Milled Rice Breakage Due to Environmental Conditions

Cereal Chem. 75(1):149–152 Milled, long-grain rice was exposed to air at temperatures (T) of 20, 30, and 40°C, and relative humidities (RH) ranging from 25 to 85%. The kernels then were subjected to a breakage test to determine the extent of damage that occurred during each exposure condition. Increasing air T levels produced higher amounts of broken kernels across the RH range. Milled rice at higher moisture content (MC) levels sustained more extensive stress crack damage at low RH conditions and less stress crack damage at high RH conditions relative to milled rice at lower MC levels. Varietal differences were also present, but were overshadowed by MC effects. Rice is a hygroscopic grain that will readily gain or lose moisture when exposed to varying environments. Moisture changes can induce tensile and compressive stresses within the kernel and often lead to stress crack development. A stress crack is a kernel failure that results from the adsorption or desorption of water by the rice kernel. Kunze and Hall (1965) and Kunze and Choudhary (1972) detailed the physics of the stresses and strains developed within the rice kernel due to moisture adsorption and desorption, respectively. Kunze and Hall (1965) proposed that when rice adsorbs moisture, the exterior layers swell, creating compressive forces at the surface. Opposite forces are accordingly produced inside the kernel causing the interior cells to act in tension and pull apart. If the internal tensile strength is exceeded, fissures will form in a straight line perpendicular to the longitudinal axis of the kernel. Kunze and Choudhary (1972) suggested that when the rice kernel loses water, the surface layers contract and tensile stresses are produced. If the tensile strength is exceeded, a pattern of cracks will form over the surface of the kernel. Stermer (1968) developed an equation relating stress crack formation to the change in the equilibrium moisture content (EMC) of milled rice kernels. The change in EMC was induced by changing the surrounding air temperature (T) and relative humidity (RH). Stermer used polarized light to accentuate stress cracks in the milled rice. Then the samples were photographed and the damage was assessed by visually inspecting the pictures. Stermer found that: 1) rice at high MC levels was more likely to crack in low EMC environments than was low MC rice, and 2) the amount and rate of damage was directly related to the magnitude of change in MC that the rice kernel was exposed to. Previous researchers (Stermer 1968, Kunze and Hall 1967) have reported kernel damage by visually inspecting kernels for fissures and surface cracks. However, the development of a stress crack does not necessarily give a measure of the strength reduction of the kernel because some cracked kernels will withstand the forces applied to them while being transported or stored. Henderson (1954) showed that “checked” kernels do not always break when milled. Matthews et al (1970) concluded also that cracked kernels were not always broken during the milling process. A goal of this overall research project was to develop a rapid and repeatable method to quantify the amount of kernel damage resulting from exposure to various environments. To fulfill this need, a mechanism was devised that applied pressure to each individual rice kernel as it passed between two rollers. The extent of damage incurred by kernels exposed to different air conditions was then recorded as the mass percentage of the original sample that was broken by the rollers. This mass percentage is herein referred to as milled rice breakage. The construction details and test performance of this mechanism are given in Siebenmorgen et al (1997). Head rice, defined as “unbroken kernels of rice and broken kernels of rice which are at least three-fourths of an unbroken kernel” (USDA 1983), is generally worth twice as much as broken kernels. Thus, there is a significant financial loss associated with the breakage of milled rice. Much of the prior work related to moisture sorption damage in rice has focused on rough or brown rice, rather than on milled rice. The objective of this research was to quantify the extent of kernel damage incurred by milled rice of various long-grain varieties over a range of MC levels when exposed to air at various T and RH levels. This research extends the earlier work of Stermer (1968) by: 1) using an improved method of quantifying the presence of stress cracks in milled rice kernels; 2) more accurately delineating the stress crack response of milled rice at given MC levels to T and RH levels of the exposure air; 3) using an exposure medium consisting of a flowing stream of air instead of a stagnant environment. MATERIALS AND METHODS Testing Apparatus The experimental procedure consisted of exposing head rice at various MC levels to a stream of air at various T and RH conditions. An apparatus was developed to ensure that the air immediately surrounding individual kernels was not allowed to stagnate during the exposure period. A system was constructed to allow 1 Published with the approval of the Director of the Agricultural Experiment Station, University of Arkansas. Mention of trademark or proprietary products does not constitute a guarantee or warranty by the University of Arkansas and does not imply its approval to the exclusion of other products that may also be suitable. 2 Professor, research assistant, and former research assistant, respectively, Biological and Agricultural Engineering Department, University of Arkansas, Fayetteville, AR 72701. 3 Corresponding author: E-mail: [email protected] Phone: 501/5752841. Fax 501/575-2846. Publication no. C-1998-0108-05R. © 1998 by the American Association of Cereal Chemists, Inc. Fig. 1. Rice exposure apparatus.