Low Molecular Weight Soluble Starch and its Relationship with Sorghum Couscous Stickiness

Soluble carbohydrates from sorghum couscous were extracted with water and subjected to low-pressure, size-exclusion chromatography (SEC). Differences were observed in their amount and composition in couscous produced from different sorghum cultivars. When chromatographed on a high cut-off Sepharose CL-2B column, the soluble carbohydrates eluted at a molecular weight lower than that of amylose. Two distinct peaks were obtained when the soluble carbohydrates were chromatographed on a low cut-off SEC (Bio-Gel P-10) column. Peak I was a high molecular weight (HMW) component which complexed iodine with maximum absorption at 540 nm. Peak II consisted of low molecular weight components that did not complex with iodine. The soluble carbohydrate fraction was debranched with isoamylase and fractionated on the Bio-Gel P-10 column. A minor fraction (Fraction I), representing long chains, and two major fractions representing intermediate (Fraction II) and short chains (Fraction III) were obtained, indicating that the soluble material contained branched starch molecules. For Fraction I, λmaxranged from 570 to 580 nm which corresponded to DPnof 50–60 glucose units. For Fraction II, λmaxranged from 510 to 525 nm which was equivalent to DPn26–30. Fraction III did not complex iodine and was assumed to be composed of low molecular weight molecules. Couscous texture (stickiness) was highly correlated with the amount of HMW soluble fraction (r=0·86, p