Initial heating temperature and native lipid affects ordering of amylose during cooling of high-amylose starches

Formation of ordered structures from disordered amylose is practically important. The thermal behavior of high-amylose maize starches was studied during cooling, following heating, and during subsequent reheating. Four commercial high-amylose genotype maize starches with varying amylose contents (ae du, ae su2, and ae [nominally both 50 and 70% amylose]) were heated to either 120, 140, 160, or 180 degrees C, cooled to 5 degrees C, and reheated to 180 degrees C in a differential scanning calorimeter. Each starch was studied with its native lipid, as well as in reduced-lipid and lipid-free form. On cooling of lipid-containing starches, two distinct exotherms were observed and attributed to amylose-lipid complex formation and to amylose chain association. A distinct exotherm at approximate to 75 degrees C was attributed to amylose-lipid complex formation. The exotherm attributed to amylose chain association on cooling varied according to the initial heat treatment, lipid level, and starch type. Starches with higher amylose contents showed larger exotherms on cooling. For initial heat treatments to 120 or 140 degrees C, a broad exotherm beginning at approximate to 95 degrees C was observed on cooling. In contrast, for initial heat treatments to 160 and 180 degrees C, a sharper exotherm with a peak temperature below approximate to 55 degrees C was observed. Upon reheating, samples that had been initially heated to 120 or 140 degrees C showed a peak at >140 degrees C that was attributed to the melting of ordered amylose. Starches initially heated to 160 or 180 degrees C did not show this peak. This work illustrates that initial heating temperature, as well as lipid content and amylose content, all affect amylose chain association during cooling. Thus, this work suggests strategies for controlling ordering of amylose during processing.