.

KGD provides a page for each kiwifruit genome assembly, typically comprising multiple categories of biological information, and submenus to access data analysis tools including tools for performing queries, BLAST searches, genome browsing, pathway analysis, and downloads of the genome resources. KGD also generates a page for each queried gene (gene feature page) that includes categories of basic information and the gene structure displayed in a genome browser (Fig. 1a ), genome/mRNA/protein sequences (Fig. 1b ), functional annotations and homologous genes (Fig. 1c ), expression profiles (Fig. 1d ), and syntenic blocks (Fig. 1e ).

The page contains different sections with different content types, including ( a ) overview of information for the gene (gene position, structure, and functional annotation), ( b ) gene/mRNA/protein sequences, ( c ) homologous genes and sequence alignments generated by BLAST, ( d ) RNA-Seq expression profiles, and ( e ) synteny blocks related to the gene

To import the expression information (read counts and FPKM values) as well as the corresponding experimental metadata into KGD, we used two Tripal extension modules: ‘SRA’ and ‘RNA-Seq’, which we previously developed 29 . The ‘SRA’ extension module is a mimic of the NCBI SRA database for the purpose of managing the meta-information of projects, samples, and experiments but does not require the storage of raw reads. The ‘RNA-Seq’ module is designed to manage and display gene expression profiles. In KGD, the ‘RNA-Seq’ home page lists all collected projects and provides mouse-over descriptions in which after an RNA-Seq project is selected, the meta-information of the project is displayed. Furthermore, a submenu including the ‘Heatmap’, ‘DEGs’ and ‘Expression Viewer’ is provided to guide users to explore and analyze the expression datasets. Additionally, gene expression profiles can be accessed under the ‘RNA-Seq Expression’ section within the gene feature page (Fig. 1d ).

The identified syntenic blocks and homologous gene pairs were loaded into KGD using the ‘SyntenyViewer’ module. The ‘Synteny’ section on the gene feature page has been designed to display all available syntenic blocks and homologous gene pairs associated with a specific gene (Fig. 1e ). Furthermore, each syntenic block can be linked to a new page that lists all genes located in the syntenic region.

A biochemical pathway database for different Actinidia species, ActCyc, was implemented within KGD using the PathwayTools web server 21 . Through ActCyc ( http://kiwifruitgenome.org/pathway ), users can easily search biochemical pathways and perform comparative analyses.

Utility and discussion

Query option.

In summary, two search categories are provided in KGD: gene search and batch query. The gene search option provides an interface for querying KGD with a gene ID or keyword associated with gene annotations. To facilitate the queries of genes and functional annotation data stored in KGD, we employed the Apache Solr search engine ( http://lucene.apache.org/solr/ ) to build indexes for different sources of annotation information, including gene functions, GO terms, InterPro domains and homologs.

In addition to the gene search option under each genome page, a global search function is provided under the main menu of KGD. This function provides a quick query against all the records stored in the database and returns results in a tabular format including the gene ID, gene type, and gene description (Fig. 2a ). From this table, users can browse the detailed feature page for each gene by clicking the corresponding gene link.

a List of genes returned from a global search using a keyword. b Interface of the homology search (BLAST) implemented in KGD. c Result page of the homology search. The bottom image illustrates the alignment of query and subject sequences

The batch query option allows users to retrieve sequences, annotations and other types of information (e.g., TFs and TRs) for a given list of genes. The batch query function in KGD was modified from the ‘Sequence Retrieval’ page of Tripal 16 .

Homology search

To provide a homology search function, we implemented the Tripal BLAST UI extension module in KGD. All genome, mRNA, CDS and protein sequences of kiwifruit species stored in KGD are available for comparison through the BLAST program. To prevent users from selecting inompatible BLAST programs (BLASTN, BLASTP, BLASTX, tBLASTN and tBLASTX) for the corresponding databases, the list of BLAST programs is automatically set up according to the selected reference database (Fig. 2b ). Options for filtering low-complexity sequences and selecting the maximum number of returned BLAST hits are provided. The BLAST function provides downloadable output files ordered by the expected values in three different formats, HTML, TSV and XML, and the results page lists all the hits, with each hit linked to a graphic output that shows the alignment coordinates between the query and the hit and a color-ranked bit score for the alignment (Fig. 2c ).

Genome browser

In KGD, we implemented JBrowse 30 , a widely used genome browser, to display genome sequences, gene models, and expression profiles. Currently, all publicly available kiwifruit genomes, predicted gene models, and gene expression profiles derived from RNA-Seq data have been imported into JBrowse. The tracks of a given gene in a reference genome are also embedded in the gene features page to provide a graphical and informative view of its sequence and structure (Fig. 1a ). Additionally, the genome browser can support other types of interesting data, such as single-base resolution genome variants, when they become available in the near future.

Synteny viewer

To view syntenic blocks and homologous gene pairs between different kiwifruit genome assemblies, we developed ‘SyntenyViewer’, an extension module of Tripal, in KGD. Syntenic blocks can be retrieved by selecting a query genome together with one or more subject genomes. ‘SyntenyViewer’ will draw circus plots to display syntenic blocks for every pair of query and subject genomes (Fig. 3a ) and simultaneously generate a full list of the syntenic blocks. For a specific syntenic block, ‘SyntenyViewer’ creates an image to display the homologous gene pairs, and the view can be zoomed in or out as desired (Fig. 3b ). The full list of genes included in the homologous gene pairs is provided with links to the detailed feature page of each gene (Fig. 1 ). In brief, the ‘SyntenyViewer’ module can not only reveal syntenic blocks between any two genome sequences but also connect homologous gene pairs in syntenic blocks. With this module, homologous members of interesting genes that are located in a specific region of one kiwifruit genome can be easily identified and intuitively viewed for the other kiwifruit genome.

a Syntenic blocks displayed in a Circos plot. The blue arc indicates the query chromosome, and the red arcs indicate the chromosomes of the compared genome. Gray lines between blue and red arcs indicate syntenic blocks identified between the two genomes. The lines of a syntenic block will become red when the user mouses over it. b Detailed view of a specific synteny block. The query and compared chromosomes of a specific synteny block are shown in orange and blue, respectively. The yellow and black lines within each chromosome indicate homologous gene pairs, which are connected by gray lines

Enrichment analysis

Large-scale genomic studies typically result in large lists of interesting genes. Interpreting such gene lists to obtain biologically meaningful information is the basic premise for understanding the underlying regulatory mechanisms of important biological processes and biochemical pathways. Enrichment analysis is a powerful and frequently used method for identifying specific families or groups of genes that are overrepresented in a list of biological entries (e.g., GO terms and biochemical pathways). We previously developed two custom-built extension modules of Tripal, ‘GO tool’ and ‘Pathway tool’, based on the hypergeometric test 29 . These two modules were also implemented in KGD to identify significantly enriched GO terms and biochemical pathways from a list of user-provided genes.

RNA-Seq expression analysis

KGD not only stores gene expression profiles derived from RNA-Seq datasets but also provides an ‘RNA-Seq’ module to allow users to perform RNA-Seq data analyses, including the identification of differentially expressed genes (DEGs) and the visualization of gene expression profiles. The two most popular DEG identification tools, edgeR 31 and DESeq 32 , were integrated into the ‘RNA-Seq’ module in KGD. The tools provide users the option of selecting their desired cutoff values for the gene expression fold change and adjusted P -value to determine the final DEGs. The result page for the DEG analysis includes the project description, parameter settings, top 100 DEGs ordered by adjusted P -values, and a download link to a file with all identified DEGs together with their relevant information (Fig. 4a ). Furthermore, the result page provides links to other modules for many downstream analyses of the identified DEGs, such as BLAST, batch query, GO term and pathway enrichment analyses, and gene functional classification.

a Statistical analysis results listing the top 100 DEGs ordered by adjusted p -values. b Heatmap showing the expression profiles of a list of user-defined genes. c Single-base resolution expression profile view in JBrowse

In addition to viewing the expression profiles of individual genes on the gene feature page (Fig. 1d ), the ‘RNA-Seq’ module provides two interactive visualization tools: a heatmap tool developed using Plotly’s JavaScript library ( http://plot.ly ) for displaying the expression profiles of a set of genes (Fig. 4b ) and an expression viewer embedded in JBrowse for displaying single-base resolution expression profiles under certain conditions (Fig. 4c ).

Conclusion and future directions

We have constructed the KGD, which serves as a central portal for kiwifruit genomics and provides comprehensive genome and transcriptome resources for kiwifruits. KGD stores the sequences of various kiwifruit genome assemblies, predicted mRNAs and proteins as well as comprehensive functional annotations, genome synteny blocks, homologous gene pairs, gene expression profiles, and biochemical pathways. The database also offers various query, analysis and visualization tools, including tools for basic and batch queries, BLAST, a genome browser, a biochemical pathway database (ActCyc), tools for GO term and pathway enrichment analysis, a genome synteny viewer and a DEG analysis tool. An important feature of KGD is that four modules recently developed by our groups, a ‘GO tool’, ‘Pathway tool’, ‘SyntenyViewer’ and ‘RNA-Seq’, have been implemented to extend the capabilities of the database.

KGD will be continuously updated when new genome, transcriptome and other types of genetic datasets of kiwifruit species become publicly available. Additionally, we will continue to develop novel extension modules that can be adopted by the Tripal community. We believe that KGD will be a global, active platform for researchers and breeders working with kiwifruit as well as other plant species.

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Acknowledgements

This study was supported by grants from the National Natural Science Foundation of China (31972474, 31671259, 31471157, 31900257, 31400049, and 90717110), the Anhui Provincial Natural Science Foundation (1808085QC68) and the National Foundation for the Germplasm Repository of Special Horticultural Crops in Central Mountain Areas of China (NJF2017-69), the National Science Fund for Distinguished Young Scholars (30825030), Key Project of the Government of Sichuan Province (2013NZ0014), Key Project of the Government of Anhui Province (2012AKKG0739; 1808085MC57), and the US National Science Foundation (IOS-1339287 and IOS-1855585).

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These authors contributed equally: Junyang Yue, Jiacheng Liu

Authors and Affiliations

School of Horticulture, Anhui Agricultural University, Hefei, 230036, China

Junyang Yue, Wei Tang, Wei Li, Ying Yang, Lihuan Wang, Congbing Fang & Yongsheng Liu

School of Food and Biological Engineering, Hefei University of Technology, Hefei, 230009, China

Junyang Yue, Xiaofeng Tang, Shengxiong Huang & Yongsheng Liu

Boyce Thompson Institute, Cornell University, Ithaca, NY, 14853, USA

Jiacheng Liu, Ya Qing Wu, Kun Zhao, Zhangjun Fei & Yi Zheng

USDA-Agricultural Research Service, Robert W. Holley Center for Agriculture and Health, Ithaca, NY, 14853, USA

Zhangjun Fei

Ministry of Education Key Laboratory for Bio-resource and Eco-environment, College of Life Science, State Key Laboratory of Hydraulics and Mountain River Engineering, Sichuan University, Chengdu, 610064, China

• Yongsheng Liu

Beijing Advanced Innovation Center for Tree Breeding by Molecular Design, Beijing University of Agriculture, Beijing, 102206, China

Plant Science and Technology College, Beijing University of Agriculture, Beijing, 102206, China

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Z.F., Y.L., and Y.Z. designed and managed the project. J.L., J.Y., Y.Z., and Y.Q.W. constructed the database; K.Z. and Y.Z. collected and analyzed the data. W.T., X.T., W.L., Y.Y., L.W., S.H., and C.F. participated in discussions. J.Y., Y.Z., Z.F., and Y.L. wrote and revised the manuscript.

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Correspondence to Zhangjun Fei , Yongsheng Liu or Yi Zheng .

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Yue, J., Liu, J., Tang, W. et al. Kiwifruit Genome Database (KGD): a comprehensive resource for kiwifruit genomics. Hortic Res 7 , 117 (2020). https://doi.org/10.1038/s41438-020-0338-9

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DOI : https://doi.org/10.1038/s41438-020-0338-9

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To describe the nutritional and health attributes of kiwifruit and the benefits relating to improved nutritional status, digestive, immune and metabolic health. The review includes a brief history of green and gold varieties of kiwifruit from an ornamental curiosity from China in the 19th century to a crop of international economic importance in the 21st century; comparative data on their nutritional composition, particularly the high and distinctive amount of vitamin C; and an update on the latest available scientific evidence from well-designed and executed human studies on the multiple beneficial physiological effects.

Of particular interest are the digestive benefits for healthy individuals as well as for those with constipation and other gastrointestinal disorders, including symptoms of irritable bowel syndrome. The mechanisms of action behind the gastrointestinal effects, such as changes in faecal (stool) consistency, decrease in transit time and reduction of abdominal discomfort, relate to the water retention capacity of kiwifruit fibre, favourable changes in the human colonic microbial community and primary metabolites, as well as the naturally present proteolytic enzyme actinidin, which aids protein digestion both in the stomach and the small intestine. The effects of kiwifruit on metabolic markers of cardiovascular disease and diabetes are also investigated, including studies on glucose and insulin balance, bodyweight maintenance and energy homeostasis.

Conclusions

The increased research data and growing consumer awareness of the health benefits of kiwifruit provide logical motivation for their regular consumption as part of a balanced diet. Kiwifruit should be considered as part of a natural and effective dietary strategy to tackle some of the major health and wellness concerns around the world.

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The Nutritional and Health Benefits of Kiwiberry ( Actinidia arguta ) – a Review

Inulin and Health Benefits

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Introduction

Kiwifruit are a nutrient-dense fruit and extensive research over the last decade on the health benefits of kiwifruit has linked their regular consumption to improvements not only in nutritional status, but also benefits to digestive, immune and metabolic health. The health benefits of consuming fruit are well documented [ 1 ]. Kiwifruit are exceptionally high in vitamin C and contain an array of other nutrients, notably nutritionally relevant levels of dietary fibre, potassium, vitamin E and folate, as well as various bioactive components, including a wide range of antioxidants, phytonutrients and enzymes, that act to provide functional and metabolic benefits. The contribution of kiwifruit to digestive health is attracting particular attention owing to a growing body of evidence from human intervention studies. There are several plausible mechanisms of action that are likely to act together including the fibre content and type, the presence of actinidin (a natural proteolytic enzyme unique to kiwifruit which breaks down protein and facilitates gastric and ileal digestion [ 2 , 3 ]), and other phytochemicals which may stimulate motility [ 4 ].

