Effects of gastrointestinal pH on the bioactivity of fruit peel extracts rich in betalains and polyphenols from six Opuntia species

For the purpose of studying the peel from Opuntia fruits as potential bio-functional ingredients, the effect of gastrointestinal pH on the composition and bioactivity of six species peel-extracts were evaluated. Results showed that incubation at gastric and intestinal pH significantly affected the content in betalains of most peel extracts. The concentration of betacyanin decreased more than the concentration of betaxanthin upon incubation at pH 2 followed by incubation at pH 7. The contents in total polyphenols were not affected, although the concentration of particular phenolic compounds changed in some of the extracts. A positive correlation was observed between ABTS+ scavenging activity and the content in polyphenols. Incubation at gastrointestinal pH affected antioxidant activity as determined using the FRAP assay, but not as determined using the ABTS+ assay. The inhibition of α-glucosidase and pancreatic lipase by the extracts was significantly affected by pH treatments, and this effect was probably due to structural changes in bioactive compounds. A positive correlation was observed between α-glucosidase inhibition and protocatechuic acid concentration. Gastrointestinal pH exerts a great effect on a variety of antioxidant and enzyme inhibiting activities of peel extracts that depend on the specie, O. megacantha peel exhibiting the highest bioactivity.