Applications of Microwave Heating of Foods

Doctoral thesis, 2015

Abstract Microwave heating of foods offers rapid and flexible heating both for consumer and industrial applications, with several advantages, such as reduced processing time (due to volumetric heating and reduced coming-up times (heating time required to reach the desired target temperature), selective heating, increased process flexibility and efficient heating (volumetric heating of the food does not require heating of a surrounding medium or package). However, successful installations require knowledge in process design. Modelling could be used to accomplish this in a way that gives the desired electromagnetic field distribution. The resulting microwave heating pattern depends in a complex way on a large number of parameters, related to the food, the package as well as the oven. Among these parameters are: food dimensions and geometry, dielectric and thermo-physical properties of the food, design and geometry of the food package, placement of food components and design of the oven. This thesis deals with selected applications of microwave heating of foods at 2450 MHz, modelling of microwave heating of foods in a lab-scale microwave oven, and the evaluation of continuous tubular microwave heating of foods in pilot-scale, in terms of temperature uniformity in a homogeneous model food and with respect to rapidness in heating of a particulate model food and its resulting product quality. Additionally, an accurate method for determining the dielectric properties of liquids is described. Dielectric properties of foods are required as input data for modelling of microwave heating, and also serve as a first basis for understanding the phenomena behind microwave heating of foods. In the food industry, process operations often require continuous processes, due to the desired high yield. This suggests a large potential for well-designed and controlled microwave continuous operations. High-temperature short-time (HTST) processing of foods could be used to achieve thermal processing as sterilisation, while exposing foods to heating for a shorter time than conventional heat treatment, with resulting minimal product degradation. It is suggested that continuous tubular microwave heating of particulate foods could offer an alternative to HTST processing for producing high-quality particulate foods with increased rapidness due to the volumetric microwave heating and the shorter coming-up times to achieve the necessary target temperature. The work presented in part of this thesis demonstrates that it is possible to achieve microwave-assisted HTST conditions in terms of continuous tubular microwave heating of a high-concentrated particulate model food product for a 12 mm tubular system at 2450 MHz. This is accomplished by combining modes that heat primarily in the centre (TM020) and in the periphery (TM120) of the tube. This process results in a more rapid and uniform heating where the product (particulates as well as the surrounding continuous phase) will reach the desired target temperature more rapidly (than in a traditional HTST system), with resulting reduced losses in product quality in terms of e.g. piece integrity. Additionally,microwave HTST processing offers product quality advantages due to reduced overheating of the continuous phase and also large process flexibility, since a wide interval of possible combinations of target temperature and holding time is accomplished in the same equipment. Moreover, combined centre and periphery heating is shown to give a more uniform temperature distribution, than periphery alone or centre alone heating could offer, in the 16 mm tubular microwave system. Furthermore, modelling of heating of foods in a microwave combination oven at 2450 MHz, in which the food is heated by combined microwave and convective heating, is described by solving for the electromagnetic fields in a model food load and then including these as a source term in a heat transfer model. Validation experiments of the numerical model showed good qualitative and quantitative agreement. Computers today are considerably faster than 10-15 years ago, which contributes to increased use of modelling tools for process design and development of optimised industrial microwave heating systems. It is suggested that this has contributed to increase the number of successful installations in food industry and will continue to support the steadily growing trend of industrial installations in the years to come.