The kiwifruit of commercial cultivation are large-fruited selections of predominantly Actinidia deliciosa cv Hayward (green kiwifruit) and an increasing range of gold varieties of various Actinidia species. The Hayward cultivar is an oval-shaped berry with a dull brown hairy skin, however, one of its most attractive features is the strikingly beautiful appearance of the bright translucent green flesh interspersed with several rows of small black seeds. As an example of a gold fleshed kiwifruit Zespri® Sungold ( Actinidia chinensis spp .) have a bright yellow flesh surrounded by a smooth, hairless, bronze-coloured skin. The flesh of the green Hayward cultivar is described as a tangy, sweet and sour combination providing a unique flavour combination, whereas the gold cultivar is described as having a sweet and tropical taste [ 5 , 6 ].

In the twentieth century, kiwifruit came a long way from being a wild species partially exploited by man to being a commercial crop of international economic importance [ 7 ]. Kiwifruit are native to the temperate forests of the mountains and hills of southwest China. Missionaries in the nineteenth century made many contributions to the advancement of botany and the distribution of horticultural plants [ 8 ]. The first botanical specimens of A. chinensis were sent to Europe by the Jesuit priest Père Pierre Noël Le Chéron d’Incarville around the 1750s and later by Robert Fortune, a plant collector. Robert Fortune was sent to China by the Horticultural Society of London (1843–1845) to “collect seeds and plants of an ornamental or useful kind”, and one of Fortune’s specimens of A. chinensis was held at the Royal Botanic Gardens at Kew, London. The first fruits of A. chinensis to be seen in Europe were sent, preserved in spirit, to Kew in 1886. Today New Zealand is a major producer of kiwifruit, and all early commercial varieties of kiwifruit plants in New Zealand and around the world can be traced back to a Church of Scotland mission station in Yichang, China, in 1878. Early in the twentieth century, the seeds and plants were regarded as ornamental curiosities with no mention of the edible fruit. The introduction of kiwifruit to New Zealand can be traced to a school teacher, Isabel Fraser, who in 1904 returned from a visit to China with seeds [ 7 ]. Around 1922, Hayward Wright, a nurseryman living near Auckland, New Zealand, offered plants in his catalogue, listing the plant as “a wonderful fruiting climber” and promoting it as a highly valuable new fruit because it ripens in the winter over a long period, thus making the fruit a valuable addition to the short supply of winter fruits.

The Hayward cultivar has been sold widely from the late 1930s and the dominance of this cultivar worldwide is now complete. The first commercial orchards and large-scale plantings occurred around this time. Orcharding kiwifruit required brave and courageous decisions as the work was hard, there were no proven patterns of management by growers and agronomic problems were faced as they occurred. World War II and then agricultural and marketing incentives from the 1950s to the present day resulted in the rapid geographical expansion of orchards in New Zealand, Australia, Chile, USA and Europe, mainly Italy, France and Greece. In Italy, the high content of vitamin C gave kiwifruit the reputation of being the “frutto della salute”—the health fruit [ 8 ].

The last 100 years have seen the domestication of the kiwifruit from being a wild plant (the so-called “Chinese gooseberry”) to a stage where it is now an important crop in several countries. The name “kiwifruit” was proposed by Turners and Growers Ltd, an exporting firm in Auckland, after the flightless bird, which is endemic to, and often taken as, the emblem of New Zealand. Servicemen were also commonly known as “Kiwis”, and by 1969 the name kiwifruit was well established and accepted.

The process of domestication of kiwifruit is a fascinating and complex story. It includes botanical identification, the collection of seeds and propagating material, cultivation techniques to grow and manage the plant, the management of a dioecious perennial climber, selection of the best cultivars, the commercial discoveries of the cultural conditions affecting yield, harvesting, storage, packing to extend the season and transporting across the globe [ 8 ].

Of all the different species of Actinidia , the main cultivar of economic importance is A. deliciosa , and all the commercial plantings in New Zealand can be traced back to the seeds introduced by Isabel Fraser. The geographic range, the diversity of the wild population and subsequent development of cultivars, including gold and red-fleshed varieties, indicate that the gene pool, mostly sourced from wild types in China, offers many opportunities for breeding programmes for many desirable attributes, including very high levels of vitamin C [ 5 , 9 ]. Whilst the kiwifruit season requires winter growing, the fruit can be stored very well once harvested and also is produced in both the northern and southern hemispheres. This means that kiwifruit is available throughout the year which is important for those interested in regular consumption for its health benefits [ 10 ].

The nutritional attributes of kiwifruit

Comprehensive and independent data on the nutritional composition of kiwifruit can be found in the USDA National Nutrient Database for Standard Reference [ 11 ] and the New Zealand Food Composition Database (NZFCD) [ 12 ]. Chemical analyses are conducted on fruit ripened to the “ready-to-eat” state to ensure that the data are reflective of what would normally be consumed. Typically, kiwifruit ( A. deliciosa and A. chinensis— “green” and “gold” cultivars, respectively) are eaten with the skin removed, and hence the analyses shown in Table 1 are completed on the edible flesh portion only. A recent update to this information in the NZFCD now includes nutritional composition of the skin, as there is anecdotal evidence of growing number of consumers who choose to eat the skin, particularly of the gold varieties since it is smoother, thinner, and hairless. Consumption of whole SunGold kiwifruit (including the skin) increases the fibre, vitamin E and folate contents by 50, 32 and 34%, respectively [ 13 ].

The total ascorbic acid content is the most distinctive nutritional attribute of kiwifruit [ 12 ]. The levels in the Hayward green cultivar are typically between 80 and 120 mg per 100 g fresh weight [ 14 ]. This natural variation of amounts of vitamin C in fruit, including kiwifruit, is due to numerous factors including growing region and conditions, use of fertilisers, maturity at harvest, time of harvest, storage and ripening conditions [ 15 ]. In terms of nutritional value, using scoring models that rank and compare the amount of important nutrients present in foods, kiwifruit score well against other fruit. This provides a useful means for communicating those nutritional benefits to consumers [ 16 , 17 , 18 ], and should be noted that the high nutrient density score is largely driven by their high vitamin C content [ 12 ]. Figure  1 compares the vitamin C content of various fruits to that of Hayward and SunGold kiwifruit cultivars. The SunGold kiwifruit contains 161.3 mg vitamin C per 100 g—almost three times the amount found in oranges and strawberries on an edible flesh weight basis.

Graph comparing the vitamin C content of kiwifruit with other commonly consumed fruit

From the technical and sensory perspectives, the high ascorbic acid and low tannin content in kiwifruit are thought to explain why the cut fruit does not develop the typical browning reaction that occurs in most other fruits [ 14 ].

Vitamin C (ascorbic acid) is an essential dietary nutrient for humans, as we lack the terminal enzyme l -gulono-γ-lactone oxidase in the ascorbate biosynthetic pathway [ 19 ]. There is an absolute requirement for Vitamin C for a range of biological functions. Vitamin C is a cofactor of metallo-enzymes necessary for the biosynthesis of collagen, l -carnitine, catecholamine, neurotransmitters, and peptide hormones such as oxytocin [ 20 , 21 ]. Vitamin C in involved a in the regulation of transcription factors [ 22 ]. The strong antioxidant properties of Vitamin C are well documented, it scavenges free radicals and other reactive oxygen and nitrogen species, with a capacity to regenerate other small molecule antioxidants from their respective radicals [ 23 , 24 ]. Thus, it protects biomolecules such as lipids and DNA against oxidative damage [ 25 , 26 ].

There is evidence from in vitro, as well as animal and human intervention studies that supports the role of vitamin C in the functioning of the immune system. Leukocytes, which are cells responsible for defending the body against invading pathogens, contain high levels of vitamin C, indicating a vital function in the immune system [ 27 ]. These cells include neutrophils, the first cellular responders to inflammatory challenge. Their primary function is to destroy invading microorganisms and thereby prevent systemic infection [ 28 , 29 ].

A recent Cochrane systematic review [ 30 ] upholds the role of vitamin C in improving immune function and reducing the duration of common cold symptoms in the ordinary population. A Gold kiwifruit intervention study showed enhanced plasma vitamin C concentration and reduced severity and duration of upper respiratory infection symptoms in 32 elderly people supplemented with four kiwifruit per day for 4 weeks [ 31 ].

An effectively functioning immune system is crucial for maintaining physiological integrity, and the European Food Safety Authority (EFSA) Panel on Dietetic Products, Nutrition and Allergies (NDA) [ 32 ] considers that maintaining normal immune function is a beneficial physiological effect. Given the multiple roles of the immune system providing defences against infections and allergic manifestations such as asthma, urticaria and eczema, the specific effect on immune function is required for scientific substantiation of health claims on a food/constituent. The requirements for substantiation of health claims on maintaining normal immune function in a population group considered to be at risk of immunosuppression (e.g., older adults, individuals experiencing stress or engaging in heavy physical exercise, or after exposure to ultraviolet radiation) are provided in the scientific opinion of EFSA [ 32 ].

The vitamin C content of green and gold kiwifruit is 92.7 and 161.3 mg per 100 g, respectively [ 11 ]. In the European Union, the Reference Intake (RI) for vitamin C for labelling purposes is 80 mg [ 33 ]. For “source” and “high” nutrient content claims for vitamin C, the amounts required for the claims are 15% RI, or 12 mg, and 30% RI, or 24 mg, per 100 g, respectively. Hence the levels of vitamin C in kiwifruit qualify them as being high in the vitamin, and eligible for authorised health claims in the European Union (EU) for vitamin C nutrient functions (Table 2 ). Amongst a background of a large number of antioxidant species, the vitamin C content of kiwifruit has the highest correlation with total antioxidant activity of kiwifruit [ 34 ].

High levels of vitamin C in kiwifruit can improve iron bioavailability [ 35 ]. Poor iron status remains one of the most common micronutrient concerns globally [ 36 ], and is associated with a number of adverse health consequences [ 37 ]. In a study of individuals with low iron status [serum ferritin (SF) ≤ 25 µg/L and haemoglobin (Hb) ≥ 115 g/L], eating kiwifruit with an iron-fortified breakfast cereal was found to improve iron status [ 35 , 38 ]. In this study, 89 healthy women were randomised to receive iron-fortified breakfast cereal, milk and either two Zespri gold kiwifruit or one banana at breakfast every day for 16 weeks. After 16 weeks, median serum ferritin significantly increased from 17.0 µg/L at baseline to 25.0 µg/L, compared to the banana group, which had a median serum ferritin level of 16.5 µg/L at baseline that rose to 17.5 µg/L at the end of the study ( P  < 0.001). Importantly, the 10 µg/L increase in serum ferritin in the women who ate kiwifruit increased levels to within the normal reference range of 20–160 mg/L. Additionally, median soluble transferrin receptor concentrations significantly decreased by − 0.5 mg/L for kiwifruit versus 0.0 mg/L for banana ( P  = 0.001) [ 35 , 38 ].

Significant proportions of the population around the world, including the UK [ 39 ], have very poor fruit and vegetable intakes that result in suboptimal vitamin C status. The maintenance of the body pools and of plasma and cellular vitamin C concentrations are considered criteria for establishing the requirements for vitamin C based on the assumption that saturation of body pools and plasma concentrations are associated with fulfilling the essential functions of vitamin C in the body [ 26 ]. Saturating plasma levels, now considered to be associated with optimal health and wellbeing, are found in only around 20% of the normal, healthy population. Carr et al. [ 40 ] showed that consuming kiwifruit had a strong effect on plasma and muscle [ 23 ] vitamin C levels. To measure the contribution of gold kiwifruit to dietary vitamin C intake, plasma vitamin C levels were measured in a group of 14 male students with low vitamin C status (average baseline plasma, 38 mM). Participants were asked to consume half a kiwifruit per day for 4 weeks, two kiwifruit per day for 6 weeks and finally three kiwifruit per day for 4 weeks. The addition of as little as half a kiwifruit per day resulted in a significant increase in plasma vitamin C. However, one kiwifruit per day was required to reach what are considered to be healthy levels [ 40 ]. At two kiwifruit per day, plasma levels approached saturation, with no further increases with three kiwifruit per day. This observation was confirmed by increased urinary output of vitamin C at two kiwifruit per day, which coincided with plasma levels reaching around 60 mM. These results confirmed the pharmacokinetic data of Levine et al. [ 41 ] and indicated that plasma vitamin C levels in humans saturate at an intake of about 200 mg/day. This is equivalent to eating approximately two kiwifruit per day.

Furthermore, vitamin C and increased consumption of fruits and vegetables have been shown to be associated with enhanced feelings of wellbeing and vitality [ 42 , 43 , 44 , 45 ]. It is well established that fatigue and lethargy are common early symptoms of subclinical vitamin C deficiency and can be resolved with vitamin C supplementation [ 46 ]. The effects of vitamin C on fatigue are likely explained by its in vivo function as an enzyme cofactor for the synthesis of important biomolecules such a dopamine, neurotransmitters and hormones synthesised by the nervous system and adrenal glands [ 47 ].

Kiwifruit contain relatively high levels of vitamin E [ 12 , 48 ], compared to other commonly consumed fruit. SunGold and green kiwifruit contain 1.40 and 1.46 mg per 100 g [ 11 ], respectively, of the main form, α-tocopherol present in the flesh [ 49 ]. These levels are sufficient to permit the use of nutrient function claims for Vitamins E in the EU (Table 2 ). Fiorentino et al. [ 49 ] showed that α-tocopherol is found in the flesh of kiwifruit, possibly associated with cell membranes and therefore potentially bioavailable. Fiorentino et al. [ 49 ], also identified a new form of vitamin E in kiwifruit, δ-tocomonoenol, noting that its radical scavenging and antioxidant capacity contributed to the total antioxidant activity of kiwifruit. Studies showing that the consumption of both green and gold kiwifruit correlates with increased plasma vitamin E concentrations, suggest the vitamin E in kiwifruit is bioavailable [ 31 , 50 ].

