88C-2 |
Glass transition of rice kernels by dynamic mechanical thermal analysis |
Z. SUN1, W. Yang, and T. J. Siebenmorgen. (1) Department of Food Science, University of Arkansas, 2650 N. Young Ave., Fayetteville, AR 72704 Rice kernel fissuring is one of the major problems facing the rice industry at both farm and processor levels, since fissuring reduces head rice yield that has direct economic consequence. A new theory - glass transition hypothesis - has been proposed recently, with plausible experimental verifications, for explaining fissure formation in rice kernels. A key component to the hypothesis is accurate presentation of glass transition temperature (Tg) of rice kernels. The best method for Tg measurement, as literature suggests, is dynamic mechanical thermal analysis (DMTA). Our objective was to accurately measure the glass transition temperature of rice kernels by DMTA. Temperature range was –40°C to 120°C with a heating rate of 3°C/min. A brown rice kernel was trimmed and polished at both ends to produce a rice cylinder to fit to the sample holder. To reduce moisture evaporation during tests, rice cylinders were covered with a thin layer of aluminum foil on the circumferential surface. When temperature was below the Tg, storage modulus, E’, changed little because of slow moisture evaporation. When temperature passed Tg, E’ started to drop sharply. The curve of E”/E’ vs. T showed two transitions: a weak peak around 15°C and a broad peak that was related to the glass transition and water evaporation. The relation of E”/E’ vs. T was little affected by moisture evaporation, although the relation of E’ vs. T was affected pronouncedly by moisture evaporation. A method has been developed for determining Tg based on the E”/E’ ~ T curves. The glass transition temperature of rice kernels decreased with the increased moisture content. The Tg measured by DMTA was slightly higher than that by a TMA, but both Tg values were in the same magnitude. The Tg measured by DMTA will facilitate the application of the glass transition hypothesis to the rice drying process to better understand rice fissuring mechanism and subsequently help find ways to design better dryers to improve rice milling quality.
Session 88C, Food Engineering: Physical and Chemical Properties
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