Chalky and translucent rice grains differ in starch composition and structure and cooking properties

Chalk is an important quality characteristic in rice and occurs most commonly when high temperatures are experienced during grain development. The aims of this report are to determine whether chalk affects cooking quality and to attempt to explain the effects on the basis of starch and protein in chalky and translucent grains. Three cultivars of rice were grown in the glasshouse at either 38/21 degrees C or 26/15 degrees C (day/night temperatures). Rice grown at the higher temperature contained more chalky grains. Grains in the inferior position were more susceptible to forming chalk than were those in the superior position. The presence or absence of chalk affected cooking quality but neither amylose content, amylopectin structure nor protein composition explained the differences in cooking quality. However, the shape, size, and packing of amyloplasts and cells in chalky grains differed from those in translucent grains and might offer an explanation for the differences in cooking quality. It seems likely that the processes involved in the initiation or packing of amyloplasts are susceptible to high temperatures.