Determination of the in vitro protein digestibility of some commercial hard fruits and their effects on the large boot microbiota

The purpose of this study was to determine the total phenolic contents, antioxidant capacities, protein digestibilities and dietary fiber contents of commercially important nuts (almond, cashew, hazelnut, pistachio and walnut) and to investigate their effects on colonic microbiota compositions and functions in vitro. The highest total phenolic content was obtained in walnut samples, which was followed by pistachio, hazelnut, cashew, and almond, respectively. As aspected, the samples posessing higher total phenolic contents has the higher antioxidant capacity. Among the samples, the highest dietary fiber content was determined in pistachios (16.75%), followed by almonds (14.97%), hazelnuts (14.25%), walnuts (11.60%) and cashews (9.92%). The protein digestibilities of the samples were determined by calculating the amount of nitrogen remaining in the medium and the amount of primary amino acids measured by using the Kjeldahl and TNBS methods, respectively, after digestive enzyme treatments and the results revealed that cashew samples had the highest protein digestibility, while walnut samples had the lowest protein digestibility. Interestingly, it was determined that there was no significant relationship between phytic acid values and protein digestibilities of the samples; however, an inverse relationship between total phenolic content and protein digestibility was detected. The effects of the samples on the colonic microbiota were determined by measuring the short-chain fatty acids formed by the fecal microbiota as a result of fermentation using gas chromatography and the composition of the microbiota as a result of the fermentation of the nuts were examined through next generation sequencing. The effects of the samples on the colonic microbiota were found to differ. Specifically, the cashew sample was found to be fermented faster than the other samples and provides more butyric acid formation. It is thought that the high amount of butyric acid formation in cashews is due to the fact that cashew induces the growth of microbial groups (i.e. OTU30 Butyricimonas and OTU34 Collinsella aerofaciens) that are capable of producing butyric acid. This study shows that the nutritional quality of nuts differs; the cashew sample has higher protein digestibility and promotes the formation of beneficial microbial metabolites in the large intestine and the growth of beneficial microorganisms.