Theoretical and experimental analysis of microwave heating processes

Abstract

Thermal processing is the major processing technology in the food industry and its purpose is to extend the shelf life of food products without compromising food safety. Apart from the positive effect of food treatments, such as the inactivation of pathogens, there are also some limitation by way of partial destruction of quality attributes of products, especially heat-labile nutrients, and sensory attributes. The technological revolution, nutritional awareness, and continuous demand of the new generation have necessitated search for new or improved food processing technologies. Presently, several new food processing technologies, including microwave heating, are investigated to improve, replace, or complement conventional processing technology. Microwave has been successfully used to heat, dry, and sterilize many food products. Compared with conventional methods, microwave processing offers the following advantages: 1) microwave penetrates inside the food materials and, therefore, cooking takes place throughout the whole volume of food internally and rapidly, which significantly reduces the processing time; 2) since heat transfer is fast, nutrients and vitamins contents, as well as flavor, sensory characteristics, and color of food are well preserved; 3) ultrafast pasteurization or sterilization of pumpable fluids minimizes nutrient, color, and flavor losses; 4) minimum fouling depositions, because of the elimination of the hot heat transfer surfaces, since the piping used is microwave transparent and remains relatively cooler than the product; 5) energy saving because of the absence of a medium between the sample and the MW; in addition, if the system is well projected, high efficiency can be reached (some authors showed the reduction of the energy costs during drying processes using microwaves, with a further improvement using air dryer and microwaves in sequence; moreover, consider the possibility to use alternative energy sources, eg. photovoltaic); 6) perfect geometry for clean-in-place system; 7) low cost in system maintenance; 8) space saving, if the system is compared with the traditional ones, based on boilers and surface heat exchangers. On the other hand, there are some problems which prevent the diffusion of this technique; among them: 1) uneven temperature patterns of the food processed, due to the uneven temperature field inside the microwave cavity; 2) temperature readout and control problems, because traditional probes fail: in particular, the thermocouples disturb the measurement and are damaged by the electric field, while fiberoptic probes allow to know the temperature only in few points; 3) difficulties in predicting the temperature field, because of coupling of three physical phenomena, that is, electromagnetic wave propagations, heat transfer and, in most of cases, fluid motion. Consider that sizing, during the design phase, and the control, during the operating phase, could be based on theoretical predictions, avoiding the so called “trial and error” approach. To address the critical points mentioned above, during the thesis work, theoretical models were developed and experimental tests were performed, with reference to “batch” and “continuous flow” processes. In particular, after a brief description of the principles of microwave heating, some batch processes have been analysed, that is, apple drying and in-package heating of water and oil. In both cases, the use of infrared technique allowed to obtain the spatial temperature distribution of the samples under test. ... [edited by Author]