Study On Improving Temperature Uniformity Of Microwave-reheated Instant Rice With Metallized Packaging Film

Instant rice(IR)is easy to carry and can be eaten after reheating,so it can meet the demands for consumers who are busy and fast-paced.At present,the most common IR in the market is packaged in rectangular boxes for cold storage.Microwave heating has the advantages of simple operation,fast heating,energy saving and high efficiency,so it is suitable for the reheating of rectangular-boxed IR.However,in microwave reheating the rectangular-boxed IR,due to the focusing effect of microwave at the corner of the material processed and the attenuation of microwave energy with the increase of material depth,the non-uniform issue of temperature distribution occurred such as the higher temperature at the corner and the lower temperature in the center of the material processed.This seriously affects the taste and quality of IR after reheating.In order to improve the temperature uniformity of microwave reheating IR,a method of metallization packaging coated on a rectangular instant box was proposed.The effects of metallized packaging film on the temperature uniformity of IR heated by microwave were investigated based on bench experiments and computer simulation.The variations of thermophysical parameters(specific heat capacity,thermal conductivity,etc.)and dielectric parameters(dielectric constant and loss factor)with increasing temperature were measured and analyzed in microwave reheating of refrigerated IR.The variations of microwave penetration depth inside the material were also explored.This provides the basic parameter basis for analyzing the interaction mechanism between microwaves and materials.The microwave reheating model of refrigerated IR within the rectangular boxed was constructed and verified by bench experiments.Clarifying the reasons for the temperature inhomogeneity of IR in the reheating process may provide references for improving the temperature uniformity.Based on the analysis of the experimental and simulation results of the temperature uniformity of the material,the design of metalized packaging film coated on the rectangular box was put forward and it was used to explore the influences on the temperature uniformity of IR in microwave reheating.The main research contents and conclusions are as follows:(1)The thermophysical parameters of IR were measured and analyzed.The thermal conductivity and specific heat capacity of IR increased with the increase in temperature;the dielectric constant first increased and then decreased with the increase in temperature;the dielectric loss factor decreased with increasing temperature.The microwave penetration depth decreased with increasing temperature.(2)Based on the measured thermophysical parameters,the model of microwave reheating IR was developed and the numerical simulation of the heating process with material rotation was carried out to explore the position and formation law of uneven temperature distribution.The electric field distribution at a single discrete position depended on the relative position of the material processed and the microwave waveguide,and the higher intensity in the electric field was caused by the standing wave of the microwave inside the material.In the early stage of heating,the temperature distribution of the material processed was similar to that of the electric field,and the material temperature at the high intensity of the electric field was higher than that at other weak areas.With the rotation of the material in the microwave oven,the position between the material and the cavity of the microwave oven was changed.Different modes of electric field were generated inside the material,and the electric field distribution with different intensities was formed inside the material.These electric field modes at each discrete position interacted,and the focusing effect inside the material was gradually exhibited.The distribution of electric field is the main reason determining the temperature distribution of material processed in microwave heating.(3)Based on the position and formation law of non-uniform temperature distribution,a metalized packaging scheme was proposed to improve the temperature distribution uniformity of IR in microwave reheating.The influence of metalized packaging on the temperature uniformity of microwave-reheated IR was explored by combining the experiment measurement and simulation method.The temperature of IR decreased at the corner and increased near the center of material processed due to the metallized packaging.The ’hot spot’ and ’cold spot’ areas reduced significantly,which could effectively improve the uniformity of temperature distribution and microwave utilization in microwave reheating IR.The maximum RMSE between simulation data and experimental values was less than 10 °C.This suggested that there was a good agreement between the simulation data and the measured ones.Metallized packaging could alter the distribution mode of electric field applied to the material processed,and reduce the formation of standing wave to improve the uniformity of electric field distribution.This promotes the uniform absorption of microwave energy by the material processed,and further to ameliorate the temperature uniformity of IR after microwave reheating.The above research results are helpful for the design of packaging structure and the quality improvement for instant food under microwave reheating.