Kiwifruit are often referred to as being a good source of dietary folate. The folate content of kiwifruit green and gold cultivars compared with other fruits are shown in Fig.  2 . The folate content of 31 µg per 100 g in gold kiwifruit meets the criteria of EU Regulation to make a “source” claim as it exceeds the 15% of the Reference Intake of 200 µg/day. In other countries, where the recommended daily intake is often higher (e.g., 400–500 µg/day in Nordic counties, 400–600 µg/day in the USA Australia and NZ), such nutrient content claims cannot be made. The authorised health claims in the EU for folate nutrient functions are shown in Table 2 .

Graph comparing the folate content of kiwifruit with other commonly consumed fruit

As folate is extremely labile and its presence in green leafy vegetables is easily destroyed by cooking, fresh kiwifruit can make a useful contribution to the total diet, especially during pregnancy when it is difficult to meet folate requirements. During pregnancy, folate requirements are 600 µg/day, which can be safely achieved through the use of conventional foods, foods with added nutrients and food supplements [ 51 ].

Green and gold kiwifruit are good sources of potassium, containing typically around 301–315 mg per 100 g. These amounts are sufficient to meet the criteria of EU Regulation (EC) no. 1924/2006 on nutrition and health claims made on foods to make a natural “source” claim, as it exceeds the 15% of the Reference Intake of 2000 mg/day. The authorised nutrient function health claims in the EU for potassium are shown in Table 2 . The potassium content of kiwifruit compared to other fruit is shown in Fig.  3 . In other countries, where the recommended daily intake is often higher, such content claims cannot be made.

Graph comparing the potassium content of kiwifruit with other commonly consumed fruit

Fresh foods such as fruits and green vegetables are generally good sources of potassium and low in sodium. The sodium content of kiwifruit is only 3 mg per 100 g and can be described as naturally low in sodium. The sodium to potassium (Na + /K + ) ratio of kiwifruit is consistent with recommendations to increase potassium intake through increased consumption of fruit and vegetables, and is amongst the more favourable Na + /K + balance of selected fruits [ 52 ]. Studies have provided evidence that potassium rich diets or interventions with potassium can lower blood pressure, especially in individuals with hypertension [ 53 , 54 ], however, more recently the dietary Na + /K + ratio has been shown to be more strongly associated with an increased risk of hypertension and CVD-related mortality than the risk associated with either sodium or potassium alone [ 55 , 56 ].

Dietary fibre

The dietary fibre of kiwifruit comes almost entirely from the plant cell walls, and particularly the polysaccharides that form the major structural components of these walls. Kiwifruit contain about 2–3% of fresh weight non-starch polysaccharides [ 48 ] that make up the fruit cell walls, providing a valuable contribution of both soluble and insoluble fibre to the diet. Analysis of dietary fibre of green and gold kiwifruit has shown they comprise about one-third soluble and two-thirds insoluble fibres, although kiwi gold fruit contain considerably less total fibre than green [ 57 ]. The soluble fibre fraction contains almost exclusively pectic polysaccharides, whereas the insoluble fibre is mostly cellulose and hemicelluloses.

Changes occur in the composition and structure of kiwifruit cell walls during development and ripening. These structural changes in cell wall polysaccharides are reviewed in detail by Sims, Monro [ 58 ]. Cell wall polysaccharides are generally resistant to digestion and absorption in the human small intestine and are considered to be delivered to the colon in a chemically unaltered state. However, even minor chemical or structural changes can impact on the physicochemical properties and fermentability that determine their impact on health.

In the hind-gut, the physiological benefits of fibre are believed to arise from the products of bacterial fermentation of the soluble fibre, and from the physicochemical properties of any fibre that remains unfermented [ 59 , 60 ]. Among the most important physicochemical properties of kiwifruit fibres are the hydration properties, which include water retention, capacity and swelling, viscosity (which requires solubility), and properties that depend on the size, shape and porosity of undigested particles. Water retention is physiologically relevant because it influences transit time, faecal bulk, stool consistency and other functional benefits [ 60 ]. The high swelling and water retention of kiwifruit fibre in comparison with other forms of dietary fibre such as wheat bran, commercial preparations of sugar beet fibre and apple fibre, accentuate the value of consuming kiwifruit as a natural whole product that has had minimal processing. Kiwifruit dietary fibres are susceptible to fermentation, and so many provide benefits through the production of the short chain fatty acids [ 58 ]. Future studies on the mechanisms by which kiwifruit dietary fibres, as part of a balanced diet, modulate digestion processes and act as a substrate for beneficial colonic microbiota, may aid understanding of the actions of fibre in the gut [ 61 ] and its beneficial effects on human health.

As kiwifruit develop and ripen, the concentrations of chemical components in the tissue change. The most marked change in the physiology of the fruit during ripening leads to a rapid decrease in starch concentration and a consequent increase in fructose and glucose. Kiwifruit tissue is very hard while the fruit is developing on the vine, but flesh firmness decreases during the later stages of development [ 14 ]. Fortunately, kiwifruit that are physiologically mature but have barely started to ripen can be harvested and will continue to ripen successfully off the vine. Cool storage immediately after harvest reduces the rate of ripening. It is these particular characteristics of kiwifruit that allow producing countries such as New Zealand to store unripe fruit and ship to it distant markets over an extended period. Suitable indicators of maturity for kiwifruit are used to ensure that fruit reaches an appropriate stage of development before harvest. A “maturity value” is important, and three changes in kiwifruit are taken into account—decreasing flesh firmness, conversion of starch to sugar and soluble solids concentration (to measure sugar concentration) are all used to provide an accurate assessment of final eating quality. The predominant sugars present in Actinidia are glucose and fructose with a small amount of sucrose present when the fruit is ripe and ready-to-eat. The amount of total sugars and ratios of these sugars vary not only as a function of maturity but also with the variety of kiwifruit [ 62 , 63 ]. The ratio of fructose: glucose is important in terms of digestive health and preferably should be around 1:1 to reduce symptoms of gastrointestinal discomfort, such as bloating, caused by rapid fermentation by gut bacteria.

Interestingly, as they ripen, many fruits undergo a marked decrease in chlorophyll content, and carotenoids and anthocyanins become dominant. These visual changes indicate the stage of ripeness. On the other hand, in green kiwifruit there is little if any decrease in chlorophyll content and the internal colour remains an attractive bright green when fruit are “eating ripe”. As kiwifruit begins to ripen, starch concentration decreases from 6% of fresh weight to trace amounts, and total sugars increase to 12–15%. The concentration of soluble solids also increases to reach a plateau of 14–16% before fruit is eating ripe.

Understanding the factors affecting the rate of ripening is of considerable commercial importance for fruit quality. In fruit that is ready for consumption the sugars provide the appealing sweet flavour of kiwifruit, which is balanced by the organic acid composition [ 62 , 63 ].

From a physiological perspective, the sugar content of kiwifruit, like all fruit, may potentially influence the management of blood sugar levels following their consumption, however current research suggests the glycaemic response effects of kiwifruit as a whole food are potentially different to that which could be expected of individual components [ 64 ]. Interestingly the glycaemic index (GI) of kiwifruit is relatively low (green kiwifruit, 39.3 ± 4.8 and gold kiwifruit, 48.5 ± 3.1 [ 65 ]). The low GI value of kiwifruit is observed in both healthy human subjects and those with Type 2 diabetes [ 66 ]. The importance of managing postprandial blood sugar levels is covered in the section on metabolic health.

Antioxidants

In addition to the various nutrients in kiwifruit described above, for which there are dietary intake recommendations and well described physiological functions, kiwifruit contain a complex network of minor compounds that may also be associated with beneficial physiological functions. Various Actinidia species have been extensively analysed for their antioxidant chemical profiles [ 67 , 68 , 69 , 70 , 71 ]. As well as vitamins C and E, the other antioxidants include the carotenoids lutein, zeaxanthin and β-carotene, chlorophylls, quinic acid, caffeic acid glucosyl derivatives, β-sitosterol, chlorogenic acid, phenolics, including flavones and flavonones, to name but a few [ 72 , 73 , 74 , 75 ]. The antioxidant capacity of kiwifruit constituents has been measured by means of various in vitro chemical assays that monitor the quenching, scavenging or retarding of free radical generation [ 6 ]. For example, the total antioxidant capacity of kiwifruit was reported to be higher than apple, grapefruit and pear, but less than raspberry, strawberry, orange and plum [ 76 , 77 ]. While these in vitro studies indicate that the various antioxidants are capable of preventing or delaying some types of cell damage from the unstable free radicals created every day during normal metabolism, the detailed mechanism of how this translates to effects in vivo which are directly linked physiological changes is yet to be fully understood [ 78 ]. In a number of human studies, beneficial changes to biomarkers of CVD, have been attributed to the antioxidant compounds present in kiwifruit [ 79 , 80 , 81 , 82 , 83 , 84 , 85 ]. The stability of antioxidants during simulated in vitro gastrointestinal digestion [ 86 , 87 ], and their bioaccessibility/bioavailability [ 88 ] provide supportive evidence for the potential for physiological effects of the antioxidants in kiwifruit. There is significant variation in the types and levels of antioxidant compounds and total antioxidant activity both between Actinidia species, and as a function of extraction solvent [ 73 , 74 , 75 ]. Several studies have explored the influence of growing practices and region on the activity of bioactive and antioxidant compounds in kiwifruit. Park et al. [ 89 ] found generally higher, but not consistently significant, levels of bioactive compounds in organically grown kiwifruit, whilst in an Italian study, the geographical location of orchards did not significantly influence vitamin C or polyphenolic contents [ 90 ].

Although there are no dietary intake recommendations for antioxidants in general, the scientific data suggest that eating kiwifruit has the potential to inhibit oxidative and inflammatory processes, although the supporting data for antioxidant activities are more substantial than those related to the kiwifruit’s potential anti-inflammatory activities. The results of human studies of the antioxidant efficacy of kiwifruit are inconsistent owing to differences in experimental protocols, the cultivar of kiwifruit used, the amount and duration of the study as well as the biomarkers used [ 6 ]. Kiwifruit could undoubtedly be a useful dietary vehicle for delivering antioxidant nutrients and other phytonutrients. Future studies on kiwifruit will explore the bioavailability, metabolism, tissue distribution and biological effects of kiwifruit constituents on relevant disease markers. The emerging evidence could provide the basis for improved dietary strategies for achieving dietary antioxidant and anti-inflammatory health benefits in humans [ 91 ].

Actinidin and minor proteins

Kiwifruit contain several unique proteins and the cysteine protease actinidin, the most abundant protein in kiwifruit, of interest for their bioactive potential.

The characterisation and biochemical properties of actinidin have been extensively studied [ 92 , 93 ], and more recently its potential role in human health [ 94 , 95 ]. Actinidin is active over a wide range of pH, including those of the GI tract [ 96 ] thus having the potential to influence protein digestion, and intestinal permeability [ 97 ]. In contrast to potential benefits (see Digestive health), actinidin is also the major kiwifruit allergen [ 90 , 98 ]. Green and gold kiwifruit have been known to cause allergic reactions ranging from mild symptoms localised to the oral mucosa in the majority of individuals to anaphylactic reactions, particularly in children [ 99 ]. Very little information is available in the literature on the prevalence of kiwifruit allergy, and human intervention studies with kiwifruit have shown that kiwifruit are well tolerated without any adverse side effects [ 35 , 50 , 84 , 100 ]. The magnitude and patterns of reactivity to kiwifruit allergens appears to vary with ethnic/geographical/cultural differences, age of subjects and other clinical characteristics of individuals exposed to kiwifruit [ 6 ]. Lucas, Atkinson [ 101 ] have provided a detailed review of unresolved issues regarding kiwifruit and have suggested requirements to be met prior to designation of allergens to a database. Processing may diminish the risk of allergic symptoms in those with allergies to raw kiwifruit [ 102 , 103 ].

Kiwellin is another protein in kiwifruit, that as a function of ripening stage and postharvest treatment of the fruit is susceptible to actinidin activity, producing the peptide kissper, and and KiTH [ 104 , 105 ]. Kissper is of particular interest for human health as it displays a range of beneficial activities, including anti-inflammatory response, reducing oxidative stress at the GI mucosal interface [ 106 ], and pH-dependent and voltage-gated pore-forming activity, together with anion selectivity and channelling [ 4 ]. This suggests that kissper is a member of a new class of pore-forming peptides with potential beneficial effects on human health, including a potential effect on gastrointestinal physiology [ 4 ].

• Digestive health

Early Chinese pharmacopoeia from the Tang Dynasty onwards (AD 618–907) list a whole variety of medicinal uses for “mihoutao” fruit, the Chinese name generally used for Actinidia species, including aiding digestion, reduction of irritability and curing of dyspepsia and vomiting.

Functional gastrointestinal disorders (FGIDs) are common and distressing [ 107 ]. FGIDs include functional dyspepsia (FD) and irritable bowel syndrome (IBS), affecting an estimated 3–28% of the global population [ 108 ], particularly the elderly and women, and may severely affect the individual’s quality of life and wellbeing [ 107 , 109 ]. Upper gastrointestinal disorders include gastric reflux, stomach ache, delayed gastric emptying, nausea and vomiting, and lower gastrointestinal disorders include constipation, indigestion, bloating and diarrhoea. Current interventions for FGIDs include lifestyle and dietary modifications as well as pharmacological interventions targeting pain, motility, laxation and the gut microbiota [ 108 ].

The worldwide growth in the incidence of FGIDs has created an immediate need to identify safe and effective food-based interventions. For example, constipation may be present in up to 29% of the population, depending on the definitions used [ 110 , 111 , 112 ]. Food ingredients such as psyllium and wheat bran are the most studied for maintaining a healthy gut and to manage abdominal discomfort. Additionally, it is generally regarded that adequate intakes of fibre-rich fruits and vegetables daily with sufficient water will prevent constipation. Whole green kiwifruit have been used and promoted for many years to maintain abdominal comfort [ 113 ] and have been studied more recently under controlled settings [ 114 , 115 ]. The components found in kiwifruit have been shown to increase faecal bulking and softness and enable better lubrication, assisting the propulsion of content along the colon [ 116 , 117 ].

