Transport phnomena in food processes assisted by electromagnetic heating, in the radio frequencies

Defrosting operations (tempering/thawing) are very important in the food processing industry when raw food ingredients (vegetables, fish, meat) enter the production line at frozen state. In fact, freezing operation is the most widely used preservation method to keep the raw food safe, storable for long time and deliverable at long distance. But, before being used along the food processing line, tempering or thawing is needed in order to make these frozen products workable, subsequently undergoing cutting, slicing, chopping and etc. The defrosting operations are traditionally characterized by long processing times and they are vulnerable to quality deteriorations unless they are done properly, maintaining the characteristics of the food product and preventing its microbial contamination. Defrosting operations need to be carried out considering the end-point temperatures that comply with the quality standard regulations provided by authorities. Despite their drawbacks of long processing time, energy consumption and quality loss, conventional defrosting methods (air thawing, water thawing) are still in operation in food processing industries. On the other hand, increased demand of processed and ready to eat frozen food products in modern life style has fueled the technological advancements to propose on-demand, fast and reliable defrosting solutions. Radio frequency (RF) assisted heating has become an interest of various industries due to the possibility to reduce time for heating, improve power efficiency and eventually improve heating uniformity. It has been also proposed as process providing better quality and safety thawed food product: however, it still carries with it a problem of non-uniform heating in different food products. In regular box shaped food products, the corners and edges showed overheating while the interior portion remain at lower temperature, especially during defrosting. Therefore, it was essential to understand, investigate and analyze the physics behind RF assisted heating of foods, considering that -in the case of thawing – the process complexity is increased by the phase transition, with all the consequences related to physical properties change. Tremendous efforts were devoted to optimize the process parameters (the gap between electrodes and samples, the power cycling, the electrodes configuration, the RF unit mode batch or continuous) and factors affecting the heating uniformity considering the optimum quality of food product subjected to tempering/thawing processes. The experimental measurement of the dielectric properties of the food product at different state (frozen, tempered and thawed) was of essential importance to complete the data set required by the virtual tools built during the three years of PhD studies, thus increasing the trustability of the results provided by the various simulation scenarios. More in details, transport phenomena analysis of the RF assisted tempering/thawing of food products using both 50-ohm and free-running oscillator RF systems were performed, and computer simulation models based on commercial software were developed. The validated models were then used to investigate, analyze and design process parameters that affect heating behaviors. Power cycling using optimized power-time profiles resulted in better uniform distribution of end-point temperatures during RF thawing in 50-ohm system. For the case of free running oscillator RF heating system, the effect of moving the food load (and eventually the electrode) on power density and electric field distribution and eventually on uniformity of temperature distribution were investigated. Experimental study was carried out to compare the performance of stationary and continuous RF thawing of food products. Results revealed that, thawing of food product under moving condition slightly improved heating uniformity with respect to stationary condition. End-point temperature of -0.2 °C ± 1.5 °C and -0.4 °C ± 0.7 °C were achieved at 17 min residence time at stationary and moving condition, respectively. The problems of overheating on the corners were reduced by a temperature difference of about 10 °C. In RF heating of foods, also size and shape of the food load revealed to have great importance in the final distribution of temperature. The effects of orientation and shape on temperature distribution and power absorption during RF heating were investigated using food simulant on 50-ohm RF system. Three primitive shapes (cube, cylinder and sphere) were prepared using tylose and subjected to heating in a 50-ohm RF heating system using different RF power levels (300, 400 and 500 W). Results indicated that not only the shape but also the orientation of the load were factors playing a major role in the temperature distribution: in terms of temperature and power absorption uniformity, vertically placed cylindrical samples were convenient followed by rectangular ones, while spheres and horizontally placed samples resulted in non-uniform heating. The computer simulation developed previously was used to design a new strategy to investigate the effects of up/down movement of top electrode and movement of food product on a conveyor belt on an industrial RF system. Results showed that a combination of moving electrode and food product resulted in improved heating uniformity and was found to be more efficient in terms of reducing hot spots. This research work contributes significantly to the knowledge of RF assisted heating of food products and to the understanding of RF heating mechanism, to set up strategies to improve heating uniformity in the food and to optimize configuration of the RF system. This study also provides a significant input for the design and optimization of industrial scale RF thawing/tempering systems. [edited by Author]