Physicochemical properties and molecular structures of starches from millet (Pennisetum typhoides) and sorghum (Sorghum bicolor L. Moench) cultivars in Nigeria

Some physicochemical properties and molecular structures of starches from millet (Pennisetum typhoides, Doro and Gero) and sorghum (Sorghum bicolor, red and white) in Nigeria were examined. Starch granules of millet and sorghum were 3-14 mum and 4-26 mum in diameter, respectively. Millet cultivars had similar peak viscosities (204-205 RVU) on pasting, while sorghum showed similar minimum viscosities (155-156 RVU). The actual amylose content (%) calculated from iodine affinity (IA, g/100 g) was 20.1 and 21.4 for sorghum and 21.3 for millet. The IA of amylopectin was high (1.27-1.42) and its average chain lengths were 20-21 with beta-amylolysis limit of 56%. Amylopectins showed a polymodal molecular weight distribution on a molar basis. The distributions differed among the samples with a higher amount of larger molecules in Doro and red sorghum. Weight- and molar-based distributions of debranched amylopectins on HPSEC were polymodal with weight-based distribution showing presence of long chains. Peak DP values for A+B-1 and B-2+B-3 chain fractions were 13-16 and 42-43, respectively. The (A+B-1)/(B-2+B-3) ratio on molar basis (9.0-11.5) was similar to maize and rice amylopectins. Peak DP on molar-based distribution for white sorghum and millet amyloses were similar (490-540) and the DPn range was narrow (1,0601,300), but weight-based distribution profiles differed. The average chain lengths were 260-270 with 3.9-4.8 chains per molecule.