Cereal Chem 65:457-462   |  VIEW ARTICLE

Influence of Protein on Starch Gelatinization in Sorghum.

A. Chandrashekar and A. W. Kirleis. Copyright 1988 by the American Association of Cereal Chemists, Inc. 

Nine sorghums selected on the basis of density (as a measure of grain hardness) were decorticated and ground into flour. Kafirin content decreased with density. Alkali and acid gel consistency tests and an enzymatic measure of the degree of starch gelatinization (DG) of the cooked samples were made. Grains containing more kafirin gave higher gel consistency values and lower values for DG. The rate of gel firming during storage was greater for the hard than for the intermediate hardness grain samples. Gels from the soft sorghums firmed to a maximum value within 30 min. Addition of a reducing agent (0.05% 2- mercaptoethanol [2-ME]) before cooking decreased gel consistency values, decreased the rate of gel firming during storage, and increased the DG particularly in the hard sorghums. The effect of 2-ME on gel firming and DG was not as pronounced for the intermediate and soft samples. Scanning electron microscopy of flour particles from a hard and soft grain revealed that starch from the hard grain particles was enclosed in protein and cell wall. Flour particles from the softer sorghum contained some particles that resembled those of the hard sorghum, yet many particles were seen where the cell wall had sloughed off and the starch granules were not tightly enclosed in protein. On cooking, the protein matrix of the hard samples appeared as convoluted sheets with protein bodies buried in the matrix. The matrix of the softer sorghum expanded to a greater degree and showed a more open structure. In the hard grain sample, 2-ME opened up the matrix structure, and the sheets of protein were broken into smaller fragments. We concluded that proteins influence starch gelatinization in sorghum. More kafirin-containing protein bodies were seen in samples with lower capacities for starch gelatinization, and the way in which these protein bodies were organized around the starch granule appeared to act as a barrier to starch gelatinization. Alteration of the protein with a reducing agent increased starch gelatinization.