CHEMICAL PROPERTIES, IN VITRO STARCH DIGESTIBILITY AND THE ESTIMATED GLYCEMIC INDEX OF WATERYAM, COCOYAM, CASSAVA AND SWEET POTATO

Abstract

The study investigated the chemical properties (proximate composition, mineral content, amylose, amylopectin content, total starch, resistant starch and digestible starch, phytochemical content), in vitro starch digestibility and estimation of glycemic index of wateryam, cocoyam, cassava (TMS 0581) and sweet potato. Levels of significance (p≤0.05) between the means of the roots and tubers’ chemical characteristics were determined at 95% confidence limit (p≤0.05). The moisture content on dry basis ranged from 12.63 to 11.67% with wateryam having the highest value, while sweet potato recorded the lowest value. The highest protein content was observed in sweet potato (4.75%). The fat content was in the range of 1.14% (cassava) to 2.65% (sweet potato). The highest crude fibre and ash content (2.34% and 3.98% respectively) were recorded in sweet potato. Carbohydrate content ranged from 86.28% (sweet potato) to 92.99% (cassava). Energy value was in the range of 388.39 to 390.66 kcal/ 100 g. The mineral composition showed sodium, calcium, zinc, phosphorus, iron and manganese as the most abundant element in wateryam, while potassium was the predominant element in all the samples studied. Amylose and amylopectin contents ranged from 21.30 to 32.67% and 67.33 to 78.70% respectively. Total starch (70.67 g/ 100 g) and digestible starch (65.79 g/ 100 g) were recorded in cocoyam, while resistant starch content of 15.23 g/ 100 g was recorded for wateryam. Saponin content of 19.99 mg/ g, tannin (4.74 mg/ g) and terpernoid (13.63 mg/ g) were recorded in cocoyam, sweet potato and cassava respectively. Oxalate and phytate contents ranged from 0.22 to 1.05 mg/ g and 7.43 to 28.51 mg/ g respectively. The cyanide content was very low (0.56 to 1.15 mg/ kg). Cardiac glycoside (19.85 mg/ g) and total phenol (49.42 mg/ g) were recorded in sweet potato. Flavonoid and carotenoid contents ranged from 1.78 to 14.15 mg/ g and 18.79 to 29.72 mg/ kg respectively. The least values of in vitro starch digestibility (1.34 mg/ g) and the estimated glycemic index (53.57%) were recorded in wateryam, making the root and tuber, the most preferred in terms of nutritional and health benefits. The comparison of the in vitro starch digestibility and the estimated glycemic index of the samples showed differences in nutritional importance and could form a basis for dietary advice to diabetic subjects.