Antioxidant activities and inhibitory effects of free and bound polyphenols from date (Phoenix dactylifera L.) seeds on starch digestive enzymes

Abstract

Date (Phoenix dactylifera L.) seeds have gained interest as a valuable by-product of  the date fruit industry and have been identified as a rich source of functional and bioactive ingredients. In this study, date seeds from five varieties (Medjool, Deglet Nour, Barhee, Bou Sthammi and Dayrie) cultivated in Australia were analysed for their total phenolic and flavonoid contents, ferric reducing antioxidant power (FRAP), Trolox equivalent antioxidant capacity (TEAC), and in-vitro inhibition against α-amylase and α-glucosidase. The date seed powders (DSP) were extracted with acetone-methanol-water (2:2:1, v:v:v) to obtain free polyphenols.  The bound polyphenols were extracted from the residual solids with butanol-HCl (97.5:2.5; v/v) treatment at 100 °C. The greatest quantities of total phenols (4166±227 mg Gallic Acid Equivalents 100g^-1 DSP), total flavonoids (52.1±9.6 mg Quercetin Equivalents 100g^-1 DSP), FRAP (1589±47 µmol Iron[II] Equivalents g^-1 DSP) and TEAC (45.2±1.4 µmol Trolox Equivalents g^-1 DSP) were detected in seeds from Deglet Nour variety.

Both free and bound polyphenol fractions showed significant (p<0.05) inhibition against rat intestinal α-glucosidase with little or no inhibition against pancreatic α-amylase. Bound polyphenols showed stronger α-glucosidase inhibition compared with free polyphenols.  The IC₅₀ for α-glucosidase were reported as mg GAE mL^-1 ranged from 0.39±0.02 (Deglet Nour) to 0.68±0.02 (Medjool) for bound polyphenols, and from 0.907±0.08 (Medjool) to 1.75±0.15 (Barhee-khalal) for free polyphenols. Bound polyphenol fraction, with strong α-glucosidase inhibition and weak α-amylase inhibition, was suggested as a significant source of functional food ingredients with anti-hyperglycaemic properties.