Starch digestibility, polyphenol contents and in vitro alpha amylase inhibitory properties of two varieties of cocoyam (Colocassia esculenta and Xanthosoma mafafa) as affected by cooking

Abstract

The effect of cooking (boiling) on the starch digestibility, polyphenol contents and in vitro α-amylase inhibitory properties of two varieties of cocoyam: Colocassia esculenta (taro) and Xanthosoma mafafa (Tania) were investigated. The total starch contents of the raw and cooked tubers ranged from 25.3 to 35.6 g 100 g−1; resistant starch ranged from 9.7 to 20.9 g 100 g−1; digestible starch ranged from 11.7 to 24.2 g 100 g−1. The starch digestibility of the raw and cooked tubers ranged from 35.89 to 71.38% while their rapidly digestible starch values (expressed as g 100 g−1 of total starch) ranged from 20 to 32 g 100 g−1. The raw tubers contained considerable amounts of phenols and flavonoids and boiling of the tubers retained their phenolic and flavonoid contents. The aqueous extracts of the raw and cooked tubers demonstrated considerable α-amylase inhibition properties (32.47–46.62% over a concentration range of 10–50 mg mL−1) under in vitro conditions but which were lower than that of the antidiabetic drug—acarbose that had 69.08% inhibition of alpha amylase activity over a range of concentration of 2–10 mg mL−1. The study showed that the raw and cooked forms of the cocoyam varieties had low digestibility of their starches and exerted considerable inhibition of α-amylase activity under in vitro conditions which suggests the potential usefulness of the boiled forms of these cocoyam varieties as functional foods for people with type 2 diabetes mellitus.