It is thought that the unique combination of soluble and insoluble fibres, polyphenols and actinidin, present in kiwifruit, confers the gastrointestinal benefits, improvements in laxation and reduction of abdominal discomfort, both in individuals with either constipation-predominant irritable bowel syndrome (IBS-C) and in normal healthy people suffering from constipation without reported side effects. The putative mechanism of kiwifruit on maintenance of normal GI function has recently been reviewed [ 95 ]. The review discusses the physiological functions of the digestive system, the pathophysiological mechanisms behind functional constipation, a summary of the work covering the effects of green kiwifruit on the gut as well as hypothetical mechanisms behind the gastrointestinal effects of green kiwifruit.

Lack of dietary fibre is a contributing factor in people with constipation [ 118 ], and both soluble and insoluble fibres can add bulk, increase water retention in the colon [ 119 , 120 ] and change faecal consistency [ 121 , 122 ]. Dietary fibre can also decrease transit time [ 122 , 123 ]. Soluble dietary fibres are the main substrate for the microflora in the GI tract [ 60 ]. When setting the Dietary Reference Value (DRV) of 25 g /day for dietary fibre, the EFSA NDA Panel used the role of fibre in bowel function as the most suitable criterion [ 124 ]. Consuming 2 green kiwifruit per day would provide approximately 6 g of fibre (24% DRV), therefore, depending on habitual dietary fibre intake this may be a significant contribution to the total daily intake. Kiwifruit typically contain about two-thirds insoluble fibre, and one-third soluble fibre [ 125 ], and as previously mentioned, kiwifruit fibre has an impressive water retention capacity [ 57 , 58 ]. In the native state, the capacity of kiwifruit fibre to swell, defined as the volume fibre has in water after passively settling [ 126 ], is more than six times higher than that of apple fibre, and one and a half times higher than psyllium [ 58 ], but is significantly reduced when subjected to processing conditions such as dehydration [ 127 ]. Feeding studies in pigs [ 128 , 129 ] as well as observations in human studies [ 114 , 115 , 130 ] have demonstrated that feeding kiwifruit increases water retention and faecal bulking, however animal studies suggest the pectic substances of kiwifruit are highly susceptible to fermentation in the hind-gut [ 131 , 132 ]. Such fermentation may produce short-chain fatty acids capable of stimulating colonic motility [ 133 ] and contribute to the effects of kiwifruit, however the role of kiwifruit fibre in human digestive function is yet to be fully understood. In contrast, but consistent with earlier findings of changes associated with processed kiwifruit, the fibre of a dried kiwifruit product consumed as a part of a mixed fibre diet, did not demonstrate a significant contribution to faecal bulking in the rat [ 131 ]. A reduction in GI transit time has been linked to actinidin [ 128 ]. Although a considerable proportion of short chain fatty acids have recently been shown to be derived from the fermentation of non-dietary gut materials [ 134 ], kiwifruit fibre may also contribute to favourable changes in the human colonic microbial community [ 135 ] and their metabolites [ 136 ] which are associated with intestinal health [ 137 ].

The proteolytic enzyme actinidin from green kiwifruit has been shown in in vitro studies to aid protein digestion both in the stomach and small intestine [ 2 , 3 ]. For example, a range of common protein sources derived from soy, meat, milk and cereals were incubated with a kiwifruit extract containing actinidin and pepsin at pH 1.9 (a simulation of gastric digestion in humans) [ 3 ]. Results in this gastric digestion model showed that for milk, soy and meat protein sources, the presence of kiwifruit extract enhanced digestion to a greater extent than pepsin alone [ 13 ]. Likewise, in an in vitro, small intestine digestion model, actinidin-containing kiwifruit extract was particularly effective in improving the digestion of whey protein, zein, gluten and gliadin [ 2 ]. These studies suggest that actinidin may assist with protein digestion in the gastric and ileal regions, that may be of benefit particularly to individuals with compromised digestive function [ 138 ]. Under in vitro conditions, gastric lipase remained active, however actinidin effectively inactivated amylase suggesting that when cooked starch is consumed together with kiwifruit it is possible that starch digestion may be retarded [ 139 ].

There is growing evidence that kiwifruit have beneficial effects on digestive health and general wellbeing, a potentially important characteristic in the light of the increasing proportion of the elderly population in ageing societies that experience impaired bowel function, changes in gastrointestinal function [ 138 ], and gastrointestinal discomfort.

Table 3 summarises the findings from human clinical trials with fresh green kiwifruit. The daily consumption of two kiwifruit was found to increase stool frequency, including the number of complete spontaneous bowel motions per week, reduce gastrointestinal transit time and improve measures of intestinal comfort. These early human studies [ 50 , 114 , 130 , 140 , 141 , 142 ] were carried out in different countries and included different study populations (e.g., differing in age, health status), and the lack of a common protocol may have led to results that were not applicable to the larger normal healthy population. Most studies consider the effects of prolonged kiwifruit consumption, however recently Wallace et al. [ 143 ] investigated the acute effects of green kiwifruit on gastric emptying following consumption of a steak meal, using a computerised SmartPill™, and measures of digestive comfort. Although the SmartPill™, did not provide reliable data following the meal event, there was a significant reduction in bloating and other measures of gastric discomfort [ 143 ]. A multi-country, randomised, cross-over, controlled human intervention study is currently underway to evaluate further the effects of green kiwifruit on digestive function [ 144 ]. Changes in bowel function in the general population such as reduced transit time, more frequent bowel movements, increase faecal bulk or softer stools are considered by EFSA to be beneficial physiological effects, provided they do not result in diarrhoea [ 32 ]. Similarly, reducing gastrointestinal discomfort [e.g., bloating, abdominal pain/cramps, borborygmi (rumbling)] are considered appropriate outcome measures in human studies that include the use of validated questionnaires on severity and frequency of symptoms. The EFSA Panel on Dietetic Products Nutrition and Allergies (NDA) [ 32 ] has also stated that IBS patients or subgroups of IBS are generally considered an appropriate study group to substantiate health benefits on bowel function and GI discomfort.

Fermentable Oligosaccharides, Disaccharides, Monosaccharides and Polyols (FODMAPs) are rapidly fermentable, poorly absorbed carbohydrates found in food that can cause digestive discomfort, especially for people with IBS [ 145 ]. The action of FODMAPS is likely via multiple pathways [ 146 ], and includes the release of gases from the bacterial fermentation of oligosaccharides and the proportion (if any) of malabsorbed fructose, polyols, and lactose [ 147 ]. Symptoms associated with FODMAPs include abdominal bloating, pain, cramps, excessive flatulence and altered bowel habit [ 146 ]. Low FODMAP diets are effective in the treatment of functional gastrointestinal symptoms [ 148 , 149 ].

Kiwifruit are certified as low FODMAP fruits by the Monash University low-FODMAP diet digital application ( https://www.monashfodmap.com/i-have-ibs/get-the-app/ ), based on their relatively low proportions of fructose and fructans per single fruit serve. A recent pilot study demonstrated that the consumption of two green kiwifruit is not associated with clinically significant evidence of colonic fermentation as shown by hydrogen and methane on breath testing [ 150 ], lending support for the low FODMAP status for kiwifruit.

Metabolic health

Metabolic abnormalities such as dyslipidaemia [increased blood total cholesterol (TC), low density lipoprotein cholesterol (LDL-C), triglycerides (TG), lower high density lipoprotein cholesterol (HDL-C)], hypertension, vascular inflammation, abnormal glucose metabolism and haemostatic disorders all play important roles in the pathophysiology of the major causes of morbidity and mortality such as diabetes, cardiovascular disease (CVD), stroke and dementia [ 151 , 152 , 153 ]. A number of studies have investigated the effects of green and gold kiwifruit on some of these metabolic markers, including the effects of kiwifruit on glucose and insulin balance, and on bodyweight maintenance and energy homeostasis.

Green and gold kiwifruit have clinically measured glycaemic indices (GIs) of 39 and 48, respectively [ 65 ], which puts them in the GI “low” category (GI < 55). The glycaemic response to a fruit depends not only on GI, but also the amount of carbohydrate consumed in the fruit. As kiwifruit contains only about 12% available carbohydrate and a low GI; the impact kiwifruit produces on plasma glucose levels is low enough for the fruit to be suitable in managing diets for people with reduced tolerance to glucose. The fibre content of kiwifruit may cause a delay in carbohydrate digestion and absorption by way of swelling action that reduces the rate of glucose diffusion [ 57 , 127 ]. This reduction in glycaemic response by 200 g kiwifruit (approximately two fruits) has been demonstrated in a human intervention study conducted by Mishra et al. [ 154 ]. The authors concluded that the low in vivo glycaemic impact could partly be attributed to the carbohydrate in kiwifruit being fruit sugars (fructose) and partly to the non-digested fibre components reducing the rate of intestinal processes such as digestion, sugar diffusion and mixing of intestinal contents. This partial substitution of starch-based staple foods, such as a high carbohydrate breakfast cereal, with kiwifruit could be an effective and healthy way to improve glucose homeostasis [ 154 ]. Further exploration of this effect was investigated by Mishra et al. [ 64 ] to better understand the role of non-sugar components in kiwifruit in modulating glycaemic response. Kiwifruit consistently reduced the amplitude of the glycaemic response of participants following a series of wheat-based cereal meals adjusted to match the available carbohydrates from kiwifruit leading the investigators to conclude that components other than the available carbohydrate in kiwifruit, such as cell wall remnants or phenolic compounds, may be involved in the improved glycaemic response to co-ingested foods [ 64 ]. The energy value of foods is also an important dietary aspect in managing risk factors for metabolic syndrome. Using an in vivo–in vitro model that determines the available energy (AE) content based on ATP yield at the cellular level [ 155 ], Henare et al. [ 156 ] found the AE of green and gold kiwifruit was significantly less than that predicted by the traditional Atwater system, suggesting kiwifruit are useful in dietary weight management strategies. Further studies will explore the use of kiwifruit as an effective dietary strategy to reduce postprandial hyperglycaemia while at the same time increasing the amount of essential nutrients consumed.

Regular consumption of green and gold kiwifruit can also affect beneficially certain physiological biomarkers, particularly in individuals with metabolic abnormalities related to major causes of morbidity and mortality, such as diabetes, cardiovascular disease (CVD), stroke and dementia [ 157 ]. For example, Chang, Liu [ 158 ] investigated the effects of two kiwifruits on the lipid profile, antioxidants and markers of lipid peroxidation in hyperlipidaemic adult men and women in Taiwan. After 8 weeks of the intervention, the HDL-C concentration was significantly increased, whilst the LDL-C/HDL-C ratio and TC/HDL-C ratio were significantly decreased. Vitamin C and vitamin E, the antioxidant nutrients, together with plasma antioxidant status, also increased significantly in fasting blood samples.

Gammon et al. [ 100 ] found that consumption of two green kiwifruit per day for 4 weeks favourably affects plasma lipids in a randomised controlled trial in 85 normotensive and pre-hypertensive hypercholesterolaemic men compared with the consumption of a healthy diet alone. Small, but significant, differences occurred, including an increase in HDL-C and a decrease in TC: HDL-C ratio and TG’s. There were no significant differences, however, between the two interventions for plasma TC, LDL-C, insulin, high-sensitivity C-reactive protein (hs-CRP), glucose and blood pressure (BP). In a further exploration of the study, no beneficial effects on markers of cardiovascular function, or on BP were noted [ 159 ].

In 2012, Karlsen et al. [ 80 ] demonstrated that intake of three kiwifruit per day for 3 weeks promoted pronounced anti-hypertensive effects, as well as antithrombotic effects in male, middle-aged and elderly smokers. The authors commented that this dietary approach may be helpful in postponing pharmacological treatment in individuals with high-normal blood pressure or hypertension. From a further randomised, controlled study over a period of 8 weeks, Svendsen et al. [ 79 ] concluded that among men and women aged between 35 and 69 years with moderately elevated BP, the intake of 3 kiwifruit added to the usual diet was associated with lower systolic and diastolic 24-h BP compared with one apple a day. The authors observed these results were in contrast those of Gammon et al. [ 159 ], noting the differences in study population criteria (normotensive [ 159 ] versus moderately elevated BP [ 79 ]) may have been a contributory factor. Although Svendsen et al. [ 79 ] found no differences in measures of endothelial function in their study, they suggested that an increase in plasma antioxidant status (lutein), and in increased dietary potassium, resulting from the kiwifruit intervention, could be an explanation for the improvements in BP observed.

In vitro studies on antioxidant and fibrinolytic activities have also indicated the potential cardiovascular protective properties of kiwifruit extracts [ 160 ]. Evidence that consumption of kiwifruit can modulate platelet reactivity towards collagen and ADP in human volunteers was provided in a study by Duttaroy, Jørgensen [ 84 ]. The authors concluded that kiwifruit may have the potential to increase the effectiveness of thrombosis prophylaxis.

Habitual intakes of high levels of fruits and vegetables have long been associated with beneficial effects that lower the risk of chronic diseases, including CVD in humans [ 161 ]. The presence of antioxidant components such as vitamin C, vitamin E, polyphenols [ 162 ], a favourable Na + /K + ratio [ 52 ], and other bioactive components in kiwifruit could explain their beneficial physiological effects [ 157 ].

Concluding remarks

This review highlights the nutritional attributes and health benefits of green and gold kiwifruit. The nutritional composition, particularly the high amount of vitamin C, supports its position as a highly nutritious, low energy fruit. With the plethora of man-made, processed health foods available to the consumer, one aspect that sets kiwifruit apart is that it is a natural, whole food. Nature compartmentalises many bioactive and nutritional components within the complex structure of cell walls, cells and the matrix in between. Human digestion interacts with fresh whole foods to break down the structures and digests the complex carbohydrates slowly. Many health care professionals now recognise whole foods are ideal for the release and delivery of nutrients and health components to various locations along our digestive tract.

There is a growing body of evidence to support the beneficial effects of kiwifruit in gastrointestinal function in healthy individuals as well as in individuals with constipation and other gastrointestinal disorders [ 143 , 144 , 163 ], and recognition for the role of kiwifruit in their management [ 164 ]. This presents an evidence-based opportunity for health care professionals to adopt dietary recommendations, and for consumers to recognise the impact of diet, in particular whole foods, on specific body function, and their health and well-being. Green and gold kiwifruit are well characterised and the mechanisms of action for the benefits on gastrointestinal function and modulation of glycaemic responses now being better defined.

