Effects of domestic thermal processing on carbohydrate quality, digestive enzyme inhibition and glycaemic response of white yam (Dioscorea rotundata)

Domestic thermal processing influences starch digestibility and postprandial glycaemic response, yet its effects on the carbohydrate quality of white yam (Dioscorea rotundata), a major West African staple, are incompletely understood. This study examined the effects of boiling, roasting and deep-frying on proximate composition, carbohydrate fractions, α-amylase and α-glucosidase inhibitory activities, and glycaemic response, using rat postprandial blood glucose measurements and an in vitro enzymatic starch hydrolysis assay. Frying increased apparent crude fat (gravimetric weight-loss method; absolute values probably overestimated) from 15.56 ± 1.79 to 29.35 ± 1.89g100g−1, and carbohydrate estimated by difference fell from 68.93 ± 2.60g100g−1 in the raw flour to 51.15 ± 2.60 and 55.11 ± 1.62g100g−1 in the fried and boiled flours; boiling produced the highest reducing sugars (3.43 ± 0.04g100g−1). At 1.0mgmL−1, all methanolic extracts strongly inhibited α-glucosidase (79.54 ± 2.94% for raw to 86.41 ± 0.76% for boiled) and α-amylase (80.02 ± 0.09% for fried to 82.17 ± 0.09% for roasted), but inhibition did not track glycaemic response. Rat-derived relative glycaemic responses (RGR) were highest for roasted yam (42.8 ± 13.3), followed by boiled (39.3 ± 23.8), fried (32.6 ± 24.2) and raw (25.2 ± 31.9). In vitro, glucose release over 120min was greatest for raw flour (+64.3mgdL−1), followed by roasted (+46.9) and boiled (+13.4) flours, and the fried-flour profile was unreliable at 90–120min; no glucose reference was run in vitro. Domestic processing altered the proximate composition and sugar content of white yam, but the glycaemic responses of rats did not differ significantly among the flours (p=0.68). These findings are preliminary and require confirmation in adequately powered studies, including human trials.