REVIEW ON THE SIGNIFICANCE OF STARCH AND PROTEIN TO WHEAT KERNEL HARDNESS

Wheat starch contains large lenticular (A-type) and small spherical (B-type) granules, with some intermediate (underdeveloped A-type), composed of two structurally different polysaccharides, amylose (20-30%) and amylopectin (70-80%), plus small amounts of lipids, nitrogen and phosphorus. Vitreous character is related to air spaces between granules, whereas hardness relates to protein-starch bond strength and protein matrix continuity between granules. Grain hardness is controlled genetically by one or more major genes, modified by one or more minor genes. Environmental factors also exert effects. Most hardness measurements are grouped into categories based on grinding, crushing, abrasion or indentation. Recent methods, viz microscopy, NIR, single kernel cutting, tensile strength and an acoustical technique, seem to have promise. Three basic mechanisms of grain hardness have been postulated: (a) chemically induced adhesion between the protein matrix and starch granule, (b) continuity of the protein matrix, and (c) net charge on the protein. Some recent studies have revealed small amounts of protein as an integral component of the starch granule. The surface granule proteins showed relatively low molecular weight polypeptide bands (5-30 kDa), whereas the integral starch granule proteins had higher molecular weights (59-149 kDa). The 59 kDa protein is postulated to be the enzyme that synthesises amylose. The 15 kDa polypeptide is related to kernel softness, because it is strong in soft wheats, weak or faint in hard wheats, and absent in durum wheats. Reconstituted tablets made from hard wheat starch showed higher tensile strength than those made from soft wheat. Tablets reconstituted from soft wheat starch free from 15 kDa protein exhibited increased tensile strength, suggesting that the 15 kDa protein acts as a 'non-sticking' agent between the starch granules and the protein matrix. Some lipids are associated with amylose and the starch granule surface but their significance is not clear.