Factors Affecting the Slope of Head Rice Yield vs. Degree of Milling

Cereal Chem. 75(5):738-741 Milling data of four long-grain rice cultivars were analyzed to determine the uniformity in the slope of their curves for head rice yield (HRY) versus the corresponding degree of milling (DOM). The data set for each cultivar comprised samples that had been subjected to various drying air conditions and durations and milled over a range of moisture contents. All treatment combinations were split and milled for either 15, 30, 45, or 60 sec in a McGill no. 2 laboratory mill to obtain HRY versus DOM data. Linear relationships between HRY and DOM, as observed in past research, were confirmed. This implies that as rice is milled to greater extents (higher DOM), the HRY decreases linearly. Within the bounds of the experimental levels tested, neither the drying air condition nor drying duration affected the rate at which HRY changed with DOM. However, the cultivar and the moisture content at which the rice was milled significantly (P < 0.05) influenced this rate. At higher milling moisture contents, the decrease in HRY per unit of increase in the DOM was greater than at lower moisture contents. While not conclusive, there was an indication of a relationship between the average kernel thickness of a cultivar and the HRY versus DOM slope. A goal of rough rice processing is to achieve an intact, wellmilled final product; however, several processing operations influence milling benchmarks. After harvest, the rough rice is dried to ≈12–13% moisture content (MC), wet basis. This dried product is then hulled and milled. Rice quality is most commonly summarized in terms of the head rice yield (HRY) and milled rice yield (MRY). The HRY is the mass percentage of rough rice remaining after complete milling. Head rice is defined as kernels three-fourths or more of the original kernel length (USDA 1979). Similarly, the MRY is the mass percentage of rough rice remaining as milled rice, which includes both head rice and broken kernels. The degree of milling (DOM) is the extent to which the bran layers are removed by the milling operation. The Federal Grain Inspection Service classifies samples into one of four DOM levels (well-milled, reasonably well-milled, lightly milled, or undermilled) using visual comparison to standard samples representing each level. A more accurate, elaborate, and time-consuming method of estimating DOM is by measuring the amount of surface fat remaining on kernels through petroleum ether extraction (Hogan and Deobald 1961, Watson et al 1975). Bhashyam and Srinivas (1984) quantified DOM in terms of degree of polish by measuring whiteness using a photovoltaic reflectance meter. Recently introduced commercial instruments provide rapid estimates of DOM through transmittance and reflectance measurements. Siebenmorgen and Sun (1994) established a linear relationship between DOM readings from the MM1-B milling meter (Satake Engineering Co., Ltd., Tokyo, Japan) and surface fat concentration for each of three cultivars of longgrain rice. Milling influences the values of MRY, HRY, and DOM. The removal of bran layers as milling duration increases results in the reduction of MRY and HRY and an increase in DOM (Andrews et al 1992). Bhashyam and Srinivas (1984) noted that a 2% increase in degree of polish affected a 4% increase in breakage. Sun and Siebenmorgen (1993) established linear relationships between HRY and DOM for samples milled in a McGill no. 2 laboratory mill. Archer et al (1994) showed that drying treatment did not significantly affect the HRY versus DOM slope for a given cultivar harvested at a given MC. Separate slopes were reported for each cultivar-harvest MC combination. Several authors have documented the effect of MC at milling on HRY and DOM (Wasserman 1960, 1961; Pominski et al 1961; Webb and Calderwood 1977; Banaszek et al 1989). Banaszek et al (1989) showed that, as MC decreased, HRY increased when samples were milled for a constant duration using a McGill no. 2 laboratory mill. Within the MC range in which rice was classified as being well-milled, MC accounted for more than 10 percentage points of change in HRY. The operation of the McGill no. 2 laboratory mill has been shown to affect milling results. Velupillai and Pandey (1987) showed a dependence of the DOM on milling time by milling samples for periods of 0–60 sec in 5-sec increments. By increasing the pressure settings on the mill, Webb and Calderwood (1977) demonstrated that varying pressure settings can be used to obtain an equivalent DOM. Sun and Siebenmorgen (1993) showed that DOM and HRY were linearly related. This correlation has been used by Sun and Siebenmorgen and other researchers to compare HRY values at a common DOM level. Milling of samples for a constant duration yields similar results. However, correction to a standard DOM level further reduces variability in HRY measurement. As part of a study evaluating the effects of brown rice temperature on milling outcomes, Archer and Siebenmorgen (1995) illustrated the importance of adjusting HRY to a common DOM level. Observed differences in HRY, caused by reducing brown rice temperature, were removed by correcting HRY to a common DOM. Slope uniformity across drying conditions, cultivars, and milling MC levels was evaluated in this study. If these factors can be eliminated or the effects characterized, then the procedure for adjusting HRY to a common DOM could be greatly simplified and more readily incorporated as part of a standard routine for laboratory HRY determination. MATERIALS AND METHODS Rough rice samples were processed across a typical range of commercial processing conditions. Laboratory methods were used to simulate the commercial unit operations of cleaning, drying, conditioning, and milling. All samples were cleaned and conditioned identically, but drying and milling conditions varied. The experimental design incorporated drying air conditions representative of those used in both on-farm bins and commercial column dryers. The resulting samples were milled for various durations to produce a range of DOM, MRY, and HRY values. Sample Procurement Drying tests were performed using five cultivar-harvest MC combinations of long-grain rice. The cultivar-harvest MC combinations used were: Alan-23%, Alan-19%, Lacassine-21%, Newbonnet-19%, 1 Graduate assistant and professor, Biological and Agricultural Engineering Department, and assistant professor, Agricultural Statistics Department, respectively, University of Arkansas, Fayetteville. Mention of a commercial name does not imply endorsement by the University of Arkansas. 2 Corresponding author. E-mail: [email protected] Phone: 501/575-2841.