The effect of temperature cycling on the amylopectin retrogradation of starches with different amylopectin unit-chain length distribution

The amylopectin retrogradation of six cereal starches (wheat, rye, barley (normal-amylose, high-amylose and waxy), and waxy maize), three potato starches (two normal-amylose, and a high-amylopectin potato), and a single pea starch was studied by differential scanning calorimetry (DSC). The recrystallization of amylopectin was measured after 2 and 4 days of storage. In order to affect the crystallinity of the amylopectin, samples were exposed to different temperature cycles during the storage period, favouring the nucleation or the propagation of the crystallites. The temperature cycles during the first 2 days were: 24 h at 6 degrees C (facilitating nucleation), followed by 24 h at either 30 or 40 degrees C (promoting propagation). For the 4 days storage test the temperature cycles were repeated. Moreover, the amylopectin unit-chain length distribution of the starches was determined by high-performance anion exchange chromatography (HPAEC) after debranching of isolated amylopectins. The recrystallization of amylopectin was greatly affected when gelatinized starch was treated with different time-temperature cycles. The melting enthalpy of recrystallized amylopectin (Delta H) decreased in most cases. The onset temperature of melting of recrystallized amylopectin (T-o) was controlled by the propagation temperature (T-p), and increasing the latter resulted in an increase in T-o. As a result of the increase in T-o, the melting range of the recrystallized amylopectin (Delta T-r) decreased. A development of crystallites that melted at higher temperatures, noticed as an increase in the offset temperature (T-f) of melting, was observed for the cereal starches after all temperature treatments. The possibility of predicting how a specific T-p would affect and increase the T-o of recrystallized amylopectin when gelatinized starch was treated with the two day cycles was demonstrated. The DSC and HPAEC results showed that amylopectin unit-chains with DP 6 and the population of chains with DP 18-19 were positively correlated to Delta H, whereas the correlation to the population of chains of DP 8-11 was negative. Indications of a negative correlation between Delta H and chains of DP 22-34 as well as a positive correlation to chains with DP > 40 were also found. The changes found in T-f correlated to the same distinct amylopectin populations as the previous ones for the relation to Delta H. However, the signs of the correlation coefficients were changed. (C) 2000 Elsevier Science Ltd. All rights reserved.