LOW ALPHA-AMYLASE STARCH DIGESTIBILITY OF COOKED SORGHUM FLOURS AND THE EFFECT OF PROTEIN

The comparably low starch digestibility of cooked sorghum flours was s tudied with reference to normal maize. Four sorghum cultivars that rep resent different types of endosperm were used. Starch digestibilities of 4% cooked sorghum flour suspensions, measured as reducing sugars li berated following alpha-amylase digestion, were 15-25% lower than for cooked maize flour, but there were no differences among the cooked pur e starches. After the flours were predigested with pepsin to remove so me proteins, the starch digestibility of cooked sorghum flours increas ed 7-14%, while there was only 2% increase in normal maize; however, t here was no effect of pepsin treatment on starch digestibility if the flours were first cooked and then digested. After cooking with reducin g agent, 100 mM sodium metabisulfite, starch digestibility of sorghum flours increased significantly while no significant effect was observe d for maize. Also, starch solubility of sorghum flours at 85 and 100 d egrees C was lower than in maize, and sodium metabisulfite increased s olubility much more in sorghum than in maize. Differential scanning ca lorimetry results of the flour residue after alpha-amylase digestion d id not show any peaks over a temperature range of 20-120 degrees C, in dicating that sorghum starches had all undergone gelatinization. These findings indicate that the protein in cooked sorghum flour pastes pla ys an important role in making a slowly digesting starch.