ABSTRACT

This study investigated the effect of different cooking methods on glycemic index (GI) of three yam cultivars commonly eaten in Nigeria namely: Dioscorea rotundata, Dioscorea alata and Dioscorea dumentorum. Yam tubers were processed by, frying, pounding and boiling, and then immediately consumed by twenty-seven normal healthy adults comprising of males and females. Capillary blood samples were taken at the fasting state and at different intervals of 15, 30, 45, 60, 90 and 120 minutes from the consumption of each meal. The blood samples were analysed for plasma glucose concentration and incremental areas under plasma glucose curves were calculated. The GI varied between 22.31 –72.78 for the yam cultivars studied. Samples prepared by boiling recorded Dioscorea alata as low GI (22.31) and Dioscorea rotundata as high GI (72.78). While those processed by frying and pounding, recorded a low GI (20.95 – 44.35) and Dioscorea dumentorum having the least GI value (20.95). The study indicates that the glycemic index of the three yam cultivars (Dioscorea rotundata, Dioscorea alata and Dioscorea dumentorum) varies significantly by their methods of preparation and variety differences. Consumption of boiled and pounded Dioscorea alata could minimize postprandial blood glucose spikes and therefore, may prove to be more efficacious in the management of type 2 diabetes mellitus. Proximate composition (moisture, protein, fat, ash, fibre and carbohydrates) were determined for the raw and processed samples prior to glycemic analysis. The functional properties and amylose /amylopectin ratio of the flour sample were determined. The result of this study showed a significant difference (p<0.05) in the functional properties of the yam flour samples, the moisture content significantly differed (p<0.05) in different cooking methods and various varieties. There was no significant difference in the protein content of the different yam varieties. Low protein values observed in this study suggested that yam is not a good source of protein. At different levels of processing there was a significant difference in the fat content when compared to the raw samples with an increase on the fried samples and a decrease on boiled and pounded samples. There was a significance difference (p<0.05) in the crude fiber content of the different yam varieties at its raw state with Dioscorea dumentorum having the highest value (1.30). No significance difference was seen on the ash content of the boiled Dioscorea rotundata and Dioscorea dumentorum. The carbohydrate content differed significantly (p<0.05) amongst the yam varieties and their various processing methods. The variety main effect were significantly different (p<0.05) while the processing method main effect were also significantly different (p<0.05).