Impact of molecular structure of amylopectin and amylose on amylose chain association during cooling

The effect of amylose molecular size and amylopectin branch chain length on thermal behavior of native starch, defatted starch and isolated amylose during cooling was studied. Six starches from corn, amylomaize, rice, potato, bean and pea with varying amylose content (20–70%) were heated to 180°C at 10°C/min and subsequently cooled at 10°C/min to 5°C in a differential scanning calorimeter (DSC) in excess water. On cooling of native corn and amylomaize starches, two distinct exotherms were observed at around 70 and 30°C, which were attributed to amylose–lipid complex formation and amylose chain association, respectively. After defatting, a larger exotherm at a higher temperature during cooling was observed for corn and amylomaize starches, whereas bean and pea starches showed a smaller exotherm at a lower temperature. Amylomaize, with a large amount of lipid and longer amylopectin branch chains, exhibited the lowest onset temperature of amylose chain association in native and defatted starches among the tested starches, indicating the lipid and long branch chains in amylopectin restrict amylose chain association. The enthalpy of amylose chain association in defatted starch was positively correlated with apparent amylose content (R2=0.999). In amylose isolated from each starch, a larger exotherm with a peak temperature around 60°C was observed on cooling. Potato amylose, which had a long average chain length (DP¯n=4915), showed the lowest temperature and enthalpy of amylose chain association. The peak temperature of amylose chain association increased with increasing solid concentration of isolated amylose, whereas enthalpy reached a maximum at a concentration of 20–30% (w/w) solid.