Overall, the scientific evidence for the health benefits of kiwifruit needs to be expanded through the conduct of well-designed and executed human intervention studies that clearly define the study populations, the amount and duration of kiwifruit consumption and the specific beneficial physiological effects. A greater understanding of the mechanisms of action of kiwifruit and its bioactive constituents in promoting health also needs to be fully elucidated.

The increased research data identifying the nutritional and health benefits of kiwifruit and their growing consumer acceptance as a part of a balanced diet, will undoubtedly offer opportunities to tackle some of the major health and wellness concerns around the world.

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Richardson, D.P., Ansell, J. & Drummond, L.N. The nutritional and health attributes of kiwifruit: a review. Eur J Nutr 57 , 2659–2676 (2018). https://doi.org/10.1007/s00394-018-1627-z

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Received : 10 August 2017

Accepted : 27 January 2018

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DOI : https://doi.org/10.1007/s00394-018-1627-z

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Comparative study on physicochemical and nutritional qualities of kiwifruit varieties.

1. Introduction

2. materials and methods, 2.1. materials and reagent, 2.2. methods, 2.2.1. determinations of pectin and moisture content, 2.2.2. determinations of chlorophyll a content (cac), chlorophyll b content (cbc), total carotenoid content (tcc), 2.2.3. determination of total flavonoid content (tfc), 2.2.4. determinations of energy, carbohydrates, protein, lipid and dietary fiber, 2.2.5. determinations of total sugar and total acid content, 2.2.6. determinations of soluble sugar and organic acid content, 2.2.7. determinations of vitamin c (vc), vitamin b1 (vb1), vitamin b2 (vb2), vitamin b6 (vb6) and vitamin e (ve), 2.2.8. determination of aroma, 2.3. statistical analysis, 3.1. pectin and moisture, 3.2. cac, cbc, tcc and tfc, 3.3. energy, carbohydrates, protein, lipid and dietary fiber, 3.4. total sugar content, sucrose, glucose and fructose content, 3.5. total acid content, malic acid content, citric acid content, tartaric acid content and quinic acid content, 3.6. sugar: acid ratio, 3.7. vitamin, 3.8. aroma substance, 3.9. principal component analysis (pca), 4. conclusions, supplementary materials, author contributions, institutional review board statement, informed consent statement, data availability statement, conflicts of interest.

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Yuan, X.; Zheng, H.; Fan, J.; Liu, F.; Li, J.; Zhong, C.; Zhang, Q. Comparative Study on Physicochemical and Nutritional Qualities of Kiwifruit Varieties. Foods 2023 , 12 , 108. https://doi.org/10.3390/foods12010108

Yuan X, Zheng H, Fan J, Liu F, Li J, Zhong C, Zhang Q. Comparative Study on Physicochemical and Nutritional Qualities of Kiwifruit Varieties. Foods . 2023; 12(1):108. https://doi.org/10.3390/foods12010108

Yuan, Xinyu, Hao Zheng, Jiangtao Fan, Fengxia Liu, Jitao Li, Caihong Zhong, and Qiong Zhang. 2023. "Comparative Study on Physicochemical and Nutritional Qualities of Kiwifruit Varieties" Foods 12, no. 1: 108. https://doi.org/10.3390/foods12010108

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Overview of Potential Health Benefits

Singletary, Keith PhD

Keith Singletary, PhD, is professor emeritus of nutrition in the Department of Food Science and Human Nutrition at the University of Illinois. From 2001 to 2004, he was the director of the Functional Foods for Health Program, an interdisciplinary program between the Chicago and Urbana-Champaign campuses of the University of Illinois. Dr Singletary received bachelor’s and master’s degrees in microbiology from Michigan State University and his PhD in nutritional sciences from the University of Illinois. Dr Singletary’s primary research interests are in molecular carcinogenesis and cancer chemoprevention, specifically identifying and determining the mechanism of action of phytochemicals in fruits, vegetables, and spices as cancer protective agents. He also investigated the biological basis behind the role of alcohol intake in enhancing breast carcinogenesis. He has been recognized with the Senior Faculty Award for Excellence in Research by the College of Agricultural, Consumer, and Environmental Sciences at the University of Illinois and with the Outstanding Graduate Mentor/Advisor award from the Department of Food Science and Human Nutrition. Dr Singletary currently resides in Florida.

Funding for this article was provided in part by the California Kiwifruit Commission and the International Kiwifruit Organization.

The author has no conflicts of interest to disclose.

Correspondence: Keith Singletary, PhD, University of Illinois, 260 Bevier Hall, 905 S Goodwin Ave, Urbana, IL 61801 ( [email protected] ).

Kiwifruit belongs to the genus Actinidia (Actinidiaceae) and is derived from a deciduous woody, fruiting vine. It is composed of different species and cultivars that exhibit a variety of characteristics and sensory attributes. Kiwi plants have been grown for centuries in China, where they are known as mihoutau . Kiwi plant seeds were brought to New Zealand in the early 20th century, where it was eventually domesticated and sold worldwide. Currently, commercial growth of the fruit has spread to many countries including the United States, Italy, Chile, France, Greece, and Japan. Kiwifruit extracts have been reportedly used in traditional Chinese medicine for relief of symptoms of numerous disorders. In light of growing consumer acceptance of kiwifruits worldwide, there has been an increased attention given to identifying health benefits associated with its consumption. Potential benefits include a rich source of antioxidants, improvement of gastrointestinal laxation, lowering of blood lipid levels, and alleviation of skin disorders. Some individuals report allergic symptoms to kiwifruit, and a considerable research effort is being focused on characterizing kiwifuit’s allergenicity among various populations of people. Kiwifruit not only is rich in vitamin C but also is a good source of other nutrients such as folate, potassium, and dietary fiber. This fruit’s content of nutrients and biologically active phytochemicals has stimulated investigations into its antioxidant and anti-inflammatory actions that might then help prevent cardiovascular disease, cancer, and other degenerative disorders.

A fruit from far away has some interesting characteristics worth taking a look at

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Consumption of 2 Green Kiwifruits Daily Improves Constipation and Abdominal Comfort-Results of an International Multicenter Randomized Controlled Trial

Affiliations.

• 1 Department of Medicine, University of Otago, Christchurch, New Zealand.
• 2 Department of Behavioral Medicine, Tohoku University Graduate School of Medicine, Sendai, Japan.
• 3 Department of Psychosomatic Medicine, Tohoku University Hospital, Sendai, Japan.
• 4 Department of Medical and Surgical Sciences, University of Bologna, Bologna, Italy.
• 5 BKS Consulting Limited, Hamilton, Waikato, New Zealand.
• 6 Zespri International Limited, Tauranga, New Zealand.
• 7 The New Zealand Institute for Plant and Food Research Limited, Palmerston North and Christchurch, New Zealand.
• 8 Department of Nutrition, Sapporo University of Health Sciences, Sapporo, Japan.
• 9 Department of Clinical Nutrition, Tohoku University Hospital, Sendai, Japan.
• 10 Department of Preventive Medicine and Epidemiology, Tohoku Medical Megabank Organization, Tohoku University, Sendai, Japan.
• 11 Drummond Food Science Advisory Limited, Killinchy, Canterbury, New Zealand.
• PMID: 36537785
• PMCID: PMC10226473
• DOI: 10.14309/ajg.0000000000002124

Introduction: Consumption of green kiwifruit is known to relieve constipation. Previous studies have also reported improvements in gastrointestinal (GI) comfort. We investigated the effect of consuming green kiwifruit on GI function and comfort.

Methods: Participants included healthy controls (n = 63), patients with functional constipation (FC, n = 60), and patients with constipation-predominant irritable bowel syndrome (IBS-C, n = 61) randomly assigned to consume 2 green kiwifruits or psyllium (7.5 g) per day for 4 weeks, followed by a 4-week washout, and then the other treatment for 4 weeks. The primary outcome was the number of complete spontaneous bowel movements (CSBM) per week. Secondary outcomes included GI comfort which was measured using the GI symptom rating scale, a validated instrument. Data (intent-to-treat) were analyzed as difference from baseline using repeated measures analysis of variance suitable for AB/BA crossover design.

Results: Consumption of green kiwifruit was associated with a clinically relevant increase of ≥ 1.5 CSBM per week (FC; 1.53, P < 0.0001, IBS-C; 1.73, P = 0.0003) and significantly improved measures of GI comfort (GI symptom rating scale total score) in constipated participants (FC, P < 0.0001; IBS-C, P < 0.0001). No significant adverse events were observed.

Discussion: This study provides original evidence that the consumption of a fresh whole fruit has demonstrated clinically relevant increases in CSBM and improved measures of GI comfort in constipated populations. Green kiwifruits are a suitable dietary treatment for relief of constipation and associated GI comfort.

Trial registration: ClinicalTrials.gov NCT02888392 .

Copyright © 2023 The Author(s). Published by Wolters Kluwer Health, Inc. on behalf of The American College of Gastroenterology.

PubMed Disclaimer

Conflict of interest statement

Guarantor of the article: Richard Gearry, MD, PhD.

Specific author contributions: R.G., S.F., and G.B.: principal investigators who oversaw the trial in their respective countries and contributed to writing and review of the manuscript. B.K.S.: conducted the statistical analysis and reviewed the manuscript. J.A.: contributed to trial design and reviewed the manuscript. P.B.: conducted laboratory analysis and reviewed the manuscript. A.W.: and C.B.: involved in running the New Zealand clinical trial and reviewed the manuscript. C.C., M.R.B., and I.P.: involved in running the Italy clinical trial and reviewed the manuscript. Y.O., T.M., T.O., M.F., Y.E., Mi.K., Mo.K., N.K., and K.N.: involved in running the Japan clinical trial and reviewed the manuscript. L.D.: involved with trial design, contributed to writing, and review of the manuscript.

Financial support: Zespri International Ltd. was the principal sponsor and reviewed, approved, and funded the study design. The New Zealand study center trial was jointly funded by a grant from the New Zealand government (Contract C11X1312) and the sponsor company, Zespri International Ltd. In Italy and Japan, Zespri International Ltd. was the sole funder for each study center trial. The funder did not contribute to the study design or data analysis.

Potential competing interests: J.A. and P.B. are employed by Zespri International who part-funded the study. R.G. and L.D. sit on the Science Advisory Board, have received travel and research grants from Zespri International. SF and GB have received research travel grants from Zespri International.

Crossover study design.

Overview of study plan and…

Overview of study plan and test procedures.

Consolidated Standards of Reporting Trials.

Change from baseline in CSBM…

Change from baseline in CSBM frequency during interventions.

Weekly CSBM frequencies in the…

Weekly CSBM frequencies in the combined constipated group during each intervention.

Change from baseline in GSRS…

Change from baseline in GSRS total scores.

• In persons with constipation or IBS-C, kiwifruit vs. psyllium increased spontaneous bowel movements. Purtle BJ, Cash BD. Purtle BJ, et al. Ann Intern Med. 2023 May;176(5):JC53. doi: 10.7326/J23-0022. Epub 2023 May 2. Ann Intern Med. 2023. PMID: 37126812

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In May 2021, canker symptoms were detected on ‘Xuxiang’ kiwi trees in southwestern Shaanxi (Hanzhong municipality; 107.27° E, 33.23° N) in China. Seven-year-old trees exhibited black necrotic lesions and cracked areas in the trunk (Figure 1). The symptoms were observed in approximately 10% of the trees in 6 orchards (31 ha in total). Application of commercial fungicides did not control the advancement of the pathogen, and infected trees were removed to control the spread. Three samples, approximately 1 cm2 in size, of symptomatic tissue were collected and surface sterilized in 2% NaOCl for 1 min, and washed with sterile ddH2O. Four isolates showing white mycelium with yellow pigmentation were obtained after 4 days of incubation on PDA, containing chloramphenicol (50 µg/mL), at 28 ºC. The pathogen was isolated from all collected samples. ITS, EF1-α, TUB2, RPB1 and RPB2 genes were amplified using ITS1/ITS4, EF1-728F/EF1-986R, T1/T22, RPB1-5F/RPB1-8R and RPB2-5F/RPB2-7cR (strain NJC06), or RPB2-c7F/RPB2-11aR (strains NJC07 and NJC08), primers, respectively. Two isolates shared the same sequences (strain NJC08). Obtained sequences were submitted to GenBank under accession numbers MZ669205 and OL347898-OL347899 (ITS), OL439731-OL439733 (EF1-α), OL439734-OL439736 (TUB2), OL439737-OL439739 (RPB1), and OL439740-OL439742 (RPB2). The sequences shared >99% (ITS; F. avenaceum CBS 128538, MH864972), >99% (EF1-α; F. avenaceum 55-2, MN473124), 100% (TUB2; F. avenaceum SICAUCC 18-0001, MK253102), >98% (RPB1; F. avenaceum NRRL 26911, MG282372), and >98% (RPB2; F. avenaceum SICAUCC 18-0001, MK396098; or F. avenaceum FRC R-09495, CQ915486) homology to multiple F. avenaceum strains. Molecular phylogenetic tree (Figure 2) was constructed using MEGA7 with Fusarium strains found causing rot in various hosts (Wang et al. 2015), and other fungal species, such as Cadophora nalorum, Diaporthe ambigua, D. australafricana, and Neofusicoccum parvum, which were reported to cause cordon dieback on kiwi tree in Chile (Diaz et al. 2021). Microscope observations after cultivation of all isolates on barley-honey-tryptone medium (Song et al. 2020) showed the presence of septate mycelium, fusiform microconidia (8-15 µm in length, containing between 0 and 3 septa; n = 77) and chlamydospores (n = 21), and agree with the morphology of F. avenaceum (Zhao et al. 2020). To confirm pathogenicity, a sterilized spatula was used to make wounds (3 mm diameter, 1 mm depth) on the trunk of 3-months-old ‘Xuxiang’ kiwi trees. Solutions containing 1 × 106 spores/mL (20 µL) of the isolates were injected in the wounds. Sterile ddH2O was used for the control experiment. Inoculated plants were maintained in a growth chamber at 28 °C and 80% relative humidity for 4 days. The pathogen was recovered from the canker lesions, which were similar to those observed in the orchards, and its identity was confirmed by sequence analysis. The pathogen only infected wounded trees, and probably invaded the orchards during the pruning in February 2021. F. avenaceum was reported to cause canker on almond tree (Stack et al. 2020), stem rot on Anthoxanthum aristatum and Polygonatum cyrtonema (Pieczul et al. 2018; Xu et al. 2019), and root rot on carrot, Coptis chinensis and wheat (Le Moullec-Rieu et al. 2020; Mei et al. 2020; Ozer et al. 2020). Recently, F. avenaceum was found causing fruit blotch in kiwi fruit in Anhui (China) (Zhao et al. 2020). Here, F. avenaceum was found causing canker disease in kiwi tree, demonstrating the host and tissue promiscuity of this pathogen. Kiwi is an important crop in China with nearly 1.5 million tons produced in 2019. This report will help to better understand the pathogens reducing kiwi production in China.

