Characterization of maize amylose-extender (ae) mutant starches: Part II. Structures and properties of starch residues remaining after enzymatic hydrolysis at boiling-water temperature

GEMS-0067 maize ae-line starches developed by the Germplasm Enhancement of Maize (GEM) project consist of 39.4–43.2% resistant starch (RS), which is larger than the existing ae-line starches of H99ae, OH43ae, B89ae, and B84ae (11.5–19.1%) as reported in part I of the study. The objective of this study was to understand the mechanism of the RS formation in the GEMS-0067 ae-line starch. In the current study, we analyzed the structures and properties of the RS residues that remained after enzymatic hydrolysis of the ae-line starches at 95–100 °C. The RS residues consisted of two major components: large starch molecules of average degrees of polymerization (DP) 840–951 with a few branches and small starch molecules (average DP 59–74) with mostly linear chains. All the RS residues had a semi-crystalline structure with the B-type polymorph and displayed high onset (100.7–107.7 °C), peak (118.6–121.4 °C), and conclusion (139.7–158.8 °C) gelatinization temperatures. After the maize ae-mutant starches were defatted with methanol, the RS contents decreased to 27.8–28.9% for the GEMS-0067 ae-line starches and 9.0–11.0% for the existing ae-line starches. The RS residues were attributed to the presence of long-chain double-helical crystallites derived from amylose and the intermediate component (IC). These crystallites present in native ae-line starches had gelatinization temperatures above 100 °C and maintained the semi-crystalline structures after enzymatic hydrolysis at 95–100 °C.