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

摘要

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 potata),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:24h at 6℃(facilitating nucleation),followed by 24 h at either 30 or 40℃(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(△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(△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 teh 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 △H,whereas the correlation to the population of chains of DP 8-11 was negative.Indications of a negative correlation between △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 △H.However,the signs of the correlation coefficients were changed.