Energy and Economic Efficiency of Kiwi Fruit Production in Turkey: A Case Study from Mersin Province

Evaluating the roles of the farmer's cooperative for fostering environmentally friendly production technologies-a case of kiwi-fruit farmers in meixian, china, undifferentiated in vitro cultured actinidia deliciosa as cell factory for the production of quercetin glycosides.

Land plants produce a vast arsenal of specialized metabolites and many of them display interesting bioactivities in humans. Recently, flavonol quercetin gained great attention in the light of the COVID-19 pandemic because, in addition to the anti-inflammatory, antiviral and anti-cancer activity already described, it emerged as possible inhibitor of 3CLpro, the major protease of SARS-CoV-2 virus. Plant cell and tissue culture (PCTC) is an attractive platform for the biotechnological production of plant metabolites. This technology allows a large amount of water and agricultural land to be saved and, being free of contaminants in the process, it is suitable for scaling up the production in bioreactors. In a project aimed to generate and screen in vitro plant cells for the production of valuable specialized metabolites for commercial production, we generated various cell lines from Actinidia deliciosa (kiwi fruit tree) and Actinidia chinensis (gold kiwi fruit tree), that were able to produce relevant amounts of quercetin derivatives, mainly quercetin glycosides. Three cell lines from A. deliciosa were characterized by targeted and untargeted metabolomics. In standard growing conditions, they produce and accumulate up to 13.26 mg/100 g fresh weight (419.76 mg/100 g dry weight) of quercetin derivatives. To address future industrial applications, these cell lines should be entered into an acceleration program to further increase the amount of these metabolites by optimizing the culture conditions and elicitation.

Influence of Different Types of Polysaccharide-Based Coatings on the Storage Stability of Fresh-Cut Kiwi Fruit: Assessing the Physicochemical, Antioxidant and Phytochemical Properties

The present study focuses on studying the influence of various edible biopolymer coatings at several concentrations on physicochemical, antioxidant and lipid peroxidation activity levels of biopolymer-coated fresh-cut kiwi slices stored at room temperature (relative humidity: 90%). Kiwi slices were coated by dipping in xanthan gum (0.1, 0.2, 0.3% w/v), alginate (1, 2, 3% w/v) and chitosan (0.25, 0.50, 0.75% w/v) solutions for 2 min. Kiwi fruit slices without any treatment were designated as the control. Compared to the control, all coated samples retained higher ascorbic acid, titratable acidity, total phenolic component and antioxidant capacity levels. However, xanthan-gum-coated slices retained significantly higher amounts of total phenolics in comparison to alginate- and chitosan-coated slices (p ≤ 0.05). HPLC analysis showed the presence of neochlorogenic acid, chlorogenic acid, ellagic acid and epicatechin. The results suggest that the xanthan gum can be utilized to enhance the shelf life of fresh-cut kiwi slices without compromising quality.

Effect of Oxygen and Carbon Dioxide Concentration on the Quality of Minikiwi Fruits after Storage

The rapid increase in the production of hardy kiwi fruit (A. arguta) since the beginning of the 21st century has required the development of new cultivation technologies and postharvest handling procedures in order to extend the supply and transport of the fruit to distant markets. Fruit storage focuses on the inhibition of ripening processes regulated by ethylene activity or respiration. Both of these are effectively regulated by appropriate concentrations of O2 and CO2 in the atmosphere surrounding the fruit. In this study, the effect of the concentration of both gases in the cold room on the physico-chemical indices of fruit quality, i.e., mass loss, firmness, soluble solids and monosaccharides content, titratable acidity and acid content, and color of the peel was evaluated. Studies have shown that high CO2 concentrations inhibit ripening processes more effectively than low O2 concentrations. Softening of berries as well as an increase in soluble solid contents was recorded during the first 4 weeks of storage in the fruit. However, the increase in monosaccharides was fairly stable throughout the study period. The increase in soluble solids content as well as the loss of acidity were more strongly determined by CO2 than O2, although the acid content in a 10% CO2 atmosphere did not change. Additionally, the fruits were greener after storage in 10% CO2, but the weakness was skin dulling and darkening. The results indicate that the use of high CO2 concentrations (5–10%) effectively inhibits ripening processes in fruit. After 12 weeks of storage, the fruit was still not suitable for direct consumption, which suggests that the storage period can be extended further.

Oxidase-like Fe–Mn bimetallic nanozymes for colorimetric detection of ascorbic acid in kiwi fruit

In silico molecular docking and adme potential of kiwi fruit isolated compounds against apoptotic proteins, current phytochemical and pharmacological outlook of actinidia deliciosa (kiwi fruit).

: Worldwide health and therapeutic practices seek to amalgamate alternative medications with evidence-based medicine for an improved understanding of metabolic progression and its influences on the human body. Actinidia deliciosa, also known as “Kiwi fruit”, is a dioecious plant that is native to China and distributed widely across the Asian continent. Commercial planting of kiwifruit was started in the early 20th century when it reached New Zealand from China. In recent times, Kiwi fruit has gained a major demand due to its high content of vitamin C. Kiwi fruit also contains dietary fiber, iron, carotenoids and is a rich source of antioxidants. These may aid in lowering blood pressure, improve wound healing, blood glucose control and improve bowel health. Vitamin C, choline, lutein, and zeaxanthin are antioxidants that assist in the removal of free radicals from the body and may prevent the body from various diseases and inflammations. Herein, we state the health benefits found in diverse compounds from Actinidia deliciosa, highlighting the source, morphology, chemical constituent, cultivation, production, traditional uses, nutritional value, health benefits, toxicity studies, clinical trials, and pharmacological activities while highlighting side effects associated with kiwifruit. This review provides a bird’s eye insight mainly on the morphological, phytochemical, and pharmacological activity, which could be beneficial in making use of technological and scientific advances. This plant can be used as a current medical adjuvant for its potential. The complete plant must be broadly investigated for further future perspective.

Kiwi Fruit IoT Shelf Life Estimation During Transportation with Cloud Computing

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Valorization of Kiwi Peels: Fractionation, Bioactives Analyses and Hypotheses on Complete Peels Recycle

Francesco cairone.

1 Department of Drug Chemistry and Technology, “La Sapienza” University of Rome, 00185 Rome, Italy; [email protected] (F.C.); [email protected] (S.G.); [email protected] (M.A.C.); [email protected] (L.D.M.)

Stefania Garzoli

Luigi menghini.

2 Department of Pharmacy, University “G. d’Annunzio”, Botanic Garden “Giardino dei Semplici”, 66100 Chieti, Italy; [email protected]

Giovanna Simonetti

3 Department of Environmental Biology, “La Sapienza” University of Rome, 00185 Rome, Italy; [email protected]

Maria Antonietta Casadei

Laura di muzio, stefania cesa, associated data.

Not applicable.

Kiwi fruit samples ( Actinidia deliciosa Planch, cv. Hayward) represent a suitable and good source for fibers obtainment as well as for polyphenolic and carotenoid extraction. With this aim, in this study they were submitted to a double phase extraction to separate insoluble fibers by an organic phase containing lipophilic substances and an hydroalcoholic phase containing polyphenols and soluble fibers. Insoluble fibers could be separated by filtration and sent to be micronized and reused. Hydroalcoholic fractions were then furtherly fractionated by solid-phase extraction. Data coming from the color CIEL*a*b* and the HPLC-DAD analyses of the extracts were compared and correlate with those coming from the SPME-GC/MS analysis of either the finely shredded peels or of the extracts. The obtained extracts were also submitted to anti-radical activity evaluation and anti- Candida activity. Results show that all of the obtained residues are value added products. Hypotheses were also made about the nature and the possible recycle of the obtained purified solid residue.

1. Introduction

The term ‘kiwi’ indicates fruits produced by plants belonging to the Actinidiaceae family and specifically of the Actinidia Lindl genus, which are characterized by edible fruits, coming mainly from Actinidia chinensis var. hispida C.F. Liang (synonym A. deliciosa Chev., C.F. Liang, and A.R. Ferguson) A. chinensis and A. arguta (Siebold and Zucc.) Planch ex Miq., which represent the most commercialized. Some other less famous species, such as A. kolomikta (Maxim and Rups) Maxim and Actinidia arguta var. purpurea (Rehder) C.F. Liang and Q.Q. Chang (synonym A. purpurea , Rehder) also produce edible fruits [ 1 ].

More than fifty wild species are known [ 2 ], but only a few of these have been cultivated. The most diffuse cultivated plant of this genus is Actinidia deliciosa , whose production leaders are represented by China and Italy. Italy is also, along with New Zealand, one of the two biggest exporters of kiwis in the world. Kiwi composition boasts all of the best nutritional principles, such as proteins, lipids, carbohydrates and dietary fibers, vitamins, minerals polyphenols, and other antioxidants. Due to these properties and to its limited seasonal availability, other than as a fruit it is largely consumed as juice or puree, stored as frozen and lyophilized products, or finally used in fortified drinks and alcoholic beverages [ 3 ].

Valuable edible kiwi fruits come from the cultivar A. deliciosa “Hayward”. Fiorentino et al. [ 4 ] reported that fruits coming by eight different Actinidia genotypes were tested in relation to their polyphenols and vitamin C content, showing that the antioxidant capacity is largely influenced by species and cultivar. A. deliciosa “Hayward”, besides other cultivars, are also known for their positive health potential, show interesting properties due to the strong antioxidant, anti-inflammatory, and anti-diabetic capacity, combined with their antimicrobial, antiviral and antifungal activities. Most of these health properties are attributed to polyphenolic components such as phenolic acids (protocatechuic, gallic, caffeic, syringic, ferulic, salicylic, coumaric) and flavonoids (quercetin, procyanidins, chrisin and rutin), as well as a potent immunomodulatory effect ascribable to the very high vitamin C content [ 3 ].

Polyphenols and antioxidant molecules, are known to be present in the fleshly internal part of the fruits as well as in the peels, are considered a non-edible part of the fruit. Whole fruit A. arguta, also known as the mini kiwi, (and sometimes of A. chinensis , or golden kiwi) can be eaten. In this case, the smooth peels could also represent an edible part, as well as the peels of the most diffuse A. deliciosa , which typically represent a waste material [ 1 ]. Kiwi peels that are separated and discarded for fresh fruit consumption, as well as in industrial processing, represent an interesting source of relevant biomolecules that are only marginally investigated [ 4 ].

In this context, and according to the principles of the circular economy, great interest of the scientific community nowadays is devoted to the recycling and the valorization of agri-waste products to transform them into added value by-products. As reported by Aureli et al. [ 5 ], in a study on the food waste of an Italian family, “reducing food waste along the entire food supply chain is an important policy priority included in the United Nations Sustainable Development Goals for 2030”.

As with many others vegetal matrices, kiwi fruits and relative by-products contain interesting phytocompounds, whose recovery could represent a strategy to valorize the production and define innovative applications of economic relevance. They could also apply eco-friendly procedures in order to assure no risk for the consumers or to the environment.

The seeds can be conveniently reused as raw material for the extraction of a secondary product (such as the oil). It is noteworthy that the seeds are considered to be within the edible part of the fruit but during production of juice and jam they represent a relevant part of waste production [ 6 , 7 , 8 ].

Kiwi peels, which contain a high quantity of bioactive molecules such as polyphenols and antioxidants, represent one of the most important kiwi by-products to be recycled and valorized. In the last few years, kiwi peels extract has been successfully used to fabricate nanoparticles using a cost effective and eco-friendly green method of synthesis. Zinc oxide (ZnO), titanium dioxide (TiO 2 ), and tin oxide (SnO 2 ) nanoparticles (NPs) were prepared by exploiting the polyphenolic component of kiwi peels used as reducing and capping agent for NPs preparation and stabilization [ 9 , 10 , 11 , 12 ].

In this way there is the double advantage of recycling and valorizing kiwi peels, limiting the use of traditional toxic reducing and capping agent responsible of serious environmental pollution issues. The good antimicrobial activity and the anti-cancer activity sometimes exhibited by these nanoparticles, also makes them promising candidates for biomedical applications. In addition, polyphenol-rich kiwi peel extracts (PE) were also employed as reducing and capping agents for the synthesis of self-assembling silver@PE nanoparticles (Ag@PE NPs) incorporated into polymeric sodium alginate (SA)-based films. The presence of Ag@PE NPs successfully improved the antioxidant and antibacterial functions of SA-based films making them potential and promising candidates as multifunctional packaging films in food preservation field [ 13 ].

Different kiwi by-products for an optimized flowchart were recently revised by Sanz et al. [ 14 ]. A wide set of biological activities were reported, including the reduction of platelet aggregation, the normalization of triglycerides blood levels, protective effects against degenerative, cardiovascular, and cancer disease, efficacy in constipation as antiglycation and as nephroprotection, and their well-known antimicrobial activity. Great attention should be devoted to agricultural practices and processing due to their influence on the bioactive content of the starting materials, the health potential of the obtained products, and their impact on the environment.

A limited number of published papers have explored the effects of the extraction procedures applied to the kiwi peels to the chemical quality and biological activities of extracts. Conventional extraction based on ethanol or acetone mixed with water were applied for the flavonoids extraction and acidic hydrolysis has been applied for the pectin’s extraction [ 15 , 16 , 17 ]. The same classes of compounds were also obtained by ultrasound and microwave assisted methods [ 18 ]. The selectively extracted fractions were then analyzed and compared to evaluate the optimal extraction method in terms of yield and biomolecules preservation. In a previous work, we tested with success a double phase extraction to goji berries. In fact, these are characterized, as kiwi fruits and peels, by a lipophilic and a hydrophilic fraction which is simultaneously represented. This approach allowed us to simplify the process, saving time, money, and processed wastes [ 19 , 20 ]. In the present work, the kiwi peels were chosen as starting material to study a process of complete recycle of this interesting and overproduced agri-waste. ‘Hayward’ kiwi fruits were selected for the present study coming directly from either an Italian plantation or from a commercial sample. Markets indicate that in Italy, which represents the first European producer of kiwis, about 400 thousand tons of kiwi fruits per year are produced and about the 10% of the production is represented by discarded peels. A flowchart for the kiwi peel valorization and reuse, based on the circular economy principles, is presented and evaluated in this study.

