R. C. Bautista

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...

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.

Optimal Harvest Moisture Contents for Maximizing Milling Quality of Long- and Medium-Grain Rice Cultivars

The objective of this study was to determine the harvest moisture contents (HMCs) at which rice milling quality peaked for various rice lots. Multiple samples per field of cultivars Bengal, Cypress, and Drew were harvested at northeast and southeast Arkansas locations in 1999 and 2000. Additional field sample sets of multiple cultivars were collected in 2004, 2005, and 2006 at various locations in Arkansas, Mississippi, and Missouri. Head rice yields (HRYs) were described by a quadratic equation with HMC as the independent variable. Peak HRYs varied from 63.8% to 70.6%. Optimal HMCs, determined as the MC at which HRY peaked, varied from 18.7% to 23.5% for long-grain cultivars and 21.5% to 24.0% for medium-grain Bengal. The general range of optimal HMCs was 19% to 22% for long-grains and 22% to 24% for medium-grain Bengal. For rice lots with HMCs less than the optimal HMC, the amount of HRY reduction from peak values was strongly correlated to the percentage of fissured kernels at harvest; fissured kernel percentage accounted for 77% of the variation in HRY reduction from peak HRYs.

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.

Effects of Rough Rice Moisture Content at Harvest on Peak Viscosity

During rice kernel development, kernel size and chemical composition change with maturation, and the kernel moisture content progressively decreases (Del Rosario et al 1968; Lee and Chang 1985). Therefore, kernel moisture content is an important indicator of rice kernel development. Lee and Chang (1985) found that when starch and protein synthesis were nearly completed, the rice kernel moisture content was ≈27–29% (wb). The composition of rice kernels harvested after this point reportedly did not change dramatically, although the starch content increased slightly, causing a proportional decrease in protein content in the kernel (Del Rosario et al 1968; Kim et al 2001). Rice kernel-to-kernel development on panicles is nonuniform, which in turn produces a range of kernel moisture contents within a bulk at harvest (Chau and Kunze 1982; Kocher et al 1990; Siebenmorgen et al 1992). As such, physical properties such as bulk density and milling quality change with harvest moisture content (HMC). The grade of rice typically improved and the bulk density increased as HMC decreased (Morse et al 1967; Fan et al 1998). The number of fissured kernels increased dramatically when the HMC decreased from 19 to 13% and was especially severe when kernel moisture content decreased rapidly in a short time (Jodari and Linscombe 1996). Reports of optimum HMC for maximum head rice yield (HRY) ranges from 18 to 26% based on growing location, harvest date, and cultivar (Kester et al 1963; Huey 1977; Lu et al 1988, 1992; Siebenmorgen et al 1992; Jongkaewwattana et al 1993a; Ntanos et al 1996; Bautista and Siebenmorgen 2000). Limited research has been conducted to determine the effect of the HMC on functional characteristics of rice. Kester et al (1963) reported that the peak viscosity (PV) of short-grain rice first decreased and then increased with maturation. It was explained that amylases were responsible for these changes. Kim et al (2001) also noted differences in PV of medium-grain rice with HMC, however, the trends did not follow consistent patterns. The study reported herein was designed to quantify the effects of rough rice HMC on PV of popular several rice cultivars grown in the midSouth rice producing region of the United States.

The Role of Rice Individual Kernel Moisture Content Distributions at Harvest on Milling Quality

The objective of this study was to quantify the effects of rice kernel properties, associated with kernel moisture content (MC) distributions, on milling quality as harvest MC varied. Multiple samples of rice cultivars Bengal, Cypress, and Drew were harvested at various MCs from northeast and southeast Arkansas in 1999 and 2000. Additional sample sets of various rice cultivars were collected in 2004, 2005, and 2006 at various locations in Arkansas, Mississippi, and Missouri. Individual kernel MCs from panicles were measured immediately after each harvest. The percentages of kernels at MC levels >21%, >22%, >23%, >24%, >25%, and >26% and at MCs 22% and <14%. Based on this analysis, optimal HMCs ranged from 18% to 22% for long-grain cultivars, from 19.0% to 20.4% for medium-grain Bengal, and from 17.7% to 19.0% for long-grain hybrid XP723.

