Relative Humidity Effects on the Development of Fissures in Rice

Cereal Chem. 73(2):222-224 Rice grains may develop fissures due to internal stresses when sub- all of the milled rice fissuring at 100% exposure humidity. The CPFG jected to moisture-adsorbing environments. Rough, brown, and milled decreased sharply as the initial moisture level increased, with no grains rice grains were conditioned to three equilibrium relative humidities developing fissures when rough rice at 80% erh was exposed to 100% (erh) of 46, 62, and 80%, before being exposed to high relative humidity rh. The cumulative number of fissures (CNF) in rice grains increased (rh) environments of 65, 86, and 100% for different periods. Grains with exposure rh and exposure, time. The regression equations to equilibrated to 46 and 62% erh fissured when exposed to either 86 or describe CPFG and CNF related to erh, exposure rh, and time were de100% rh. For rice grains at 46% erh, the cumulative percentage of fis- veloped from the SAS Statistical program. sured grains (CPFG) increased as the exposure humidity increased, with The rice grain is hygroscopic and responds dynamically and physically to moisture and temperature changes in the environment. A dry grain surface adsorbs moisture in a humid environment, while a wet surface desorbs moisture in a relatively dry environment. Moisture adsorption is associated with water reentering the grain. This occurs when the vapor pressure at the surface of a grain is lower than the vapor pressure in the surrounding air. Kondo and Okamura (1930) were the first to show that a moisture-adsorbing environment caused low moisture grains to fissure. Moisture adsorption can occur in the field, in the holding bin of a combine, ahead of the drying front in a deep-bed dryer, or wherever low moisture grains are exposed to a humid environment (Kunze and Prasad 1978). Kunze and Hall (1965) observed the development of fissures when brown rice grains, originally at storage moisture, were exposed to a more humid environment. An increase in rh of 20 percentage points or greater above the conditions for grains in moisture equilibrium was sufficient to initiate fissures. Kunze and Hall (1967) showed that the thermal gradients produced by a temperature difference of 34.4 0 C did not produce fissures in grains as long as the rice was maintained at a constant moisture content. Stermer (1968) related stress cracks in milled rice to the changes in moisture content. He developed an exponential function that described the relationship between the rate of stresscrack damage and the change in equilibrium moisture content of milled rice due to changes in temperature and rh.