PHYSICAL PROPERTIES OF RICE RELATED TO DRYING THE GRAIN

Abstract

Stresses in the rice grain can be produced by mechanical equipment such as the combine, elevator, auger and the rice mill. There are, however, other and more insidious means of producing severe stresses in the grain with moisture and thermal gradients. Moisture gradients can be found 1) in the field on a humid night before harvest, 2) in a hopper of freshly harvested rice containing high-, low- and intermediate-moisture grains, and 3) in certain types of dryers ahead of the drying front. A single high humidity exposure of rough rice at storage moisture can be very detrimental to the quality of the grain. Other steep moisture gradients can be produced by rapidly drying high-moisture rice, With time, after drying, the gradients cause moisture to diffuse from the center to the surface thus causing 1) the moisture gradient to decrease, 2) the grain surface to gain moisture and expand, 3) the grain interior to lose moisture and contract, and 4) the grain to fissure several hours after it has been dried. Fissured rice will usually break when it is milled.