Structure-function relationahip of starches extracted from two cassava cultivars

Two genetically related cassava cultivars, KU 50 and Rayong 90, selected for similarities in starch yield, were grown under identical field conditions. Plants were harvested at times representing early harvest (6 months) to late harvest (16 months). The influences of harvest time and rainfall on structural and functional properties of extracted starch were determined. Properties of starch extracted from Rayong 90 were influenced by rainfall in the immediate four weeks prior to harvest. Amylose content, apparent amylose size and RVA initial paste temperature were highly correlated with amount of rainfall in the preceding four weeks (R20.84, p0.05). Rainfall during the harvest period is suggested as the key factor as cumulative predipitation for the preceding two months had little influence on starch properties. Properties of the starch extracted from KU 50 were less influenced by rainfall as only amylose size was significantly correlated. Time of harvest was not significantly correlated to any of the structural or functional properties. Apparent size of amylose was highly correlated with RVA paste temperature for both Rayong 90 and KU 50. Suggesting the influence of rainfall on pasting temperature was mediated through a sturctural characteristic as opposed to a direct influence on the functional property. Swelling power was also highly correlated to RVA paste viscosity. Genetic variability and time of harvest did not influence either gross granule morphology, as determined by scanning electron microscopy and size analysis, or amount of granule crystallinity, measured indirectly by thermal analysis and RVA paste temperature. Some properties were different between cultivars and time of harvest, for example, amylose content ranged between 18.5-21.5 percent for KU 50 and 20.5-24.5 percent for Rayong 90, apparent amylose size ranged between DP875-1150 for KU 50 and 775-1267 for Rayong 90. Intercultivar variation is suggested to be less of an influence than rainfall at harvest.