Environmental Conditions Causing Milled Rice Kernel Breakage in Medium-Grain Varieties

Milled rice kernels have been shown to rapidly fissure and eventually break when exposed to certain air conditions (Stermer 1968, Kunze and Hall 1967, Siebenmorgen et al 1998). Milled kernels rapidly gain or lose moisture from the environment depending on the air temperature and relative humidity (RH) of the surrounding air, as well as the moisture content (MC) of the kernels (Kunze and Choudhary 1972, Lu et al 1993). This moisture migration into or out of the kernel causes tensile or compressive stresses to occur in the starchy endosperm of the milled kernel (Stermer 1968). Depending on the moisture gradient between the kernel and the equilibrium MC of the surrounding air, these stresses can cause kernels to fissure during postmilling operations. This, in turn, can lead to kernel breakage and a significant reduction in head rice yield (HRY) (Autrey et al 1955). Past research (Siebenmorgen et al 1998) identified critical parameters causing milled rice kernel breakage in long-grain varieties. A test apparatus and an experimental procedure were developed that quantified kernel breakage as a function of air temperature and RH, kernel MC, exposure duration, and kernel temperature in long-grain varieties. As a continuation of identifying factors that lead to milled rice breakage, the relative fissuring response of two medium-grain varieties with a range of MC exposed to various air conditions representing milling environments were determined.