Heat-moisture treatment effects on sweetpotato starches differing in amylose content

Sweetpotato starch from two genotypes, Taiwan (15.2% amylose) and 93-006 (28.5% amylose), were exposed to heat-moisture treatment (HMT) of 25% moisture at 110°C for various exposure times at ‘as-is’ (pH 6.5–6.7) and alkaline pH (pH 10) conditions. In both starch samples at ‘as-is’ pH, there was a shift from a Type A pasting profile (characterized by a high to moderate pasting peak, major breakdown after holding time at 95°C and low cold paste viscosity) to a Type C pasting profile (characterized by lack of a pasting peak and no breakdown, with high cold paste viscosity in 93-006; and a slight breakdown in Taiwan). With HMT at pH 10, the pasting peak viscosity was increased and low hot paste viscosities and high cold paste viscosities were observed. Under both pH conditions after HMT, there were marked increases in gelatinization temperatures and broadening of the DSC gelatinization endotherms, and considerable decreases in swelling volume and solubilities. Gel textures of HMT starch samples appeared to be related to amylose content. Taiwan starch gel had a marked increase in hardness and adhesiveness, while that of 93-006 did not show significant differences in hardness after HMT. Both starch samples showed a marked reduction in resilience, indicating a shift from a long stringy nature to short paste consistency. Starch gels exposed to HMT under alkaline conditions showed a high degree of syneresis.