2. Materials and Methods

2.1. standards and reagents.

Bidistilled water, acetone, ethanol, acetic acid, acetonitrile, n -hexane and methanol were purchased from Merck life Science s.r.l (Milan, Italy). Reference compounds for HPLC analysis, (+)-catechin, epicatechin, caffeic acid, benzoic acid, vanillic acid and 2,2-diphenyl-1-(2,4,6-trinitro-phenyl) hydrazine (DPPH) were purchased from Merck life Science s.r.l (Milan, Italy).

2.2. Sample Preparation

Kiwi fruits cv. Hayward, were collected manually from an organic plantation in Campania Region, (Italy) consisting in adult plants productive since more than five years. Collection was carried out in July 2020, and fruit selection (Sample Series “ sel ”) was carried out on the basis of ripening stage defined by the farmers as optimal for the commercialization. A comparative sample of commercial fruits from conventional agriculture were purchased from local market. In the label, the producer defines the Hayward variety and the Italian origin (Sample Series “ com ”). Samples were stored at 4–6 °C for the time strictly necessary to complete the experimental procedures (about one week). Moisture content of peels was also evaluated (amounting to 78 ± 1%).

The fruit epicarps (peels) were manually separated from the internal fleshy pulp (consisting of mesocarps and endocarps) using a common potato peeler in order to obtain samples of uniform thickness. These were immediately homogenized by wet grinding, obtaining a paste with particles millimeters in size, which were submitted to extraction and forwarded to the experimental investigations.

2.3. Solid Phase MicroExtraction (SPME) of Peels

The sampling by SPME technique was performed following Vitalini, et al. [ 21 ] with some modifications. Representative samples of peels (~2 g) were individually placed into a 20 mL glass vial with PTFE-coated silicone septum.

For the extraction of volatiles compounds, a SPME device from Supelco (Bellefonte, PA, USA) with 1 cm fiber coated with 50/30 μm DVB/CAR/PDMS (divinylbenzene/carboxen/polydimethyl siloxane) was used. Before use, the fiber was conditioned at 270 °C for 30 min. Each sample was equilibrated for 30 min at 40 °C before sampling. Later, the fiber was exposed to the headspace of the samples for 30 min at 40 °C to collect and concentrate the volatiles compounds. Lastly, the SPME fiber was inserted in the GC injector maintained at 250 °C in split mode for the desorption of the captured components.

2.4. Double Phase Extraction

For the extraction of the bioactive compounds from kiwi peels, a double phase extraction was performed according to our previous work [ 20 ]. About 25 g of kiwi peels were extracted with a 50 mL of n -hexane and 50 mL of a hydroalcoholic mixture (ethanol:water acidified with 5% of acetic acid, 70:30 v / v ) for 3 h at room temperature under stirring. The two phases were separated and concentrated under reduced pressure at 40 °C with a rotary evaporator, weighed and stored at 4 °C until analyzed. The resulting extracts are identified as HA com and HA sel for hydroalcoholic fraction and HE com and HE sel for hexane fraction obtained from commercial sample or local cultivation, respectively.

2.5. Colorimetric Analysis

The extracts obtained from double phase extraction of peels, were submitted to colorimetric analysis at room temperature (20 °C ± 1), with a colorimeter X-Rite MetaVue TM , equipped with a full-spectrum LED illuminant and an observer angle of 45°/0° imaging spectrophotometer. The cylindrical coordinates were calculated according to a previous work [ 22 ]. pH of hydroalcoholic extracts was 4.3 ± 0.1.

2.6. Solid-Phase Extraction (SPE)

The hydroalcoholic extracts ( HA com , HA sel ) were subjected to solid-phase extraction using a Discovery ® DSC-18 SPE Tube column (Merck Life Science s.r.l., Milan, Italy) according to Cairone et al. [ 23 ] with some modifications. The column was previously activated with methanol and then, conditioned with water acidified with 5% of acetic acid. 1 g of hydroalcoholic extract was dissolved in 5 mL of water and loaded into the column. The column was washed with 5 mL of water acidified with 5% of acetic acid and then eluted with 5 mL of methanol and 5 mL of ethanol. The obtained fractions ( HA-SP com , HA-SP sel ) were concentrated under reduced pressure at 40 °C with a rotary evaporator, weighed and stored at 4 °C until analyzed.

2.7. Partitioning in Ethyl Acetate

The hydroalcoholic extracts, ( HA com , HA sel ), dissolved in 10 mL of water, were extracted thrice in a separating funnel with 10 mL of ethyl acetate. The organic fraction (HA-EA com , HA-EA sel ) was concentrated under reduced pressure at 40 °C with a rotary evaporator, weighed and stored at 4 °C until analyzed.

2.8. GC-MS Chemical Analysis

The chromatographic analyses of the peels and of all of the obtained extracts were carried out on Clarus 500 model Perkin Elmer (Waltham, MA, USA) gas chromatograph coupled with a mass spectrometer equipped with a flame ionization detector (FID) and a Varian Factor Four VF-1 capillary column. The operative conditions were the following: oven temperature at 40 °C for 2 min, then increased to 220 °C at 6 °C/min and finally held for 10 min at this same temperature (for the pulps and peels headspace); oven temperature program at 60 °C then increased to 170 °C at 6 °C/min, increased to 250 °C at 8 °C/min and finally held for 10 min at this same temperature (for the liquid phase of the extracts); injector temperatures: at 250 °C for the peels and 270 °C for the direct injection of the extracts liquid phase.

Helium was used as carrier gas at a constant rate of 1 mL/min. The mass spectrometer was operated at 70 eV (EI) in scan mode in the range 40–400 m / z . Ion source and the connection parts temperature were 220 °C.

The identification of volatile compounds was performed by matching their mass spectra with those stored in the Wiley 2.2 and Nist 02 mass spectra libraries database and by calculating the Linear Retention Indices (LRIs) using a series of alkane standards analysed under the same conditions used for the samples. LRIs were then compared with available retention data reported in the literature. The peak areas of the FID signal were used to calculate the relative concentrations of the components expressed as percentage, without the use of an internal standard and any correction factor. All analyses were carried out in triplicate.

2.9. Antioxidant Activity by DPPH (2,2-Diphenyl-1-picryl-hydrazyl) Method

According to Cairone et al. [ 23 ] a solution 100 µM of DPPH was prepared in methanol. Then, 2 mL of this solution were added to 1 mL of methanol, stored in the darkness, and monitored by UV/VIS Lambda 25 spectrophotometer (Perkin Elmer Waltham, MA, USA), at the wavelength of 515 nm, until the absorbance value was stable.

0.5 mL of a sample solution (5 mg/mL) were added with 2 mL of the same DPPH solution and 0.5 mL of methanol. The absorbance at 515 nm was controlled, following the same conditions described above, and the reduction of DPPH absorbance after 30 min was detected. Finally, a calibration curve was constructed to quantify the antioxidant activity by adding 1 mL of gallic acid (from 0.9 to 6.5 µg/mL), at different concentrations, to 2 mL of the DPPH solution, following the previous described conditions. A calibration curve was constructed (y = 0.6473e −378.5x ) and the antioxidant capacity was expressed as gallic acid equivalents.

2.10. Antifungal Activity Assay

Growth inhibition assays were performed according to standardized methods for yeast using the broth microdilution method (CLSI M27-A3, 2008; CLSI, 2012) [ 24 ]. The strains C. albicans ATCC24433, coming from the American Type Culture Collection (ATCC, Rockville, MD, USA), and C. glabrata PMC0849, PMC0822, PMC0806 PMC0843, coming from the Pharmaceutical Microbiology Culture Collection (PMC, Sapienza, Rome, Italy), were tested. The strains were grown on Sabouraud dextrose agar (Sigma Aldrich, St. Louis, MI, USA) at 35 °C for 24 h. The suspension of Candida cells was prepared, and the final concentration of the inoculum was 1.0 × 10 3 –1.5 × 10 3 CFU/mL (CLSI. M38-A2, 2008). The extracts were dissolved in dimethyl sulfoxide at concentrations 100 times higher than the highest tested concentration. The extracts were then serially diluted 2-fold across the 96-well plates in RPMI 1640 medium (Sigma-Aldrich, St. Louis, MI, USA) and the final concentration ranged from 1024 µg/mL to 0.5 µg/mL. The plates were incubated at 35 °C. After 24 h, the lowest concentration of extracts that caused ≥50% growth inhibition (MIC) was determined. The experiments were performed three times in duplicate. The results are expressed as median.

2.11. HPLC-DAD Analysis

Kiwi peels hydroalcoholic extracts were weighed, dissolved in methanol and analysed with a HPLC-DAD (Perkin Elmer, Milan, Italy), equipped with a Series 200 LC pump, a Series 200 DAD and a Series 200 autosampler, including a TotalChrom Perkin Elmer software for plotting data. The analyses were performed on a Luna RP-18, 3µ, with a linear gradient constituted by acetonitrile and water acidified by 5% formic acid, from 100% of aqueous phase to 35% in 55 min, at flow rate of 0.9 mL/min. Calibration curves were expressed in µg/mL and were constructed for catechin (y = 5.18 x–− 24.29; R 2 0.9997), epicatechin (y = 5.01x + 21.6; R 2 0.9995), caffeic acid (y = 26.03x + 20.37; R 2 0.9974), sinapic acid (y = 11.37x + 9.92; R 2 0.9984).

2.12. Statistical Analysis

Each assay was replicated at least three times. Data are expressed as mean ± SEM and statistical significance was determined using the XLStat 2021, software (New York, NY, USA).

3. Results and Discussion

Pomace, constituted by peels, seeds, and other parts resistant to the squeezing process, represents the primary by-product of the kiwi juice industry [ 25 ]. It represents a valuable source for the recovery of useful dietary fibers, as well as for other metabolites of health and economic values such as polyphenols, carotenoids, chlorophylls, and aroma compounds.

In the present work, a work-flow on the kiwi peels was studied with the aim to deep the knowledge about their aroma character, the potential added value deriving from the use as source of extracts and biomolecules for cosmetic and pharmaceutical applications. Finally, we aimed to evaluate the possible applications of the residue of the applied extraction, in view of the obtainment of zero impact by kiwi wastes.

3.1. SPME-GC/MS of Separated Kiwi Peels

Comparison SPME-GC/MS analysis data of kiwi peels obtained from commercial and locally produced fruits (selected peels) shows a relatively simple aroma mixture in the latter (See also chromatograms, Figure S1 in Supplementary Material ), represented for more than 90% by linalool (50%), ocimenol (16%), α -terpineol (16%) and β-myrcene (10%). More complex results the aroma mixture from the commercial sample constituted by twelve molecules represent in percentages ranging between 24 and 3%, with α-terpineol being the most abundant (24%) followed by myrcenol (22%) and p -menth-1-en-9-al (8%). (See also Figure 1 and Table S1 in Supplementary Material ).

GC-MS and SPME-GC-MS analyses of the most significant components of kiwi peels and extracts.

Impact compounds on the kiwi aroma, as reported in the review by Garcia et al. [ 26 ], are usually obtained by simultaneous distillation extraction, vacuum distillation, and methods based on both SPME and head space analysis. Few molecule examples correspond to our experimental data if the previously reported results only refer to whole kiwi fruits. To our knowledge, no data are available on the direct application of this technique to separated kiwi peels.

Some of the identified molecules in the peels, such as linalool, α -terpineol, myrcenol, ocimenol, and β -myrcene, are significant volatile compounds and are especially correlated with wines and other grape derivatives [ 27 ] which are used in food and perfumery aroma industry.

To our knowledge, no data are available in the literature concerning the obtainment of essential oils by kiwi peels and relative production yield, aspects which deserve to be investigated further. Regardless, kiwi peels’ pleasant flavor promotes their employment in different application fields (for example, as dietary fibers food supplements).

3.2. Bioactives Extraction and Waste Recycle by Separated Kiwi Peels

Several techniques have been reported for the extraction of bioactive compounds from kiwifruit, its by-products, and different parts of the plant. Environmentally friendly solvents such as water or ethanol are recommended for the extraction, especially if the product will be used for food and nutraceutical applications. In addition, these technologies are adequately efficient and allow short operating times [ 28 ].

In this work, the fresh kiwi peels coming from the local production or from the market samples were prepared and extracted by a double phase extraction, as reported in our previous paper [ 20 ]. The simultaneous presence of a carotenoid and chlorophyll significant fraction, in addition to the polyphenolic component, imposes the use of organic solvents other than an hydroalcoholic mixture. This one-pot approach allowed us to save time, money, and solvent quantities, thereby reducing as much as possible the used unsafe n -hexane (which was chosen as the best compromise respect to other organic and chlorinated solvents), giving excellent results in terms of yield and stability of obtained extracts.

The yields from hydroalcoholic extraction resulted in 7.5 and 7% of the fresh weight, respectively, for locally produced ( HA sel ) and commercial sample ( HA com ). Considering a water content of about 78% (known by dehydration experiments performed on the same kiwi peels), the yield in hydroalcoholic extract was about 33% for dry peels. According to Martín-Cabrejas et al. [ 25 ], but also considering the simultaneous presence of ethanol as extraction solvent, in this fraction we should separate the polyphenolic fraction, soluble sugars content, ashes, and soluble fibers. A green color residue, furtherly evaluated by CIEL*a*b and confirmed by spectrophotometric analyses, indicates a small residue of chlorophyll and carotenoid content with respect to the organic extract.

The n -hexane extracts ( HE sel and HE com ) gave about a 0.05% yield and was mainly composed by chlorophylls and carotenoids. In this case, considering a water content of about 78%, the yield in n -hexane extract was about 0.22% and the presence of chlorophylls and carotenoids was confirmed by CIEL*a*b, spectrophotometric and HPLC analysis.

