Development of a natural ingredient from kiwi epicarp with potential application in the food industry

Kiwifruit is a recognized source of bioactive compounds with beneficial properties for consumers’ health [1]. Although only pulp is normally consumed, there are several studies in literature that have been exploring the fruits epicarp, which is characterized as an excellent source of bioactive compounds. Thus, since the food industry produces annually tons of this normally discarded and unused residue [2], this work aimed to characterize two varieties of kiwi (Actinidia delicious cv. "Hayward" - green pulp kiwi variety and Actinidia spp. - red pulp kiwi variety), comparing the two pulps and epicarps. This study will valorize these bio-residues. The proximal composition (protein, fat, carbohydrates and ash contents, and energy) of the pulp of these varieties, as well as the composition in free sugars, fatty acids, organic acids and tocopherols, and the study of the bioactivities (antioxidant, anti-inflammatory, cytotoxic and antimicrobial activities) of the pulp and epicarp were evaluated. A hydroethanolic extraction was used to obtain compounds subsequently identified and quantified by HPLC-DAD-ESI/MS. The results revealed that pulps have a high water content and low amount of other macronutrients. Both epicarp varieties presented a higher concentration of phenolic compounds than the corresponding pulp. Additionally, only Actinidia spp. (red pulp kiwi variety) presented an anthocyanin, cyanidin-O-pentosylhexoside. α-tocopherol was the most abundant vitamer in all the analyzed parts. Four organic acids (quinic, malic, citric and ascorbic acids) were identified in all the studied samples. The best antioxidant activity was shown by the epicarps, highlighting the green pulp kiwi as the best variety. Similarly, only the epicarp of the green pulpkiwi variety presented positive results for all the tested cell lines, being able to inhibit the cell growth in a moderate way. Additionally, none of the extracts showed toxicity in a porcine liver cell line at the highest tested concentration (GI50> 400 μg/mL). Both kiwi varieties also demonstrated an excellent antimicrobial activity against some pathogenic bacteria. These results confirm the potential application of kiwi epicarp in the food industry as a source of functional compounds with bioactive properties, contributing in the valorization of these bio-residues.The authors are grateful to the Foundation for Science and Technology (FCT, Portugal) and FEDER under Programme PT2020 for financial support to CIMO (UID/AGR/00690/2019); national funding by FCT, P.I., through the institutional scientific employment program-contract for L. Barros and R. Calhelha’s contract, and Carla Pereira’s contract though the celebration of program-contract foreseen in No. 4, 5 and 6 of article 23º of Decree-Law No. 57/2016, of 29th August, amended by Law No. 57/2017, of 19th July; research contract of C. Caleja (Project AllNatt, POCI- 01-0145-FEDER-030463); to FEDER-Interreg España-Portugal programme for financial support through the project 0377_Iberphenol_6_E. The authors are grateful for financial support to the Ministry of Education. Science and Technological Development of Republic of Serbia, grant number 173032. The authors also thank the company KiwiCoop (Oliveira do Bairro, Portugal) for providing the kiwi samples.