PHYSICOCHEMICAL PROPERTIES OF STARCHES OF SOME TROPICAL TUBERS

ABSTRACT

The starches of corms and cormels of red and white cocoyam (Colocasia esculenta) varieties, cassava (Manihot esculenta), sweet potato (Ipomea batata), bitter yam (Dioscorea dumentorum) and water yam (Dioscorea alata) were extracted. Their proximate compositions, physicochemical properties, mineral compositions and antinutrient contents were analyzed. The results of the proximate analysis (pWCCM (corms of white cocoyam)>WYAM>SPOTA>WCCL >RCCL (cormels of red cocoyam)>CASS. The darkest starch sample was WCCL while the lightest was CASS. The results of bulk density ranged from 0.70 g/ml to 0.76 g/ml. The values obtained were higher than that of a commercial starch, Starch 1500 used in the pharmaceuticals as tablet binder. The increasing order of water absorption capacities obtained was RCCLhad the lowest ability to retrograde after cooling its paste followed by SPOTA. The most viscous starch paste was that of WYAM with paste stability of 119.58 RVU. From the results, it was generally observed that the higher the retrogradation, the lower the stability. The gelatinization temperatures were between 60.45 and 62.150C. The results of paste clarity showed that after freezing for three days, the most transparent starch paste was CASS. All the starch samples were good sources of minerals. The predominant mineral was potassium and its peak value was exhibited by SPOTA (3102.00±0.50 mg/kg), which incidentally had the highest value of phosphorus (1012.00±0.50 mg/kg) among all the samples. The peak value of iron (21.00± 0.02 mg/kg) was in WCCM. The values of zinc were low with the highest value as 4.50±0.02 mg/kg in RCCL. Other heavy metals such as Hg, Pb, Cd, Cu, Mn, As and Ni were not detected in all the starch samples. The results of antinutrients in the starches showed that the level of trypsin inhibitor ranged from 0.09 to 0.13±0.01 mg/100g. Free cyanides were also detected in all the samples at micro quantity. The range of values obtained for cyanide content was lower than the recommended value. The highest value of cyanide content was 21.03±0.02 μg/100g (≈ 0.21±0.02 mg/kg) in CASS and the least was 7.31±0.01 μg/100g (≈ 0.07±0.01 mg/kg) in RCCL.