The structure of four waxy starches related to gelatinization and retrogradation

Abstract

Starches from two waxy rices, a waxy maize, a waxy barley showed onset temperatures (To) of gelatinization in excess of water of 349.2, 340.5, 339.1, and 331.1 K, respectively, as determined by differential scanning calorimetry. The To values, as well as peak (Tp) and final (Tc) transition temperatures, decreased in the same order as their X-ray crystallinity. Annealing the starches increased To and Tp, but the differences remained the same. After lintnerization in 2.2m HCl at 30°, the acid-resistant fractions from the four starches gave the same d.s.c. thermograms and the same constitutive molecular dextrins before and after debranching, as determined by high-performance anion-exchange chromatography with pulsed amperometric detection. Gels (50% water) of the four starches all had the same, but reduced To values after retrogradation. These results indicate that the molecular structure of the microcrystalline region is the same in the four granular starches, and that To is controlled indirectly by the surrounding amorphous regions. The retrogradation of the four waxy starches appeared to be directly proportional to the mole fraction of unit chains with d.p. 14-24 and inversely proportional to the mole fraction of unit chains with d.p. 6-9. © 1992.