Predicting Rice Physicochemical Properties Using Thickness Fraction Properties

ABSTRACT Long-grain rice cultivars Francis and Wells and hybrid XL8 Clearfield were harvested from two locations at three harvest moisture contents (HMC) in 2003. The rough rice was dried, fractionated into thin, medium, and thick fractions, and milled. Physicochemical properties of unfractionated and fractionated samples were determined. The effects of HMC and location on thickness distributions were investigated and the weighted-average physicochemical properties of the thickness fractions were compared with those of unfractionated rice. Generally, the growing location and HMC affected kernel thickness distributions, green kernel content, fissured kernel content, and head rice yield (HRY). As kernel thickness within samples increased, amylose content increased and the protein content and α-amylase activity decreased. Thick fractions had greater peak viscosities than medium and thin fractions. The thin, medium, and thick fraction physicochemical property weighted averages provided good predictions of mos...

INDIVIDUAL RICE KERNEL MOISTURE CONTENT VARIABILITY TRENDS

Variation in individual kernel moisture content (MC) distributions in rice panicles were measured for rice cultivars Bengal, Cypress, and Drew. Rice panicles were harvested at different stages of maturity from foundation seed fields at two locations with widely varying soils in Arkansas during the autumns of 1998, 1999, and 2000. The objective was to quantify variability of individual rice kernel MCs as affected by maturity, planting location, and year. Individual kernel MC distributions were multi-modal until the average harvest MC (HMC) reached about 16%, at which point the distributions tended to be primarily single-modal. For all cultivars, HMC significantly affected kernel MC standard deviation (SD). For Cypress and Drew, individual kernel MC SD was also significantly affected by year and location with significant interactions between HMC and year and HMC and location. Regression equations that accounted for the variation in individual kernel MC at varying kernel maturity stages were generated for each cultivar.

Estimating the Economic Value of Rice (Oryza Sativa L.) as a Function of Harvest Moisture Content

The net value (NV) of rice, as affected by drying costs and milling quality changes associated with harvesting rice at various moisture contents (MCs), was studied using a five-year data set comprising eight cultivars harvested over a range of MCs from 11 southern U.S. locations. A quadratic relationship was used to characterize the change in NV across harvest MC (HMC); this relationship was due to the progressively-increasing fee structure for commercial drying costs and the quadratic nature of head rice yield (HRY) changes with HMC. A sensitivity analysis revealed that as the price of brokens increased, there was a slight decrease in the HMC at which NV was maximized. Relative to the price of brokens, the optimum HMC was not influenced by fluctuations in head rice price. At a given HMC, the NV of a rice bulk increased with the price of brokens, and the extent of the increase was heavily influenced by the HRY versus HMC relationship. In all instances, the optimal HMC to maximize HRY (HMCopt-HRY) was greater than the HMC corresponding to the maximum NV (HMCopt-NV). When HMC, NV and HRY were plotted regardless of cultivar, location, or harvest year, the MC at which HRY was maximized was 21.7% whereas the MC at which NV was maximized was 18.5%, representing a 3.2 percentage point difference between HMCopt-HRY and HMCopt-NV.

Milled Rice Fissure Formation Kinetics

Milled rice samples at various moisture contents (MCs) were exposed to air inside a chamber that was equipped with a video camera and monitoring system that enabled observation of fissure formation over 24-h exposure durations. The effects of milled rice kernel MC (11%, 12%, 13%, or 14%), cultivar (Bengal, Wells, and CL161), air relative humidity (RH; 10%, 20%, 30%, 45%, 60%, 75%, or 90%), and air temperature (5°C, 10°C, 15°C, 20°C, or 30°C) on the rate and amount of kernel fissuring were quantified. Rice cultivar and MC played minor roles in determining the rate and amount of fissuring relative to air temperature and RH. Fissuring rates were greatest at low (10%) and high (90%) RH levels; fissuring occurred almost immediately after kernels were exposed to these RH levels. Greater air temperatures considerably increased the rate and total amount of fissured kernels. Little fissuring was observed in an RH range of 30% to 75%, at any air temperature tested, providing a safe window for milled rice exposure to environmental conditions.