Low molecular weight soluble starch and its relationship with sorghum couscous stickiness

Soluble carbohydrates from sorghum couscous were extracted with water and s ubjected to low-pressure, size-exclusion chromatograph) (SEC. Differences w ere observed in their amount and composition in couscous produced from diff erent sorghum cultivars. When chromatographed on a high cut-off Sepharose C L-2B column, the soluble carbohydrates eluted at a molecular. weight lower than that of amylose. Two distinct peaks were obtained when the soluble car bohydrates 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 weigh t components that did not complex with iodine. The soluble carbohydrate fra ction was debranched with isomylase and fractionated on rile Bio-Gel P-10 c olumn. A minor fraction (Fraction I), representing long chains, and two maj or fractions, representing intermediate (Fraction II) and short chains (Fra ction III) were obtained, indicating thai the soluble material contained br anched starch molecules. For Fraction I, lambda(max) ranged from 570 to 580 nm which corresponded to DPn of 50-60 glucose units. For Fraction II, lamb da(max) ranged From 510 to 525 nm which was equivalent to DPn 26-30. Fracti on III did not complex iodine and was assumed to Le composed of low molecul ar weight molecules. Couscous texture (stickiness) was highly correlated wi th the amount of HMW: soluble fraction (r = 0.86, p<0.01) and with the prop ortion of debranched Fraction II of DPn 26-30 (r = 0.89, p<0.01). The decre ase in stickiness upon addition of oil was more pronounced in cultivars tha t had high proportions of Fraction II. This decrease in stickiness is likel y to be due to a lipid-starch interaction. (C) 2000 Academic Press.