Relating Rice Milling Quality Changes During Adsorption to Individual Kernel Moisture Content Distribution

Cereal Chem. 75(1):129–136 Several varieties of rough rice that were either stored for an extended period of time or freshly harvested were conditioned to initial moisture contents ranging from 10 to 17%. After the individual kernel moisture content distributions were measured, the samples were soaked in water at temperatures ranging from 10 to 40°C. The samples were then dried and milled. The bulk critical moisture content, at which head rice yield began to decline due to moisture adsorption, ranged from 12.5 to 14.9%, depending on the variety, harvest moisture content, and storage conditions. The kernel critical moisture content, determined from each sample from the cumulative kernel moisture content frequency distribution, increased with increasing sample initial moisture content. Rice quality is determined by several grading factors including head rice yield (HRY). Head rice is defined as milled rice including kernels three-fourths or more of the original kernel length (USDA 1979). The HRY is the mass fraction of rough rice that remains as head rice after complete milling. Because broken kernels have a considerably lower commercial value than that of head rice, a primary goal of the rice industry is to maximize HRY. A primary cause of HRY reduction is the development of kernel fissures resulting from stresses induced by moisture sorption. Kernels with fissures will generally break apart during milling, causing reductions in HRY. Past research has addressed this overall issue by investigating fissure formation in individual kernels, as well as by directly measuring the effects of moisture adsorption on HRY reduction. Fissuring Caused by Moisture Adsorption Previous research has shown that rapid moisture adsorption leads to kernel fissuring and subsequent breakage during milling. Kunze and Choudhary (1972) showed that when kernels are suddenly exposed to a step increase in relative humidity (RH), moisture is adsorbed at their surface. This results in a swelling of the cells in the surface layers, producing compressive stresses that are balanced by tensile stresses in the inner portion of the kernel. If the compressive stresses in the surface layers develop to the extent that the resulting tensile stresses exceed the tensile strength of the central portion of the kernel, fissuring occurs. Kunze (1977) concluded that when low moisture content (MC) rice kernels in preharvest and postharvest conditions are subjected to environments causing moisture adsorption, the kernels develop internal fissures perpendicular to their long axis. Kondo and Okamura (1930) found that field rewetting increased the number of cracked kernels, and that the percentage of cracks increased with the duration of exposure to moisture. Stahel (1935) showed that the percentage of whole grains in field rice begin to decline at an average field MC of 17–20% (all moisture contents, unless otherwise stated, are expressed on a wet weight basis) and that moisture adsorption produces fissures below the critical point of 14%. Breese (1955) reiterated Stahel’s statements in his study of hysteresis in hygroscopic equilibria of rough rice. Srinivas et al (1978) reported that fissuring of long-grain rough rice during soaking was related to the initial moisture contents (IMC) and soaking water temperatures and durations in four longgrain rice varieties: Halubbulu, Kaddi Bhatha, IR 20, and Madhu. The water temperatures in the study ranged from 5 to 85°C. An increase in the soaking water temperature generally produced higher fissuring rates and greater numbers of fissures, although sufficiently high temperatures induced gelatinization and apparent fissure healing. At 5°C, fissuring occurred after 45 min, while at 80°C, maximum fissuring occurred within 15 min. Effects of Moisture Adsorption on HRY Siebenmorgen and Jindal (1986) quantified the effects of moisture adsorption directly on head rice yield reductions (HRYR) for long-grain rough rice. All adsorptive conditions, including high RH air, soaking, and mixing with high MC rice, resulted in significant HRYR when the IMC was <13%. Remoistening of rice at MC >16% had no discernible effect on HRY. Chen and Kunze (1983) exposed rough rice samples at MC levels of 8.6 and 10.7% to RH environments of 64, 72, 82, and 92%, and temperatures of 20 and 30°C. Exposure duration, IMC, and RH were significant in reducing HRY. Banaszek and Siebenmorgen (1990) developed an equation to predict HRYR for rough rice exposed to moisture adsorptive conditions. They found that RH and IMC are significantly related to the HRYR caused by moisture adsorption. Temperature was of minimal importance under the adsorptive conditions used. Variation of Individual Kernel MC Chau and Kunze (1982) found significant variability in individual kernel MC levels in the field. They concluded that the longer rice is left in the field, the greater the probability that the kernels with lower MC will fissure before harvest. Kocher et al (1990) observed that individual kernel MC distributions showed two to three modes on early harvest dates, but that the high MC modes gradually diminished as time progressed. They concluded that this field variability was the primary reason that individual rice kernels respond differently to both field and postharvest operations. Kunze and Prasad (1978) showed that there was a wide span of individual kernel MC levels within a mass of freshly harvested rice. Wadsworth et al (1982) found that the MC levels of thickerkernel fractions, which included 90% of the weight of their samples, were not significantly different, but the MC levels of the remaining thinner kernel fractions were significantly higher than the bulk rice and increased with decreasing thickness. Siebenmorgen et al (1990) showed that there was considerable variation in kernel MC levels within a bulk, even after three months of equilibration. They also reported greater kernel MC variability at higher average MC levels. 1 Published with the approval of the Director of the Agricultural Experiment Station, University of Arkansas, Fayetteville, AR. 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 graduate assistant, respectively, Biological and Agricultural Engineering Department, University of Arkansas, Fayetteville, AR. 3 Corresponding author. E-mail: [email protected] 4 Associate Professor, Agricultural Statistics Department, University of Arkansas,