Starch granule surface proteins

In the high water content of the immature kernel, proteins and lipids compete for the starch granule surface. The granule surface would be expected to be less polar (more hydrophobic) than the aqueous medium. Puroindolines are hydrophobic proteins and would therefore tend to adsorb at the starch granule surface. By the mass action law, their amounts would be greatest in the wild phenotype (i.e., with both puroindolines present). 

The exact quantity of protein and lipid present depends on the species and variety of starch however, typical well washed cereal starch samples contain —0.3% protein and up to 1.0% lipid, while a typical root or tuber starch may contain —0.05% protein and 0.05-0.1% lipid.63 These values result from bulk analysis of starch in reality, the actual distribution of the components is not uniform, with both protein and lipid enrichment believed to be present towards and at the granule surface.58,64,65... 

Starch changes during cooking of pasta are reported to vary from a hydration-driven gelatinization process in the outer layer to a heat-induced crystallite melting in the center.525 It is speculated that both the state of the starch and the surface structure contribute to the development of the elastic texture and stickiness of pasta. Interactions between starch and the surrounding protein matrix are evident in the outer and intermediate layer. In the center of cooked pasta, wheat starch granules retain their shape due to limited water diffusion, and the protein network remains dense. 

Greenwell, P., Evers, A. D., Gough, B. M., and Russell, P. L. 1985. Amyloglucose-catalysed erosion of native, surface-modified and chlorine-treated wheat starch granules The influence of surface protein. J. Cereal Sci. 3, 279-293. 

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