ANTIOXIDANT PROPERTIES, GLYCEMIC INDEX AND INHIBITORY ACTION OF SOME YAM (Dioscorea spp.) PRODUCTS ON KEY ENZYMES LINKED WITH TYPE–2 DIABETES

Abstract

Postprandial glycemic control is important in the prevention and therapy of type-2 diabetes and related diseases. Carbohydrate is the main proportion of food consumed daily and the main energy source, but proper management of type-2 diabetes requires an adequate and proper selection of carbohydrates. Therefore, this study sought to assess the effect of processing of yam flour [white yam (Dioscorea rotundata); water yam (Dioscorea alata) and yellow yam (Dioscorea cayenensis)] to paste on the glycemic index, antioxidant properties, phenolic constituents and melanoidin contents. The yam varieties were prepared into flour and paste from which; aqueous extracts were subsequently produced. The glycemic index, starch, sugar, amylose, amylopectin contents and amylose/amylopectin ratio were determined. The antioxidant properties were investigated using a number of assays (reducing power, DPPH and ABTS* radical scavenging abilities, Fe2+ chelation ability and OH* scavenging ability). Also, the phenolic constituents and melanoidin contents were determined using gas chromatography (GC). Inhibitory action of the yam products on key enzymes linked to type-2 diabetes (α-amylase and α-glucosidase); as well as enzymatic and non-enzymatic antioxidant status of rats in vivo were determined. The pasting process caused a significant (P < 0.05) decrease in the starch, sugar and amylose contents, but a significant (P < 0.05) increase in amylopectin and glycemic index of all the yam varieties except in water yam. The sugar contents of the yams ranged from 4.36% (yellow yam) to 6.44% (white yam) for flour and 3.72% (yellow yam) to 5.07% (white yam) for paste. Also, the amylose/amylopectin ratio ranged from 0.54 (water yam) to 0.80 (white yam) for flour and 0.33 (water yam) to 0.53 (white and yellow yams) for paste. However, processing of the flour into paste caused a significant decrease (P < 0.05) in the antioxidant properties (except in Fe2+ chelating ability) of the yam varieties. The total phenol contents of the yams ranged from 12.04 mg/g (yellow yam) to 15.06 mg/g (water yam) for flour and 9.67 mg/g (water yam) to 12.19 mg/g (white yam) for paste. Also, the total flavonoid content ranged from 4.84 mg/g (yellow yam) to 11.17 mg/g (water yam) for flour and 2.50 mg/g (white yam) to 9.34 mg/g (water yam) for paste. GC-characterisation revealed the presence of some phenolic acids and flavonoids (vanillic acid, p-hydrobenzoic acid, caffeic acid, ferulic acid, piperic acid, curcumin, myricetin, eriocitrin, daidzein, genistein, quercetin, rutin). The pasting process caused a significant (P < 0.05) increase in the melanoidins contents of the yams. The yam extracts inhibited α-amylase and α-glucosidase activities in vitro in a dose-dependent pattern (1- 4mg/mL); however, the pasting process caused significant (P < 0.05) increase in the α-amylase and α-glucosidase inhibitory activities. Therefore, it thus appears that processing of yam varieties into paste (browned) would decrease the starch, sugar, amylose contents and amylose/amylopectin ratio and antioxidant capacity, while the melanoidin content, inhibition of α-amylase and α-glucosidase activities; and glycemic index increase (except water yam). Also, water yam seems to be a better carbohydrate source for diabetic patients of all the yams studied.