| With the improvement of living standards,the acceleration of the pace of life,and the change of diet and lifestyle,there is an increasing demand for microwaveable food.Currently,microwaveable food in the market is mostly packaged in rectangular lunch box and stored at low temperature,and it is easy to eat for consumers after microwave reheating.As a typical convenient-fast and microwaveable food,there is an undesirable uneven heating drawback for multi-component instant meal during microwave reheating,which affects food safety and quality.To improve the heating uniformity of multi-component instant meal(MCIM)under microwave reheating,the MCIM prepared by representative raw materials instant rice(IR)and mashed potato(MP)was taken as the research object.Based on the theories of electromagnetism,heat and mass transfer and food science,the research methods of theoretical analysis,computer simulation and bench test were employed to study the heating uniformity of MCIM under microwave reheating.Firstly,the physical properties(moisture content,apparent density and porosity),thermal properties(thermal conductivity and specific heat),dielectric properties(dielectric constant and dielectric loss factor)and microwave penetration depth into MCIM(instant rice and mashed potato)were measured and analyzed;Then,the temperature distribution and formation mechanism of MCIM during microwave reheating were clarified;Thirdly,the effects of metallized packaging on microwave reheating of IMCM were investigated;Finally,the microwave reheating process of MCIM was simulated by COMSOL software to reveal the characteristics and formation mechanism of non-uniform temperature distribution.Based on the reflection or absorption of microwave active materials(aluminum and silicon carbide),a metallized packaging design was proposed to ameliorate the microwave reheating effects and improve the heating uniformity of MCIC after reheating.The main contents and conclusions are as follows:(1)The physical and dielectric properties of MCIM as affected by temperature were measured and analyzed.In microwave reheating,the changes of physical characteristics of MCIM were not significant(p>0.05).The thermal and dielectric properties changed significantly with increasing temperature(p<0.05).With the increase of temperature,the thermal conductivity of MCIM increased gradually.The specific heat of IR increased firstly and then decreased,and the specific heat of MP showed great fluctuations.The thermal conductivity and specific heat of MCIM were significantly affected by adding salt content(p<0.05).The thermal conductivity and specific heat of MP decreased with the increase of salt content in the range of 10?40?,while it was not significant(p>0.05)in the range of 40?80?.Reasonable salt addition into the MCIM can control the changes of thermal conductivity and specific heat of sample during microwave reheating,and further affect the temperature distribution.(2)The temperature distribution and formation mechanism of MCIM during microwave reheating were clarified.Considering the influences of microwave power and lunch box volume on the uniformity of temperature distribution of MCIM in microwave reheating,the appropriate microwave reheating parameters were determined as microwave power of 800 W and lunch box volume of 650 m L.In the process of microwave reheating MCIM,"hot spots"appeared on the upper and bottom surface of the sample,and"cold spots"appeared in the center cross-section.The vertical temperature distribution was affected by the height of the sample,but less than that of the length and width of the sample.In the reheating process of MP,the temperature distribution on the upper and bottom surfaces of the sample is symmetrical and there is corner heating effect.The surface heating phenomenon of MP is stronger than that of IR.Combined with the law of temperature distribution and the regulation of microwave active materials on microwaves,the metallized packaging structure for IR and MP were designed.By comparing the temperature distribution of MCIM with four layouts under different salt content,the best arrangement for MCIM was determined as up-and-down(0.2%salt content of MR on the upper side and IR on the lower side)to achieve the best uniformity of temperature distribution.(3)The effects of metallized packaging on microwave reheating of MCIM were investigated.The formula of metallized packaging for IR and MP was determined.The ratio of film-forming solution was 100 m L deionized water and 6 g CMC;The formula of aluminum film solution was100 m L deionized water,6 g CMC and 12 g Al powder;The formula of Si C membrane solution was100 m L deionized water,6 g CMC and 6 g Si C.The metallized packaging for IR and MP was in the form of aluminum film(the aluminum film solution is used according to 3.5×10-4 m L/mm2 for aluminum film pattern spraying,with aluminum film thickness of 0.03 mm).Aluminum film can alter the transmission of electromagnetic wave incident into the lunch box and the distribution of electromagnetic field in the microwave cavity,effectively reduce the high region of electric field strength in the lunch box,improving the uniformity of electric field distribution to ameliorate the uniformity of temperature distribution.(4)The mechanism of improving microwave reheating uniformity of MCIM by metallized packaging film was revealed.The established simulation model can greatly describe the microwave reheating process of MCIM.By analyzing the reheating effect of simulation models with four different packaging structure for IR and MP,the reasonable metallized packaging structure was determined,namely the aluminum film was coated in 50 mm width and 0.03 mm thickness at the corners of the box.The introduction of metallized packaging design can greatly improve the uniformity of electric field and temperature distribution.After metallized packaging,the average temperature of upper and bottom surface of IR decreased by 8.03?and 7.42?,and the uniformity of temperature distribution increased by 35.71%and 72.22%,respectively;The average temperature on the upper surface of MP decreased by 9.58?and the average temperature on the bottom surface increased by 5.48?.The uniformity of temperature distribution on the upper and bottom surfaces increased by 22.58%and 80%respectively.Metallized packaging can effectively improve the uniformity of temperature distribution for MCIM after microwave reheating.In addition,the rotation motion of sample may reduce the excessive absorption of microwave energy in the local area,and reduce the"hot spots"in the temperature distribution for IR during reheating.In microwave reheating,reasonable design of metallized packaging structure can effectively improve the distribution of electric field inside material,and then ameliorate the uniformity of temperature distribution.The research results may provide a theoretical basis for the industrial production of multi-component instant food and related metallized packaging,and enrich the theoretical research and technological innovation of multi-component food in microwave reheating. |
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