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Effect of Milling Temperature and Postmilling Cooling Procedures on Rice Milling Quality Appraisals

¹Department of Biological and Agricultural Engineering, University of California, Davis, One Shields Avenue, Davis, CA 95616, U.S.A. ²Processed Foods Research Unit, USDA-ARS Western Regional Research Center, 800 Buchanan St., Albany, CA 94710, U.S.A. ³Corresponding author. Phone: (510) 559-5861. Fax: (510) 559-5851. E-mail: zhongli.pan@ars.usda.gov or zlpan@ucdavis.edu ⁴Department of Agricultural Engineering, Faculty of Agriculture, Suez Canal University, Ismailia, Egypt.

The objective of this research was to study the effects of different milling conditions and postmilling handling procedures on appraised milling quality of rough rice. Rough rice (M202) with moisture content of 11.5 ± 0.2% was used for this study. The samples were milled with a McGill number 3 mill under four milling conditions, including normal milling, milling at high temperature, milling with cooling using ice water, and room temperature water. The milled rice samples were cooled in closed and open plastic containers and in open pans with three temperatures: 15, 23, and 35°C. The effects of milling and postmilling conditions on milled rice temperature, moisture loss, cooling rate, single and multiple fissuring rates, total rice yield (TRY), head rice yield (HRY), whiteness index (WI), and total lipid content (TLC) were evaluated. Results showed that high single and multiple fissuring rates and low TRY and HRY were inherent in improper milling and postmilling conditions. Single fissuring rates were 15.9 and 17.6% and multiple fissuring rates were 3.5 and 7.2% for rice samples milled under normal and high-temperature conditions, respectively. Cooling methods that used open containers and pans had more moisture losses and further resulted in lowering appraised milling quality than methods that used closed containers. Low-temperature milling conditions followed by cooling in closed containers significantly reduced single and multiple fissuring rates and improved TRY and HRY by 0.9 and 1.5 percentage points, respectively. The effects of tested milling and postmilling conditions on WI and TLC were not significant. Obtained results constitute valuable information for developing milling and cooling procedures to achieve consistent, accurate, and reliable milling quality appraisals for rough rice.