Isolation, structure, and characterization of the putative soluble amyloses from potato, wheat, and rice starches

Amylose, a putative linear α-(1→4)-glucan and a component of most starches, was isolated from potato, rice, and wheat starches by forming the 1-butanol complex in a solution of the starches. It previously had been found that these amyloses were incompletely hydrolyzed by β-amylase, indicating that it was partially branched. Solubilization of the butanol complex in water and steam distillation of the 1-butanol, followed by cooling to 4 °C gave precipitation of the double helical, linear, retrograded amylose over a 15 h period, leaving the soluble amylose in solution. The soluble amyloses were precipitated with two volumes of ethanol, and the precipitate was solubilized and reprecipitated to remove traces of linear amylose. The precipitated, soluble amyloses, were partially branched and had properties intermediate between linear amylose and amylopectin. The water solubility of the potato amylose was 10.52 mg/mL, with a number-average degree of polymerization (DPn) of 8440 and 2.1% branch linkages that had a DPn of 48; the water solubility of the rice amylose was 8.83 mg/mL, with a DPn of 2911 and 1.4% branch linkages that had a DPn of 72; and the water solubility of wheat amylose was 6.33 mg/mL, with a DPn of 1160 and 1.6% branch linkages that had a DPn of 64. The three soluble amyloses have structures and properties intermediate between the nearly water insoluble (⩽1 mg/mL), linear amylose, and the highly water-soluble, 4–5% branched, amylopectin.