Acid hydrolysis of native and annealed wheat, potato and pea starches - DSC melting features and chain length distributions of lintnerised starches

Annealing tone and two step) slightly decreased the susceptibility of potato starches to acid hydrolysis (lintnerisation, 2.2 M HCl, 35 degrees C); however, it hardly affected that of wheat and pea starches. Annealing had a very pronounced effect on DSC gelatinisation of wheat, potato and pea starches: endothermic peaks narrowed and shifted to a higher temperature. However, already after 10.5 h of acid hydrolysis, differences in DSC transition temperatures and peak widths between native, one step and two step annealed starches became much smaller and almost disappeared after further hydrolysis, indicating a substantial contribution of the amorphous regions to annealing. The observation that 7-day lintnerised wheat and pea starches were less susceptible to annealing than the corresponding 24-h lintnerised starches supported this view. No changes in chain length distribution were observed between lintnerised (20 days) native and annealed potato starches and between lintnerised (7 days) native and annealed pea starches. For all other lintnerised starches, the distribution profiles showed a higher average degree of polymerisation (DP), and a slightly higher ratio of singly branched (DP 22-30) to short linear (DP 10-18) chains (except for 20-day lintnerised pea starches! for the annealed than for the corresponding native starches. This indicates that, as a result of annealing, the amylopectin branch points become more resistant to acid attack. From the above, it was concluded that the amorphous parts in the granule indeed play an important role in the molecular mechanism(s) of annealing, but that structural changes within crystalline parts may also take place. (C) 1998 Elsevier Science Ltd. All rights reserved.