Starch from hull-less barley: V. In-vitro susceptibility of waxy, normal, and high-amylose starches towards hydrolysis by alpha-amylases and amyloglucosidase

Waxy (CDC Alamo), normal (CDC Dawn), and high-amylose (SB 94893) hull-less barley (HB) starches were isolated and their susceptibilities to porcine pancreatic α-amylase (PPA), Bacillus species α-amylase (BAA), and Aspergillus niger amyloglucosidase (AAG) were determined. Scanning and transmission electron micrographs showed that the patterns of enzyme hydrolysis (surface erosion, endoerosion, and the erosion at the equatorial groove plane) were dependent on the enzyme source and starch types. The outer layers of normal and high-amylose HB starch granules were more resistant to enzyme hydrolysis. The hydrolysis rate of waxy HB starch by the three amylases at 37 °C was significantly higher than those of normal and high-amylose HB starches. The degree of hydrolysis by PPA, after 72 h, reached 91–97% in all three starches. However, BAA and AAG showed significantly lower degrees of hydrolysis in normal (<30%) and high-amylose (<37%) HB starches than in waxy HB starch (>67%). Glucose, maltose, and maltotriose contents of the hydrolysates, collected at different time intervals, were determined by HPLC. The concentration and composition of soluble sugars in hydrolysates varied with enzyme source, starch types, and condition of hydrolysis. The study suggested that both morphological and ultrastructural features influence in-vitro hydrolysis and the ultrastructure of waxy HB starch may be more open than those of normal and high-amylose HB starches.