The Formation of Resistant Starch during Acid Hydrolysis of High-amylose Corn Starch

Abstract

Four types of corn starch (waxy corn starch (WC), normal corn starch (NC), high-amylose corn starch class 5 (HAC class 5) and high-amylose corn starch class 7 (HAC class 7)) were hydrolyzed with 1.5% hydrochloric acid and the resistant starch (RS) content was measured. The acid-hydrolyzed HAC class 5 and class 7 show significantly higher RS content. The change in RS content, X-ray crystallinity, molecular size distribution, thermal property and appearance of HAC class 7 after up to 100 h acid hydrolysis were analyzed. The RS content increased to 69.3 from 40.5% at 16 h and then decreased gradually, while crystallinity continued to increase during acid hydrolysis. The fractionation profiles indicated that with the decrease of the amylose fraction correlating well with the increase in RS content. Granules hydrolyzed for 24 h retained their natural shape, whereas after 100 h hydrolysis granules were damaged. The enzyme-digested residues of native and acid-hydrolyzed HAC class 7 were recovered. The crystalline regions of HAC class 7 acid-hydrolyzed for 24 h were only slightly digested by enzymatic treatment, whereas native and HAC class 7 acid-hydrolyzed for 100 h were extensively digested. Thermal analysis showed the transition peak in high temperature region increased with acid hydrolysis and this peak was also shown in enzyme-digested residues. Acid hydrolysis may influence RS content by two mechanisms: (1) moderate acid hydrolysis increases the crystalline regions and thus resistance to enzymatic digestion of the hydrolyzed granules and (2) excess acid hydrolysis damages granular structure and decreases resistance to enzymatic digestion.