Glycemic index of different varieties of yam as influenced by boiling, frying and roasting.

Yam is one of the commonly consumed carbohydrate staples. The objective of this work was to investigate the effect of boiling, roasting, and frying on the glycemic index (GI) of white yam (Dioscorea rotundata), yellow yam (Dioscorea cayenesis), and water yam (Dioscorea alata). Yam tubers were obtained (peeled, sliced, washed) deep fried in vegetable oil for 35-40 min and boiled in water for about 35-45 min. Sliced unpeeled tubers were also roasted at about 120°C for 40-45 min. The cooked yam samples were fed to 10 healthy subjects aged between 20-50 years. A glucometer was used to measure the blood glucose concentrations of the test individuals before consuming the yam diets and after the 15th, 30th, 45th, 60th, 90th, and 120th min of consumption. The average incremental area under the curves (IAUC) obtained from the recorded blood glucose concentrations were used to calculate the GI of various yam diets. The GI of the yam diets were found to be in the following increasing order: White-yam-boiled (44.26%) < Water-yam-boiled (50.12%) < White-yam-roasted (50.62%) < Water-yam-roasted (54.04%) < White-yam-fried (59.13%) < Yellow-yam-fried (65.08%) < Water-yam-fried (69.16%) < Yellow-yam-roasted (70.62%) < Yellow-yam-boiled (75.18%). White yam diets relatively had lower GI compared to yellow yam and water yam. Boiling was found to give generally lower GI in the white and water yams and could therefore be applied in the preparation of lower GI diets for diabetics.