Glass Transition Temperature of Rice Kernels Determined by Dynamic Mechanical Thermal Analysis

Abstract

A protocol was developed to measure the glass transition temperature (T g) of brown rice kernels using dynamic mechanical thermal analysis (DMTA). In order to accommodate the sample requirements of the dynamic mechanical thermal analyzer and prevent moisture loss during experiments, kernels were first wrapped with a thin layer of parafilm and then a thin layer of aluminum foil. The wrapped kernels were then filed at both ends with fine sandpaper to produce 3 mm long rice cylinders with parallel ends. The dynamic mechanical properties of brown rice kernels were determined over a temperature range of −40°C to 120°C. During each DMTA test, the onset temperature of glass transition was measured as the T g of a brown rice kernel, since moisture diffusion and other physical properties begin to change drastically above the onset temperature. No dramatic changes in both the storage modulus (E4) and the loss modulus (E,) of brown rice kernels were found around T g , indicating that relaxation of the amorphous phase of the rice kernel was quite limited. However, E, / E4 (tand) showed a much better transition for brown rice kernels, and thus the E, / E4 vs. temperature curves were used to determine T g values of rice kernels. It was found that the T g values of rice kernels decreased linearly with increases in moisture content (MC) within the tested range. The T g of rice kernels was not affected by variety.

A Polymer Science Approach to Physico-Chemical Characterization and Processing of Pulse Seeds

“…The concept of the glass transition (Tg) has been used to explain rice kernel fissure/crack formation during drying and subsequent breakage during milling (Siebenmorgen et al, 2004). At temperatures below Tg, grain seeds exist in a glassy solid state, starch granules are compact, water is relatively immobile and diffusion of moisture inside a grain seed is slow while at temperatures above Tg, starch exists in a rubbery state, free volume is increased, the water associated with starch has greater mobility, and the diffusion of moisture is enhanced (Siebenmorgen et al, 2004).…”

“…At temperatures below Tg, grain seeds exist in a glassy solid state, starch granules are compact, water is relatively immobile and diffusion of moisture inside a grain seed is slow while at temperatures above Tg, starch exists in a rubbery state, free volume is increased, the water associated with starch has greater mobility, and the diffusion of moisture is enhanced (Siebenmorgen et al, 2004). During rice drying, the temperature will increase and moisture will diffuse from the grain seed and a temperature and moisture content gradient will develop from the surface to the center of the grain seed (Cnosssen et al, 2003).…”

“…If a sufficient moisture content gradient exists when the tempering/equilibrating environment is one that produces a change of state of the starch (transitioning from the rubbery to the glassy state), the different sections of the grain seed (surface, midpoint, centre), resulting from the moisture content gradient, pass through the Tg at different moisture content values, which is depicted by situation B of Figure 7 (Cnossen & Siebenmorgen, 2000;Cnossen et al, 2003). Due to large differences in kernel properties between the rubbery and the glassy states, specifically the thermal expansion coefficients (Perdon et al, 2000;Cnossen et al, 2003), differential stresses within the grain seed will likely cause fissuring/cracking (Siebenmorgen et al, 2004). If the tempering/equilibrating air temperature is above Tg, then the moisture content will equilibrate between the different sections of the kernel (surface, midpoint, centre) and the moisture content gradient will cease to exist.…”

“…A large amount of work has been performed in the area of understanding the effect of drying temperature on rice milling quality (Cao et al, 2004;Cnossen et al, 2003;Cnossen & Siebenmorgen, 2000;Iguaz et al, 2006). The concept of the glass transition has been used to explain rice kernel fissure formation during drying and subsequent breakage during milling of rice (Siebenmorgen et al, 2004). The effect of drying temperature on the handling quality of whole green lentil seeds in terms of seed breakage upon handling has been investigated (Tang et al, 1990), however little work has been done on the effect of drying temperature on the milling quality of red lentils.…”

Comparison of mathematical models to predict glass transition temperature of rice (cultivar IRGA 424) measured by dynamic mechanical analysis

“…Sun, Yang, Siebenmorgen, Stelwagen, and Cnossen (2002) used thermomechanical analysis (TMA) and differential scanning calorimetry (DSC) to investigate the thermal transitions in Drew (long‐grain) rice kernels. Siebenmorgen, Yang, and Sun (2004) measured Tg of Drew and Bengal rice samples using DMA. Thuc, Fukai, Truong, and Bhandari (2010) applied a thermal mechanical compression test (TMCT) to measure the Tg of rice flour and individual rice kernels of YRM64 rice, an Australian genotype.…”