Studies on the Production and Evaluation of Starch from Yam (Dioscorea spp.) and Cocoyam (Colocasia esculenta) Tubers Cultivated in Nigeria

The starches of yam (Dioscorea spp.), and cocoyam (Colocasia esculenta) were extracted with water and with 0.03M conc. ammonia solution. Their whole flours were also prepared which served as the control and cassava starch was used as the reference standard. All data obtained were subjected to analysis of variance and Duncan multiple ranges was used to compare the means. Their proximate compositions, functional and physicochemical properties, mineral elements, water-soluble vitamins and antinutrients were analyzed using standard laboratory procedures. The extracts/starch obtained showed that the percentage yield of starch was higher in yam (50.05, 50.56 for ammonia and water respectively) and lower in cocoyam (43.80, 34.48 for ammonia and water respectively). The use of ammonia significantly increased (P < 0.05) the yield of cocoyam starch from 34.48 % 43.80%. The results of the proximate analysis showed that the moisture content (10.99, 9.85 cocoyam and yam respectively), ash (3.22 and 2.10), crude protein (4.60 and 3.60) and fibre content (7.09 and 11.99) of the flours were significantly higher than in their corresponding starches (with the exception of YSW which showed a higher moisture content of 14.22 above its flour). Lower values of flours than the starches were recorded for carbohydrates (73.48, 85.05, 84.73 and 71.70, 85.33, 80.30 for cocoyam and yam respectively) and energy (314.95, 315.69, 315.49 and 304.29, 350.43, 329.54 for cocoyam and yam respectively). The results of the functional and physicochemical properties showed that the water absorption capacity (with values 2.16 g/ml, 1.04 g/ml, 1.04 g/ml, and 1.36 g/ml, 1.02 g/ml, 0.42 g/ml for yam and cocoyam flours and starches respectively) and bulk density (0.65 g/ml, 0.63 g/ml, 0.76 g/ml, and 0.57 g/ml, 0.54 g/ml, 0.51 g/ml for yam and cocoyam flours and starches respectively) were generally low. The pH of the flours ranged from 8.42 to 8.49 while that of the starches from 7.31 to 5.39. Phytic acid levels were significantly low (from 0.82% to 0.11% for cocoyam and yam respectively) and this signifies a better nutritional value. The differences between the water and ammonia extracted starches in amylose (27.73, 27.78 %T, and 16.32, 15.80 %T yam and cocoyam respectively) and amylopectins (72.36, 72.21 %T and 83.62, 84.31 %T yam and cocoyam respectively) were not significant at P>0.05. Swelling power increased with increase in temperature and cocoyam had the highest swelling power of 3.53 g/g at 90°C while yam had lowest value of 2.07 g/g at the same temperature. Also, cocoyam starch water exhibited the highest paste clarity of 60.94 %T and yam starch water exhibited the lowest (49.32 %T). Both yam and cocoyam flours and starches were highly viscous at both share rates used but cocoyam was the most viscous. All the functional and physicochemical values obtained were comparable to the standard. The levels of the trace and heavy metals analyzed were within the safe limits. Vitamins B1, B2 and B6 levels were higher than the recommended daily allowances and Vitamin C level was very low and in most cases not detected.