A. G. Cnossen

The glass transition temperature concept in rice drying and tempering: effect on milling quality.

Research on rice drying and tempering has shown that high drying temperatures (up to 603C) and high moisture removal rates (up to 6 percentage points moisture content) can be used without reducing milling quality as long as sufficient tempering at a temperature above the glass transition temperature (Tg) is allowed between drying passes. Using drying air temperatures above the Tg of the rice significantly reduces drying and tempering durations since kernel moisture diffusivity is much higher above Tg. Understanding the effects of glass transition is important in optimizing the drying and tempering processes in terms of overall required drying durations to achieve given moisture removals. The objective of this study was to investigate the effect of Tg on drying rates when using drying air temperatures above and below Tg. Both medium–grain and long–grain rice was harvested during 2000 and dried under various air conditions above and below the Tg of the rice. Results showed that rice dried significantly faster above Tg than below Tg. In addition, high temperature/low relative humidity drying air conditions, which result in a low equilibrium moisture content, apparently caused the surface of the kernel to transition from a rubbery to a glassy state and reduced the drying rate.

Rice fissuring response to high drying and tempering temperatures

The objective of this study was to determine the effect of drying and tempering treatments on rice kernel fissuring at temperatures above and below the glass transition temperature (Tg). This information was correlated with head rice yield (HRY) data to determine optimum drying and tempering strategies to maximize milling quality and kernel physical integrity. Samples were dried under three different drying air conditions for various durations and then tempered for various durations at the temperature of the drying air. Results showed that the percentage of fissured kernels decreased with increasing tempering duration for typical drying durations (i.e., 3–5% points moisture content reduction in one pass). Some samples still had many fissured kernels after extended tempering, yet had a high HRY, equivalent to the control sample. This indicates that the tempering duration required for preventing kernel fissuring might be longer than the tempering duration required for maintaining a high HRY. 2003 Elsevier Science Ltd. All rights reserved.

INTRA-KERNEL MOISTURE RESPONSES OF RICE TO DRYING AND TEMPERING TREATMENTS BY FINITE ELEMENT SIMULATION

Moisture content patterns inside rice kernels are important in understanding rice fissure formation, especially when glass transition effects are considered. Unsteady and non–linear partial differential equations were employed to describe two–dimensional temperature and moisture distributions within a single rice kernel during drying and tempering processes. Moisture content gradients (MCGs) inside the kernel were examined. Results showed that the maximum MCG (MMCG) appeared in the direction of the short axis. During the tempering process, moisture content on the kernel surface had a much faster and greater change than that at the kernel center. The intra–kernel MCG decreased considerably during the first 40 min of tempering, after which it decreased slowly to approach zero. A duration of about 40 min of tempering at 60³C helped eliminate about 90% of the MCGs created inside the rice kernel during drying, and this simulation result correlated favorably with the published tempering data in the literature. The findings from this study provided useful information for determining optimal drying and tempering conditions of rice to enhance its milling quality.

Relationship of Kernel Moisture Content Gradients and Glass Transition Temperatures to Head Rice Yield

The relationship of glass transition temperature Tg and moisture content (MC) gradient of rice kernels to head rice yield (HRY) variation was investigated. Mathematical models describing heat and moisture transfer inside rice kernels during drying were developed and solved using the finite element method. Moisture distributions inside a kernel were simulated and verified using thin-layer drying experiments, and the intra-kernel MC gradients during drying were accordingly determined and analysed. Results showed that in the glassy region, rice did not incur measurable HRY reduction after drying. However, when rice was dried in the rubbery region and then cooled down immediately without being tempered following drying, HRY decreased markedly after MC gradients exceeded certain levels. It was found in this study that the time when the percentage point of moisture removal reached a maximally allowable level before HRY decreased dramatically coincided with the time at which the curve of kernel MC gradients versus drying duration reached its peak. Such a relation was verified with the HRY data of two varieties (Cypress and M202) as measured in this study and cited from literature. The HRY trends for these two varieties were well explained through the behaviour of glass transition and MC gradients of rice.

AN APPLICATION OF GLASS TRANSITION TEMPERATURE TO EXPLAIN RICE KERNEL FISSURE OCCURRENCE DURING THE DRYING PROCESS

Fissure formation during rice drying is a major cause of rice milling quality reduction. This work has applied principles of polymer science in studying thermal and hygroscopic properties of rice kernels, particularly the glass transition temperature (Tg ). This data was used to develop a hypothesis that explains the occurrence of rice kernel fissuring as a result of drying. The drying process was mapped onto a state diagram to illustrate the changes in state that a kernel could incur through drying and tempering operations. An experiment was designed to validate the hypothesis in which the effect of the Tg on rice drying and tempering in terms of milling quality was determined. Results showed that drying air temperatures up to 60°C and high moisture removal rates could be used without reducing the milling quality, as long as sufficient tempering was allowed at a temperature above the Tg of the rice. *Published with the approval of the Director, Agricultural Experiment Station, University of Arkansas. Mention of a commercial name does not imply endorsement by the University of Arkansas.

