Research On The Law Of Temperature Distribution And Heat Transfer Of Microwave Reheating Of Instant Rice

Instant rice already attracts many consumers that were accustomed to rice diet because it is easy to carry, convenient to edible, safe and healthy. Currently, as a new heating method, microwave heating is fast, volumetric heat generation and less nutrition loss for food materials, and will be applied in rice reheating more and more widely. Temperature uniformity was the hot issue in microwave reheating of rice, which was affected by the thermal properties and dielectric properties of food, its shape and size. We studied the thermal and dielectric properties of rice related to microwave heating, and also studied the effect of radius and height to diameter ratio (L/D) of a cylinder on the law of temperature distributions in microwave reheating rice. Through standard deviation and the coefficient of variation (COV) to measure the temperature uniformity, the optimum size of rice was selected. Then the mathematical model was used for prediction of the temperature distribution of rice during microwave reheating, and the mechanism of it was explored.Firstly, this paper designed the measurement device for determining the thermal conductivity of rice according to the unsteady hotline method. The specific heat of cooked rice dependent to temperature was measured using differential scanning calorimeter (DSC), and a vector network analyzer with an open-ended coaxial cable connected to a probe was used to obtain its dielectric properties related to frequency and temperature. At all temperatures, nonlinear relationships were estimated by regression analysis and analysis of variance. The results showed that temperature had significant effect on the specific heat and dielectric loss of the rice while the thermal conductivity and dielectric constant were unaffected. The dielectric constant of rice at 915 MHz was larger than that at 2450 MHz and dielectric loss factor showed the opposite result.The effect of sample size on the law of temperature distribution of microwave reheating of rice was studied. Three radiuses and four L/D of the cylinder rice samples were selected as the research object, and measured internal temperature profile by microwave workstation and fiber temperature probes. We found that in the same radius, hot-spot temperature of rice was shifted from geometric center region to surrounding border with L/D increased. When the L/D was equal or lesser than 0.75, the increase of radius caused hot point to focus on center area. When the L/D was between 0.75 and 1.25, hot-spot temperature transferred to edge and surface regions as the radius increased. Temperature uniformity was analyzed by COV based on the standard deviation, and the optimum size of cylinder rice which resulted in the most desirable temperature distribution in microwave reheating was radius in 4 cm, height in 4 cm.Heat transfer model of microwave reheating of cylinder rice was established to base on the energy conservation law and the Fourier equation and such as heat transfer theory, and combined with the initial and boundary conditions of rice in microwave reheating. A finite difference method (FDM) was used to calculate the temperature distribution during microwave reheating and applied Lambert's law to obtain the power absorbed by the rice sample. A MATLAB program was applied to solve the resulting set of unsteady state equations. The mathematic model and calculated parameters were validated and perfected through the calculated temperatures comparing with the experimental results, and finally achieved a good match of the simulated temperatures with the measured results. This research not only provided theoretical basis for the design of microwaveable rice, but so laid the foundations for the study and development of traditional rice and noodle food applying microwave technology.