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 degr ees C); however, it hardly affected that of wheat and pea starches. An nealing had a very pronounced effect on DSC gelatinisation of wheat, p otato and pea starches: endothermic peaks narrowed and shifted to a hi gher temperature. However, already after 10.5 h of acid hydrolysis, di fferences in DSC transition temperatures and peak widths between nativ e, one step and two step annealed starches became much smaller and alm ost disappeared after further hydrolysis, indicating a substantial con tribution of the amorphous regions to annealing. The observation that 7-day lintnerised wheat and pea starches were less susceptible to anne aling than the corresponding 24-h lintnerised starches supported this view. No changes in chain length distribution were observed between li ntnerised (20 days) native and annealed potato starches and between li ntnerised (7 days) native and annealed pea starches. For all other lin tnerised starches, the distribution profiles showed a higher average d egree of polymerisation (DP), and a slightly higher ratio of singly br anched (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 a mylopectin branch points become more resistant to acid attack. From th e above, it was concluded that the amorphous parts in the granule inde ed play an important role in the molecular mechanism(s) of annealing, but that structural changes within crystalline parts may also take pla ce. (C) 1998 Elsevier Science Ltd. All rights reserved.