A Comparative Study Between the McGill #2 Laboratory Mill and Commercial Milling Systems

Cereal Chem. 86(4):470–476 The degree of similarity between rice milled in a McGill #2 laboratory mill and commercial milling processes was evaluated using eight physical, physicochemical, and end-use properties. There was no statistical difference between the two milling systems with respect to color parameters L* and a*, final viscosity, texture, and end-use cooking properties (α = 0.05). Overall, the kernel dimensions of length, width, and thickness were less in the McGill #2 laboratory-milled rice than the same rice milled commercially. The incidence of bran streaks and peak viscosity values were each higher when the rice sample was milled commercially in 27, and 28, respectively, of the 29 samples by means comparison. The decrease in kernel dimensions and incidence of bran streaks were attributed to the more aggressive nature of the single-pass, batch milling system of the McGill #2 laboratory mill as compared with multipass, continuous milling systems that are used commercially. Finally, as surface lipid content (SLC) decreased, L* increased and a*, b*, and the incidence of bran streaks decreased for both milling systems. The goal of rice milling is to remove the germ and bran layers from the exterior of the rice kernel caryopsis while maximizing head rice yield (HRY), the main quality indicator of rice. Head rice yield is the mass percentage of rough rice that remains as three-fourths or more of a whole kernel after milling (USDA 2005). The extent to which the bran layers are removed from the caryopsis influences the quality and processing characteristics of rice. Large-scale commercial mills process several tons of rice per hour and remove the bran layers by friction, or abrasion forces, sometimes with the aid of a water mist. Rice processing research, however, is usually conducted using small, representative samples that are typically milled using laboratory mills. One such mill, the McGill #2 laboratory mill, is commonly used by research facilities and the laboratories of commercial milling operations, and primarily removes bran layers by frictional forces. There is a lack of evidence that laboratory milling produces rice of the same physicochemical profile as that of commercial milling processes, thereby posing questions as to the representativeness of laboratory milling. The degree to which bran layers are removed from the caryopsis can be measured and categorized as the degree of milling (DOM). The Federal Grain Inspection Service (FGIS) separates DOM into three categories: reasonably well-milled, well-milled, and hard-milled (USDA 2005). Qualification is made predominantly on the basis of color; rice will lighten as DOM increases (Chen and Siebenmorgen 1997; Mohapatra and Bal 2005; and Saleh and Meullenet 2007). Because the majority of extractable rice kernel lipids are in the bran layers and germ (Matsler and Siebenmorgen 2005), the surface lipid content (SLC) is another way to measure milled rice DOM. The analysis of milled rice using the SLC method is more quantifiable compared with the FGIS DOM qualification system and has been used for comparative studies in rice processing (Perdon et al 2001; Siebenmorgen et al 2006; Saleh and Meullenet 2007). The SLC, as a measurement of DOM, affects the physical, physicochemical, and end-use properties of rice. First, HRY decreases as milling duration increases, and SLC correspondingly decreases (Bennett et al 1993; Reid et al 1998; Saleh and Meullenet 2007). Furthermore, HRY decreased, on average, 9.4 percentage points for every one percentage point decrease in SLC when milling was accomplished with a McGill #2 laboratory mill (Cooper and Siebenmorgen 2007). As above, the color of rice will normally whiten as DOM increases and SLC decreases. Mea surements of color parameters using the International Commission on Illumination (CIE) L* a* b* color system show that as the outer bran layers are removed, yellow and red pigments decrease to a point of equilibrium (Lamberts et al 2005). The prevalence of bran streaks, the linear mark on the dorsal edge of milled rice kernels, decreases as the SLC decreases (Bhattacharya and Sowbhagya 1976). Viscosity increases as SLC decreases (Dautant et al 2005). Perdon et al (2001) reported that peak viscosities increased with greater DOM, whereas final viscosities did not consistently increase or decrease. Lastly, Saleh and Meullenet (2007) concluded that as SLC decreases, rice firmness decreases, and water uptake and rice stickiness increase. The McGill #3 laboratory mill, a larger version of the McGill #2, has been designated as “an approved device” to determine milling yields by the FGIS (USDA 2005). The approved procedure involving the McGill #3 laboratory mill entails starting with 1 kg of rough rice, a greater amount than is typically available for many research studies. The McGill #2 mill is a friction-based, batch mill that typically processes 150 g of rough rice per batch (Andrews et al 1992) and has been used extensively for research with smaller samples. Andrews et al (1992) and Bennett et al (1993) established milling quality trends, in terms of HRY and SLC, respectively, when using a McGill #2 laboratory mill under various operational conditions. Moisture content (MC) of the rough rice was the most significant attribute affecting SLC and HRY, while milling duration, mill chamber pressure, and sample size affected SLC and HRY to a lesser degree. Surface lipid content values decreased when MC, milling duration, or chamber pressure increased. Milling a small amount of rice in laboratory mills is assumed to represent commercial milling processes. Though, commercial mills are continuous, multibreak, sequential systems that can mill several tons of rice per hour. Chen et al (1998) concluded that a multibreak abrasion and friction commercial milling system produced fewer broken kernels than a commercial single-break friction system. That same study also stated that for commercially milled (CM) rice with a low DOM level, corresponding to a high SLC, the bran content was greater on thinner kernel fractions than thicker fractions. Beneficial results from research using laboratory mills have been slow to be adopted, in part due to the lack of evidence that laboratory mills adequately represent commercial milling processes. The objective of this study was to quantify the degree of similarity between CM and McGill #2 laboratory-milled rice 1 Graduate assistant, university professor, and postdoctoral associate, respectively, University of Arkansas, Dept. of Food Science, 2650 N. Young Ave., Fayetteville, AR 72704. 2 Corresponding author. Phone: 479-575-2841. Fax: 479-575-6936. E-mail: tsiebenm