In order to isolate the polyphenolic fraction from the hydroalcoholic extract, further extraction and purification steps were carried out, such as a solid-phase extraction on the two cultivars (samples HA-SP sel and HA-SP com ) with both affording an extraction yield of 1%.

A partitioning between water and ethyl acetate was also performed (samples HA-EA sel and HA-EA com ), affording smaller yields of about 0.1%.

Performing these two kinds of further extraction, we obtained watery fractions representing about the 99% (SPE technique) or the 99.9% (water/ethyl acetate repartition) of the initial hydroalcoholic extract. These fractions represent furtherly purified water-soluble fibers, respectively obtained as first eluted phase by SPE column or by partitioning.

Although our attention has been in the past mainly directed towards the polyphenolic fraction, which here represents the remaining 1 or 0.1%, the waste fraction which is rich in soluble fibers is not negligible. In fact, dietary fibers, although not absorbed, are considered a vital nutrient for the organism, as they have been shown to play a role in the maintenance of a good health state, favoring peristalsis and preventing of several illnesses such as colon inflammation and cancer, blood and cardiovascular diseases, diabetes, and other related comorbidities [ 29 , 30 ].

This waste fraction, rich in soluble fibers (containing soluble hemicelluloses, gums, mucilages, and pectin substances), could also be used in the formulation of supplements, due to its role in increasing viscosity, as well as the insoluble fibers (rich in lignin, cellulose, chitosan and insoluble hemicellulose). From a recycling perspective, it could be used as a substrate for the preparation of nanocellulose (NC). Cellulose, in the form of nanostructures, is considered one of the most promising green materials of recent acquisition [ 31 ]. NC materials have many advantageous properties, such as chemical inertness, excellent mechanical properties, large specific surface area, and the availability of numerous hydroxyl groups that can be readily functionalized via chemical reactions [ 32 ].

All of these properties make nanocellulose a promising material for a multitude of applications in the biomedical and engineering fields (and in many other emerging fields). In the biomedical field, owing to its biocompatibility, low cytotoxicity, and tunable surface features, it is becoming increasingly popular for the preparation of hydrogels, scaffolds for tissue engineering, and innovative drug delivery systems. Particularly, in the field of drug delivery it can be used as excipient (but also as starting matrix) to prepare NC-based oral, transdermal and local drug delivery systems, which are able to control drug release and improve both drug stability and therapeutic effects [ 33 , 34 , 35 , 36 ].

The obtained extracts were then subjected to the above reported GC-MS analysis ( Figure 1 ), to HPLC-DAD and to DPPH assay for the evaluation of anti-radical capacity. The GC-MS analysis on the hydroalcoholic extracts, both showing a prevalence of Maillard products, reflects the greater complexity of the commercial respect to the locally produced kiwifruits, also confirmed by the furtherly purified extracts (HA-EA, HA-SP). A relatively simple mixture was shown by the n -hexane extracts (HE).

On the basis of the afforded yields and subsequent results by HPLC analysis and DPPH assay, a scheme of the better recycling is proposed in Figure 2 .

Scheme of peels recycling and indicative estimate of the obtained quantities.

3.3. Colorimetric Analysis

CIEL*a*b data, obtained from colorimetric analyses of kiwi hydroalcoholic and n -hexane extracts and the resulting reflectance profile curves are shown in Figure 3 .

Colorimetric data and reflectance curves of the hydroalcoholic (HA) and organic (HE) extracts. The RSD values, evaluated on triplicates, were <5%.

The lightness, L* values, contained in a narrow range, between 72 and 77, show samples characterized by a high brightness. The a* values, negative for sample greenness and ranging around −7 in the HA and −10 in the HE extracts, suggest the chlorophylls’ pigments. Nevertheless, the yellowish appearance of the hydroalcoholic extracts are represented in both types of samples. Spectrophotometric measures (not reported) confirmed the slight presence of chlorophylls pigments, highly concentrated in the kiwi samples, which were also retained in the hydroalcoholic extracts (in which they represented about one tenth respect to the n -hexane extracts). The curves related to the extracts in n -hexane show a trend at 660 nm, which is associated with a higher content of darker green pigments.

The very high b* positive values, ranging between 54 and 57 in the hydroalcoholic and between 83 and 92 in the n -hexane extract, could be related to a significant polyphenolic and carotenoid fraction, respectively. Carotenoid fraction was also quantified by the HPLC analyses, which showed the presence of about 28 mg/g of carotenoids dry extract. Carotenoids were quantified as lutein equivalents in hexane extracts. (See Supplementary Material, Figure S2 and Table S2 ).

No differences were evidenced from a colorimetric point of view between the locally produced and the commercial analyzed samples. It was not possible to compare the obtained results with those reported in the literature, since the only available papers analyzed the color of whole or sliced fruits, evaluating the browning after storage or thawing [ 37 , 38 ].

3.4. DPPH Assay

Many diseases such as Alzheimer’s disease, inflammation, atherosclerosis and Parkinson’s disease are associated to reactive oxygen species (ROS) production, which can damage the biological macromolecules, generating radical chain reactions [ 39 ].

In our studies, we used DPPH assays to evaluate the radical scavenging activity of kiwi peels hydroalcoholic extract ( HA sel , HA com , HA-SP sel , HA-SP com , HA-EA sel , and HA-EA com ). To perform the analyses, a DPPH solution was monitored in the darkness at room temperature until its absorbance values at 515 nm was stable. Afterwards, a known concentration of the different extracts was added and its antiradical activity was evaluated by spectrophotometric analysis. The obtained results, expressed as mg equivalents of gallic acid/g extract (mg GA/g), are reported in Figure 4 . As shown therin, the samples coming by SPE purification presented the highest DPPH value (53.3 mg GA/mL in HA-SP com and 42.1 mg GA/g in HA-SP sel ), followed by HA-EA com and HA-EA sel (26.7 mg GA/g and 37.7 mg GA/g respectively). The lowest DPPH values were recorded for hydroalcoholic extracts (7.2 mg GA/g in HA com and 12.1 mg GA/g in HA sel ). This is due to the lower analyte concentration in the hydroalcoholic extract, which justifies the lower antiradical activity (as further confirmed by HPLC analysis). The HA-EA fractions result in lower antiradical activity and lower extraction yield compared to the extracts obtained by SPE, suggesting a limited efficiency of the ethyl-acetate partitioning process for the massive recovery of bioactive purified fraction. Our present findings suggest selecting only the SPE as a key role in the proposed scheme of pomaces recycling, giving higher yields and higher anti-radical activities with less of an impact on time and solvents.

Comparison of anti-radical capacity related to different extraction methods. Values are expressed as mg equivalents of gallic acid/g of obtained extract.

Several DPPH assays, related to kiwi peels and available in the literature, reported values in agreement with our results by SPE purification, highlighting a strong antioxidant power and confirming the high added value of this waste materials [ 4 , 39 , 40 , 41 , 42 ].

Our results overlap with those from Fiorentino et al. [ 4 ], which reported a% inhibition of DPPH ranging between 60 and 90% for ethanolic extracts by Haywards kiwi peels. Our results, analogously calculated, ranged between 70 and 80% in HA-SP and HA-EA extracts. Hădărugă et al. [ 42 ] reported different results (22–80% inhibition) according to the hydroalcoholic composition, with values rising with ethanol percentages. These results are also in agreement with the inhibition shown by ours HA extracts, ranging from around 20 to 25%.

3.5. Anti-Candida Activity

The antifungal activity of kiwi extracts was tested in vitro against one strain of Candida albicans and four strains of C. glabrata. An interesting activity, growing from the raw hydroalcoholic extracts towards the more purified SPE and ethyl acetate extracts, was revealed. Moreover, the extracts obtained from local cultivation were more effective.

The strongest activities, characterized by MIC 50 ranging between 4 and 32 µg/mL, were shown by the ethyl acetate extracts coming from this selected cultivar. Of particular interest was the efficacy, at 4 µg/mL, against PMC806 and PMC843 strains, which are more resistant to fluconazole, (MIC 50 of 2 µg/mL). Promising activity was also exerted on C. albicans (MIC 50 8 µg/mL vs. 2 µg/mL of fluconazole),

These results ( Table 1 ), on the whole, seem to be relevant for potential application of kiwi ethyl acetate extracts in antifungal therapies, always remembering to pay attention to the phytocomplex quali-quantitative composition.

Antifungal activity of kiwi samples. Activity was determined according to Clinical and Laboratory Standards Institute guidelines (CLSI document M38-A2, 2008). Minimal inhibitory concentration (MIC) was determined. MIC 50 , the lowest drug concentration that prevented 50% of growth with respect to the untreated control. The values shown are the median from three independent measurements).

3.6. HPLC-DAD Analysis

Crude and purified extracts obtained from the kiwi peels were subjected to HPLC-DAD analysis, and a chromatogram was recorded at 280 nm for the identification of hydroxycinnamic acids (caffeic acid and sinapic were found and quantified) and flavanols, (catechin and epicatechin were found and quantified) and at 360 nm for the identification of flavonols. However, no peaks were shown at this last wavelength (See also Table S3 and Figure S3 in Supplementary material ).

As shown in Figure 5 and Table 2 , the catechin amount varied between 3 and 24 mg/g of the extract (minimum value detected in HA com and maximum in HA-SP sel ). Epicatechin, between 3 and 9 mg/g, was only recorded in SPE purified extracts and was more represented in the commercial sample. Caffeic acid varied between 0.5 and 9 mg/g, and along with sinapic acid (1.6–1.7 mg/g) was found only in SPE extracts.

Example chromatograms at 280 nm of: A, hydroalcoholic extracts; B, HA-SP extracts; C, HA-EA extracts. Quantified peaks: 1, catechin; 2, epicatechin; 3, caffeic acid; 4, sinapic acid.

HPLC-DAD data of the obtained peel kiwi extracts. The results are expressed in mg/g of dry extract. BLD, below limit of detection.

The obtained results, only partially agree with those reported in the literature. Satpal et al. [ 2 ], in a review, confirms the main presence in peels from various kiwi cultivars, of caffeic acid and its derivatives, sinapic acid, catechins and derivatives. Other works also report a small presence of flavonoids, such as chrysin and quercetin [ 16 , 43 ], not yet identified in our extracts.

Very variable values are reported in literature according to the analyzed cultivars, mostly expressed as mg/g fruit fresh weight. The sum of reported flavanols contents ranged between 0.003 and 0.148 mg/g fw and hydroxycinnamic acids between 0.01 and 0.7 mg/g fw. Taking in to account the differences due to the expression in fresh weight respect to dry weight, the available quantitative data, only partially overlapped with our results [ 16 , 44 ]. The presence of quercetin is sometimes reported, but was not found in our samples.

Comparing data, a strong analytes concentration was afforded in SP extracts, both of commercial and selected kiwi fruits, corresponding to about 19-fold increase in catechin and 6-fold increase in caffeic acid. Moreover, this concentration results in the presence of epicatechin and sinapic acid extracts, not identified in hydroalcoholic extracts. The presence of a more active phytocomplex in SPE extracts, justifies the higher anti-radical capacity shown by DPPH analyses.

As regards the difference between the phytocomplexes of the two samples, the main difference is represented by epicatechin that is present in concentration three folds higher in the peels from commercial sample, perhaps in accordance with the stronger anti-radical activity expressed.

4. Conclusions

In this study, a green and complete approach for the recycling of kiwi peels was suggested, affording the potential recovery of tons of wasted material to be reused as high added value new compounds.

Considering that millions of tons of kiwi waste worldwide are thought to be produced in a year, the proposed extraction method could afford amounts in the order of tons per year of extracts enriched in carotenoid, chlorophylls, and polyphenolic extracts.

Insoluble and soluble fibers, representing the main part of wastes, could also be recycled and used as interesting emerging materials for pharmaceutical and other health uses. Moreover, considering the real content of analytes and taking in to account the principles of the circular economy and the best green procedures, the hydroalcoholic extracts could also represent an interesting compromise. In the detailed case we report, it contains about 100 mg/100 g catechin and 16 mg/100 g caffeic acid, still retaining a pleasant aroma by HMF and derivatives. It could be easily formulated as an enhanced mix of soluble fibers and antioxidant compounds or as a food supplement.

Moreover, the ethyl acetate extracts, although affording estimated quantities of 220 tons per year, deserve to be taken in to account as potential anti- Candida agent.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/foods11040589/s1 . Figure S1: GC/FID chromatograms of kiwi peels “selected” (Panel A) and “commercial” (Panel B). Figure S2. Example chromatogram of HE extracts at 450 nm. Figure S3: Example chromatogram of hydroalcoholic extracts at 360 nm (Panel A), of standard compounds at 280 nm (Panel B, 1. Chlorogenic acid; 2. Catechin; 3. Epicatechin; 4. Caffeic acid; 5. p-Coumaric acid; 6. Sinapic acid; 7. Ferulic acid) and at 360 nm (Panel C, 8. Rutin; 9. Quercetin-3-gal; 10. Myricetin; 11. Quercetin; 12. Kaempferol. Table S1. HPLC-DAD quantitative analysis of carotenoids in HE sel and HE com . Table S2: HPLC-DAD quantitative analysis of carotenoids in HEsel and HEcom, expressed as sum of the peaks identified at 450 nm. Lutein calibration curve in µg/mL: 13.29x + 2.46; R2 = 0.9999. Table S3: Polyphenolic standard compounds taken in to account.

Author Contributions

Conceptualization, S.C.; data curation G.S., F.C., and L.D.M.; funding acquisition S.C., G.S.; investigation, F.C., S.G., and G.S.; methodology, F.C., L.D.M., and M.A.C.; project administration, S.C.; resources: S.C., S.G., G.S., and M.A.C.; software: F.C.; supervision, M.A.C.; writing—original draft S.C., F.C., and L.D.M. writing—review and editing L.M. All authors have read and agreed to the published version of the manuscript.

This work was financially supported by funding from “Sapienza” University of Rome, Ateneo 2020: RP120172B414F81B and Ateneo 2020, MA2201729D6327B0.

Institutional Review Board Statement

Informed consent statement.

Informed consent was obtained from all subjects involved in the study.

Data Availability Statement

Conflicts of interest.

The authors declare no conflict of interest.

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