Date of Graduation
Doctor of Philosophy in Food Science (PhD)
Second Committee Member
Third Committee Member
Fourth Committee Member
Drying Cost, Drying Efficiency, Drying Energy Requirement, Heat and Mass Transfer, Microwave Drying, Parboiled Rough Rice
Microwave (MW) heating offers an energy-efficient, fast method to dry high moisture content (MC) parboiled rice to safe storage MC. However, there is limited research that describes the fundamentals of heat and mass transport in rice kernels exposed to MW energy at 915 MHz, the most promising heating frequency for industrialized processing. This information is vital to explain the implications of MW technology on dried rice quality. The overall objective of this study was to develop a microwave heating technology that can sufficiently dry high MC parboiled rough rice kernels in one pass using a 915-MHz industrial microwave system. An industrial type MW system operating at 915 MHz frequency was used to dry high MC long-grain parboiled rough rice samples that were harvested at initial MC of 23% to 24% wet basis (w.b). Long grain rough rice samples were soaked in a lab-scale hot water bath set to soaking temperatures of 71 oC, 73 oC and 76 oC for 3 hours. After soaking, the wet rough rice was steamed in a lab-scale autoclave set to a temperature of 113 oC and a corresponding pressure value of 67 kPa for 5, 10 and 15 minutes (mins). The MW drying was accomplished at MW specific powers that ranged from 0.37 to 8.77 kW. [kg-DM]-1 (power per unit dry matter mass of the grain). During drying, fiber optic sensors were placed within the rice bed to collect real-time parboiled rough rice surface temperature. Results indicate that rough rice should be soaked at temperatures slightly below that of the onset gelatinization temperature of that rice cultivar and steamed for 10 min for optimal physiochemical and milling properties prior to drying by MW. Parboiled rough rice at initial MC of 35.88% reduced to a FMC of 13.48% after being treated with MW power level of 2 kW and drying duration of 31.5 min (MW specific energy of 3780 kJ.[kg-grain]-1) and at a low specific power of 2.92 kW.[kg-DM]-1. Increased MW specific power has a positive effect on parboiled rough rice MC reduction but negatively effects the rice milling characteristics. The head rice yield (HRY) obtained from the treatment was dependent on the specific energy input and reduced at higher specific energies. The drying rate was highest during the beginning of drying then slowed down during the end and can be divided into 2 periods, a first falling rate period (1.5 min to 7.5 min), and the second falling rate period (7.5 min to 31.5 min). Of the Page, Newton, Logarithmic, and Henderson & Pabis semi-empirical drying models, the logarithmic model best represented the MW drying behavior of parboiled rough rice kernels as determined by the R2, Adjusted R2, Reduced χ 2 and RMSE values. The effective moisture diffusivity was determined to be 5.04 × 10-11 m2.s-1. The activation energy was determined to be 3.02 kW.kg-1. The energy consumption was determined to be 1.05 kWh.[kg-grain]−1 with a drying efficiency of 18.89%. The drying cost for a ton of parboiled rough rice was $88.31 at a commercial energy rate of 8.41 cents per kWh in the state of Arkansas (2020). The models and parameters found in this study can be applied to industrial designs and act as an operational guide for the MW drying of parboiled rice.
Smith, D. L. (2020). Heat and Mass Transfer in Parboiled Rice during Heating with 915 MHz Microwave Energy and Impacts on Milled Rice Properties. Theses and Dissertations Retrieved from https://scholarworks.uark.edu/etd/3814