Study of the Molecular Properties of Amyloses and Amylopectins from Tropical Starches

Abstract

Four kinds of tropical starches were separated into four fractions, i. e., an amylose, two amylopectins, one of which precipitated and the other which did not precipitate in 66% ethanol solution and a C-fraction, which precipitated with thymol and did not precipitate in 10% 1-butanol. Potato and corn starches were used as controls. Iodine affinity, weight average molecular weight (MW), radius of gyration (RG), and intrinsic viscosity [η] were determined for each fraction. To determine MW and RG, one concentration approximation light scattering method was used. For amyloses, iodine affinities, MW, RG, and [η] were 19-21 I2 g/100g, 4 × 105 -2 × 106, 30 × 10-7 -97 × 10-7 cm, and 0.6-1.7 dl/g, respectively. Potato amylose had the highest MW, sago and corn amyloses had the lowest MW. Larger RG values were obtained for edible canna and corn than for others when compared in the same MW. Amylopectins (iodine affinities, 0.25-2 I2 g/100 g ; MW, 4 × 107-44 × 107 ; RG, 150 × 10-7-570 × 10-7 cm ; [η], 0.8-5 dl/g) were classified into three groups. The first one contained potato and edible canna amylopectins having medium MW in the three groups. It was thought that they might have the lowest degree of branching in the three groups. The second group contained arrowroot and cassava amylopectins having the highest MI. It was thought that they might have the highest degree of branching. The third group contained sago and corn amylopectins having the lowest MW. It was thought that they might have medium degree of branching. Six kinds of C-fraction (iodine affinities, 1.5-3.3 I2g/100 g ; MW, 3 × 107-6 × 107 ; RG, 150 × 10-7-270 × 10-7 cm ; [η], 0.8-2 dl/g) had similar Mw to the low Mw fractions in amylopectins, i. e, sago and corn amylopectins.