T. J. Siebenmorgen

Effects of commercial processing on antioxidants in rice bran

ABSTRACT Rice bran contains high amounts of beneficial antioxidants including tocopherols, tocotrienols, and oryzanols. Current rice milling technology produces rice bran from different layers of the kernel caryopsis. Under current practices, these layers are combined and then steam-extruded to form a stabilized rice bran pellet that is storage-safe prior to oil extraction. Each of these rice bran intermediates can vary in antioxidant content. The objective of this study was to investigate the changes in selected antioxidants in rice bran from both long- and medium-grain rice during commercial milling and bran processing. Rice bran collected from various milling breaks of a commercial system had varying antioxidant levels. Bran collected after milling break 2 had the highest levels of tocopherol and tocotrienol. Oryzanol concentration was significantly higher in outer bran layers. Results also indicate that the long-grain rice bran averaged ≈15% more antioxidants than the medium-grain rice bran.

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.

Rice Milling Quality, Grain Dimensions, and Starch Branching as Affected by High Night Temperatures

ABSTRACT Important rice grain quality characteristics such as percentage of chalky rice kernels are affected by both high and low night temperatures and by different day and day/night temperature combinations. High nighttime temperatures have also been suspected of reducing rice milling quality including head rice yields. Experiments to confirm or refute this have not been reported. A controlled climate experiment was conducted. Conditions in the chambers were identical except between 2400 and 0500 hours (midnight and 5 am). For those times, two temperature treatments were imposed: 1) 18°C (low temperature treatment) and 2) 24°C (high temperature treatment). Two cultivars were tested: LaGrue and Cypress. The high temperature treatment reduced head rice yields compared with the low temperature treatment. Grain widths were reduced for the high temperature treatment compared with the low temperature treatment. There was no effect of temperature on grain length or thickness. Amylopectin chain lengths 13–24 we...

Effects of Oven Drying Temperature and Drying Time on Rough Rice Moisture Content Determination

ABSTRACT THE effects of oven drying temperature and drying time on whole-kernel, long-grain rough rice moisture content determination were investigated for different moisture content levels ranging from approximately 9 to 22% (w.b.). The results showed that a simplified oven method can be used for rapid moisture measurement with accuracy comparable to that of a standard Association of Official Analytical Chemists (AOAC) method. An equation was developed that relates the apparent moisture content determined using a given drying time and temperature to the moisture content determined by the standard AOAC method.

Effects of Nighttime Temperature During Kernel Development on Rice Physicochemical Properties

ABSTRACT Rice quality can vary inexplicably from one lot to another and from year to year. One cause could be the variable temperatures experienced during the nighttime hours of rice kernel development. During the fall of 2004, a controlled temperature study was conducted using large growth chambers, testing nighttime temperatures of 18, 22, 26, and 30°C from 12 a.m. until 5 a.m. throughout kernel development, using rice cultivars Cypress, LaGrue, XP710, XL8, M204, and Bengal. As nighttime temperature increased, head rice yields (HRY) significantly decreased for all cultivars except Cypress and Bengal, for which HRY did not vary among nighttime temperature treatments. Kernel mass did not vary among temperature treatments for any cultivar. Grain dimensions generally decreased as nighttime temperature increased. The number of chalky kernels increased with an increase in nighttime temperature for all cultivars but Cypress. The amylose content of Cypress and LaGrue was significantly lower at the nighttime tem...

Glassy state transition and rice drying: development of a brown rice state diagram.

ABSTRACT The effect of moisture content (MC) on the glass transition temperature (Tg) of individual brown rice kernels of Bengal, a medium-grain cultivar, and Cypress, a long-grain cultivar, was studied. Three methods were investigated for measuring Tg: differential scanning calorimetry (DSC), thermomechanical analysis (TMA), and dynamic mechanical analysis (DMA). Among these methods, TMA was chosen, because it can also measure changes in the thermal volumetric coefficient (β) of the kernel during glass transition. TMA-measured Tg at similar MC levels for both cultivars were not significantly different and were combined to generate a brown rice state diagram. Individual kernel Tg for both cultivars increased from 22 to 58°C as MC decreased from 27 to 3% wb. Linear and sigmoid models were derived to relate Tg to MC. The linear model was sufficient to describe the property changes in the MC range encountered during rice drying. Mean β values across both cultivars in the rubbery state was 4.62 × 10-4/°C and ...

Effects of rough rice storage conditions on the amylograph and cooking properties of medium-grain rice cv. Bengal

ABSTRACT Rough rice (cv. Bengal) was stored at four moisture contents (8.8, 10.7, 12.9, and 13.6% MC) and three temperatures (3, 20, and 37°C) for up to six months. The amylograph overall paste viscosity of the milled rice increased during storage. This increase was most apparent in all samples stored at 37°C. For rice stored at 20 and 37°C at all MC levels, a 30–50% increase in peak viscosity (PV) was observed during the first three months of storage. PV subsequently leveled off for rice stored at 12.9 and 13.6% MC but declined for samples stored at 8.8 and 10.7% MC. The final viscosities also increased during storage. The water-absorption ratio of the samples during cooking in excess water increased by an average of 15% over six months of storage. The amylograph and cooking properties were significantly affected (P < 0.05) by rough rice storage duration, temperature, MC, and their respective interactions.

Quality Characteristics of Long-Grain Rice Milled in Two Commercial Systems

ABSTRACT Long-grain rice variety Kaybonnet was milled to three degree of milling (DOM) levels in two commercial milling systems (a single-break, friction milling system and a multibreak, abrasion and friction milling system) and separated into five thickness fractions. For both milling systems, the surface lipid content (SLC) and protein content of the milled rice varied significantly across kernel thickness fractions. SLC was influenced by DOM level more than by thickness, while the protein content was influenced by thickness more than by DOM level. Particularly at the low DOM levels, the thinnest kernel fraction (<1.49 mm) had higher SLC than the other kernel fractions. Protein content decreased with increasing kernel thickness to 1.69 mm, after which it remained constant. In both milling systems, thinner kernels were milled at a greater bran removal rate as indicated by SLC differences between the low and high DOM levels. For rice milled to a given DOM level, the multibreak system produced fewer broken...

Neglecting Rice Milling Yield and Quality Underestimates Economic Losses from High-Temperature Stress

Future increases in global surface temperature threaten those worldwide who depend on rice production for their livelihoods and food security. Past analyses of high-temperature stress on rice production have focused on paddy yield and have failed to account for the detrimental impact of high temperatures on milling quality outcomes, which ultimately determine edible (marketable) rice yield and market value. Using genotype specific rice yield and milling quality data on six common rice varieties from Arkansas, USA, combined with on-site, half-hourly and daily temperature observations, we show a nonlinear effect of high-temperature stress exposure on yield and milling quality. A 1°C increase in average growing season temperature reduces paddy yield by 6.2%, total milled rice yield by 7.1% to 8.0%, head rice yield by 9.0% to 13.8%, and total milling revenue by 8.1% to 11.0%, across genotypes. Our results indicate that failure to account for changes in milling quality leads to understatement of the impacts of high temperatures on rice production outcomes. These dramatic losses result from reduced paddy yield and increased percentages of chalky and broken kernels, which together decrease the quantity and market value of milled rice. Recently published estimates show paddy yield reductions of up to 10% across the major rice-producing regions of South and Southeast Asia due to rising temperatures. The results of our study suggest that the often-cited 10% figure underestimates the economic implications of climate change for rice producers, thus potentially threatening future food security for global rice producers and consumers.