Thermomechanical Transitions of Rice Kernels

Cereal Chem. 79(3):349–353 Thermomechanical analysis (TMA) and differential scanning calorimetry (DSC) were used to investigate the thermal transitions of long-grain rice kernels. Three distinct thermomechanical transitions were identified as rice kernels were heated from 0 to 200°C. The identified transitions were a low temperature transition with onset at ≈45°C, an intermediate temperature transition at ≈80°C, and a high temperature transition at ≈180°C. Low temperature transition with onset from ≈60°C at 5% moisture content (MC) to 30°C at 20% MC was identified as the glass transition of the rice kernels. Intermediate temperature transition from 60 to 100°C, depending on MC, may be caused by rapid evaporation of moisture in the rice kernels. High temperature transition was associated with melting of the crystalline structure of rice starch. The temperatures of all three transitions decreased as MC increased, confirming that moisture acted as a plasticizer in rice kernels.

A STUDY OF RICE FISSURING BY FINITE–ELEMENT SIMULATION OF INTERNAL STRESSES COMBINED WITH HIGH–SPEED MICROSCOPY IMAGING OF FISSURE APPEARANCE

Finite–element analysis was performed to simulate stress distributions inside a rice kernel during drying. The distributions of radial, axial, tangential, and shear stresses were mapped and analyzed. It was found that during drying, two distinct stress zones existed inside a rice kernel: a tensile zone near the surface, and a compressive zone close to the center. Although as drying proceeded, radial, tangential, shear, and axial stresses all decreased in magnitude after they peaked, the first three (i.e., radial, tangential, and shear) stresses approached zero in magnitude and became neutral (i.e., neither tensile nor compressive) after 60 min of drying at 60 ³ C, 17% relative humidity (RH). Only axial stress remained at a pronounced level even after 60 min of drying at 60 ³ C, 17% RH, which helps explain why most fissures form perpendicular to the longitudinal axis of rice kernels. The results were well supported by the fissure appearance caught in this study with high–speed microscopy imaging and by other evidences on rice fissuring published in the literature.

DEVELOPMENT OF COMPUTER SIMULATION SOFTWARE FOR SINGLE GRAIN KERNEL DRYING, TEMPERING, AND STRESS ANALYSIS

The quality traits of a single grain kernel can produce an index of the overall quality of the bulk grain, as individual kernels comprise a grain bulk. Therefore, single–kernel drying behavior is important in understanding the overall quality of grain. A Matlab based software package with C++ compiler was developed in this study for describing single–kernel drying, tempering, and internal stress analysis. Non–linear and transient partial differential heat and mass transfer equations describing the single kernel drying and tempering processes were solved using the finite element method. Temperature– and moisture–dependent viscoelastic properties were applied to the stress analysis of a single rice kernel. A graphical user interface and 2D/3D graphics for temperature, moisture content, and stress changes were developed. This software package is a useful tool for engineers, operators, and educators to predict temperature, moisture content, and internal stress distributions inside a single kernel.

Development of Computer Simulation Software for Single Grain Kernel Drying, Tempering and Stress Analysis

The quality traits of a single grain kernel can give index to the overall quality of a grain bulk, because it is individual kernels that comprise a grain bulk. Therefore, the single- kernel drying behavior is important to understand the overall quality of grain. A Matlab and Visual C++ based software package was developed in this study for describing single-kernel drying, tempering and internal stress analysis. Non-linear and transient partial differential heat and mass transfer equations describing the single kernel drying and tempering processes were solved using the finite element method. Temperature- and moisture-dependent viscoelastic properties were applied to the stress analysis of a single rice kernel. A graphical user interface, and 2D/3D graphics for temperature, moisture content and stress changes were developed. The simulation results were validated by thin-layer drying data and subsequent tempering experiments of rough rice. This simulation tool can be used for predicting temperature, moisture content and internal stress distribution in a single kernel.

USING DYNAMIC SHRINKAGE TESTS TO STUDY FISSURE PROPAGATION IN RICE KERNELS

Dynamic shrinkage behavior of rice kernels during drying was studied with a thermomechanical analyzer (TMA). Shrinkage kinetics data were obtained for brown rice kernels heated/dried in a TMA chamber. Seventeen rice kernels of long–grain variety Drew were tested for length change, and another seventeen kernels of the same variety were tested for thickness change. Each kernel was initially heated from room temperature to 59.7³C at a rate of 15³C/min and then maintained at 59.7³C to dry for 400 min. The shrinkage behavior of rice kernels could be described by an exponential decay relationship. Rice kernels did not shrink uniformly in thickness and length during the drying process but shrank a greater percentage in thickness than in length. Because of the non–uniform shrinkage, a tensile force along the direction of the longitudinal axis is speculated to exist during drying. This result may help explain why, with a small number of exceptions, fissures observed in rice kernels are perpendicular to the longitudinal axis of the kernel.