Modelling and simulation of heat and mass transfer during the heating of foods in microwave ovens

CAMPAÑONE L; PAOLA A; MASCHERONI R.H

Portugal

Congreso; EFFOST/EHEDG Conference 2007; 2007

The use of microwaves for food processing provides high heating rates due to their ability to transfer energy directly to the sample producing a volumetric heating, without the need of heat transfer to or within the food. Problems arise associated to the uneven increase of temperatures within the food. One of them is the generation of hot spots in different inner zones, depending mainly of food shape and size. It is the main practical problem during the implementation of microwave heating at industrial or home level. For instance, during baking the uneven distribution originates dried and burnt zones, meanwhile other parts of the food may be still too cold and uncooked. This is particularly serious in food sterilization, because the process cannot secure that all points in the food reach sufficiently high temperature for complete microbial death, and – normally – overprocessing is needed with the related loss of quality and increased energy usage.   With the aim of characterizing temperature distribution within heated products, researchers are studying – from a mathematical point of view – the problem of heating and cooking of foods by means of microwaves, because the solution of the resulting differential equations allows to predict product thermal behaviour during microwave heating. The coupled microscopic balances of energy and mass need to be solved to evaluate the temperature and water content profiles that arise during microwave heating. Adequate boundary conditions are also needed. In the case of heat transfer the microscopic balance must include the term of internal generation due to the interaction with the electromagnetic field. In many previous research papers the heat generation term for microwave heating has been modelled by applying the Lambert´s law, which assumes an exponential attenuation in the direction of propagation. In the case of products with sizes lower than a critical value, the solution of Maxwell equations, which describe radiation propagation through dielectric media, is needed, which allows to adequately describe the interaction of radiation with the food. Based on the previous considerations, the objectives of this work involve: 1.         To characterize temperature and humidity distribution within the foods, solving the coupled mass and energy balances, paying special attention to electromagnetic interaction with the food. 2.         To use the model to evaluate the thermal response of foods with different compositions and sizes, under diverse operation conditions: incident radiation intensities, power cycles, rates of forced air and the use of difference of phase angle to improve the temperature uniformity.