Effect of Rice Chilling on Drying, Milling, and Quality Characteristics

Abstract

HighlightsThis study experimentally simulated drying of rough rice subjected to various cold storage/chilling conditions.The rice was dried using slightly-heated air and high temperature air with procedures set to mimic those practiced by commercial systems.The moisture removal, drying rate constant, material state transition, milling and quality characteristics of rice upon drying conditions was investigated.The study provides important reference information for growers and rice processors using the relatively new rice cooling technology.Abstract. High temperature (field heat) and moisture content of freshly harvested rough rice promote excessive respiration and microbial growth. Therefore, the rice risks significant deterioration of quality due to delayed drying at peak harvest time when drying capacity becomes limited. The U.S. rice industry has identified that cooling/chilling the rice prior to drying to remove the excess heat, immediately after harvest, significantly preserves the quality of milled rice. This study experimentally simulated drying of rough rice after cold storage/chilling. The rice was dried using slightly-heated air and high temperature air with procedures set to mimic those practiced by commercial systems. Rough rice at moisture contents (MCs) of 16%, 19%, and 21% (wet basis) were stored at storage temperature (Ts) of 10°C, 15°C, and 20°C for up to 4 months. Following retrieval, the samples were dried at drying temperature (Td) of 35°C, 45°C, and 60°C and relative humidity (RH) of 20%. Each drying run comprised of two 20-min drying passes with the rice tempered at the drying Td for a duration of 4 h following every drying pass. Following the drying, the rice was conditioned in an equilibrium MC chamber (T=26°C, RH=56%) to 12.5% MC and then milled to evaluate milled rice yield and quality characteristics. The highest percentage points of MC removal (6.77% points) occurred following drying at air temperature of 60°C for samples with initial MC at 21% and stored at 20°C. Drying with air at 60°C decreased head rice yield (HRY) especially for samples with initial MC at 21% and stored at 10°C (HRY=51.4% versus those at 45°C and 35°C, HRY=55.95% and HRY=58.8%, respectively). Drying air temperatures studied (35°C, 45°C, 60°C) had no significant effect of causing discoloration of samples within the range of the studied initial MCs and storage temperatures. Peak and final viscosities of samples with different initial MCs (16%, 19%, and 21%) stored at 20°C followed by high temperature air drying (60°C) were significantly different from those of samples stored at 10°C and 15°C. The results provided insight into the drying, milling, and quality characteristics of rice after cold storage/chilling. The information provide foundation for development of new recommendations to improve quality of milled rice. Keywords: Conventional drying, Cooling/chilling, Milling quality, Rough rice, Storage.