In vitro Protein Digestibility, Phenolic Contents and Antioxidant Capacities of Commercially Important Tree-nuts

The aim of this study was to evaluate and compare the in vitro protein digestibility, phenolic contents, and antioxidant capacities of commercially important tree-nut species (almond, cashew, hazelnut, pistachio, and walnut). To evaluate the protein digestibility, upper gastrointestinal digestion was simulated in vitro in which tree-nuts were treated with pepsin and pancreatin. After the simulation, protein digestibility was calculated by measuring the remaining (undigestible) protein using Kjeltec nitrogen analyzer and by quantifying the primary amino acids formed through TNBS (2,4,6 trinitrobenzene sulfonic acid) method. Total phenolic contents and antioxidant capacities of tree-nuts were assessed spectrophotometrically. Our results revealed that that tree-nuts differ in their protein digestiblities in vitro. Based on the remaining protein calculations, cashew nut had the highest in vitro %protein digestibility (%91.79), followed by almond nut (%87.71), hazelnut (%86.10), pistachio nut (%81.15), and walnut (%44.75). TNBS results also partially agree with this that the lowest primary amino acid content after the digestion was found in walnut samples. The highest phenolic content was obtained in walnut (6.8 mg GAE/g), while cashew nut exhibited the lowest phenolic content (1.3 mg GAE/g). Both antioxidant capacity assays correlated with the total phenolic content; the samples possessing higher phenolic content revealed the higher antioxidant capacity. This study shows that in vitro protein digestibility, total phenolic contents, and antioxidant capacities of commercially important tree-nuts show variations from species to species, with walnut possessing the lowest protein digestibility, the highest phenolic content and antioxidant capacity, but the opposite was true for cashew nut.