Advanced Computational Methods in Heat Transfer VI, C.A. Brebbia & B. Sunden (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-818-X Computational methods in food processing: Applications in heat transfer S. Yanniotis Department of Food Science and Technology Agricultural University of Athens, Greece Abstract Calculation of heat transfer rates in processing of foods is essential in order to design optimum processing conditions and the required equipment. The use of computational methods in food processing heat transfer applications that has been published in recent years is briefly presented in this paper. Introduction The aim of the food industry today is to provide safe, nutritious and of high sensory quality food to the consumers. This aim is achieved by applying basic concepts of chemistry, microbiology and engineering to food processing. One of the most important and more common engineering disciplines in food processing is heat transfer. There are many unit operations in the food industry where steady or unsteady state heat transfer is taking place e.g. pasteurization, sterilization, blanching, dehydration, evaporation, cooling, freezing etc. Heat transfer in these operations is of primary importance and affects the hygienic, nutritional and sensory quality of the product. In applying heat transfer knowledge to food processing, one must take into account that food industry usually deals with difficult raw materials. As biological materials, in many cases they are non-uniform and of variable consistency. The shape of the products is often irregular and some times changes during processing. Many food products undergo physical property changes during heating and some even composition changes. Some, are shear- dependant, others are time-dependant. In some cases, the materials are anisotropic. In addition, very often heat transfer is combined with mass transfer and/or momentum transfer. The effect of heat treatment on the food product Advanced Computational Methods in Heat Transfer VI, C.A. Brebbia & B. Sunden (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-818-X 596 Advanced Computational Methods in Heat Transfer VI depends on the combination time-temperature because, in addition to temperature changes during processing, biochemical and microbiological changes take place. Heat transfer and reaction kinetics considerations must be taken into account in designing thermal processes of foods. Overprocessing, by applying high heat treatment, has a negative effect on the product quality and its nutritional value. Underprocessing may result in spoilage of the product. Because of these complexities, the unsteady state heat transfer partial differential equation can be solved analytically only with several simplifying assumptions. Numerical solutions by finite differences and finite elements methods in canning, freezing, drying and other food processing operations have been used. There are several publications on simple shape bodies using the finite difference method, while in more irregular geometries, the finite element method has been used. Recently, Computational Fluid Dynamics (CFD) codes have been applied to food engineering problems. The present paper attempts to outline the recent scientific activity on the applications of computational methods in heat transfer research in the field of food processing and preservation. Current State Sterilization The required processing time in commercial canning is usually calculated with semi-empirical methods. It can be calculated also by analytical solutions of the heat conduction equation for simple cases. For more complicated situations, numerical solutions have been applied. Most of the existing work with numerical solutions applied earlier is focussed on solid foods or very viscous foods like purees and concentrates which are usually assumed to be heated by pure conduction. One of the earliest applications of numerical methods to thermal processing of foods, was that of Teixeira et al.[l]. Recently, Welt et al. [2] compared the time step restrictions in a heat transfer simulation in a cylindrical can with an explicit finite difference approach using both the capacitance and the non-capacitance surface approaches. Akterian [3] developed a mathematical model for conduction heating of cans with convective boundary condition. He solved the problem with finite differences. Banga et al. [4] developed a model for the thermal processing of anisotropic and non- homogeneous conduction-heated canned foods and applied it to canned tuna fish. The mathematical analysis in sterilization of stationary cans containing liquid foods is more difficult because in this case the heat is transferred inside the can by natural convention. Since the fluid motion is due to the buoyancy force, the velocity in the momentum equations is coupled with the temperature in the energy equation. Therefore, the momentum equations must be solved simultaneously with the energy equation in order to calculate the temperature profile, the velocity profile and the slowest heating zone of the can. Numerical solution of this problem has attracted the interest of some researches. Datta and Advanced Computational Methods in Heat Transfer VI, C.A. Brebbia & B. Sunden (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-818-X Advanced Computational Methods in Heat Transfer VI 597 Texeira [5] numerically predicted transient temperature and velocity profiles during natural convection heating of canned liquid foods. Yang et al. [6] used a CFD code for the simulation of natural convection heating of starch dispersions in a cylindrical still can. Ghani et al. [7] have applied CFD to calculate transient temperature and velocity profiles in sterilization of canned liquid foods in a still- retort. Quarini and Scott [8] used a CFD package to predict the thermal- hydraulic behaviour of a non-Newtonian liquid food heated in a vertical still can. Aseptic processing and filling has been developed in recent years primarily to improve the quality of products conventionally heated in autoclaves. High temperature-short time heating of the product is applied before the container is filled and sealed aseptically. This method is applied to liquid foods and to liquid foods that contain particulates, which can be pumped through heat exchangers. Residence time distribution in holding tubes, where the product stays for certain time at a predetermined temperature in order to receive the required heat treatment, and in heat exchangers is an important processing parameter not only for liquids that contain paticulates but in continuous flow systems in general. The minimization of residence time scattering from the mean value is desirable in process optimization. The more narrow the residence time distribution, the more uniform will be the heat treatment and the better the nutritional and organoleptic quality of the product. The number of surviving microorganisms in a heat treated product is determined by the least heated part of the product. To be on the safe side, processors usually base the heat treatment on the center temperature of the fastest moving part through the system, but this results in overprocessing of the main percentage of the product which induces quality decrease. Yang et al. [9] calculated the lethality distribution within particles in the holding section of an aseptic processing system using a finite difference method. Sandeep et al. [10] wrote a three-dimensional finite difference program to compute the liquid and particulate velocities in a suspension passed through a holding tube. The program incorporated the effect of particulates on the liquid velocity, the interactions between particulates, the non-Newtonian behavior of the liquid and the effect of particulate concentration and temperature on the viscosity of the suspension. A finite element program was used to determine the temperature distribution within the particulates in the heat exchanger, the holding tube and the cooling section of an aseptic processing system. Electroheating methods Electroheating methods, mainly microwave heating and ohrmc heating, are volumetric heating methods and thus, they do not depend on heat transfer from 'the surface to the center but all the mass of the product heats simultaneously. Microwave heating of foods is a well-established heating method on domestic scale. However, on industrial scale the use of microwave heating is limited. It is being used mainly for tempering of meat, fish and butter. The main concern in microwave heating of foods in industrial scale is the uniform temperature distribution in the food. For safety reasons, all the points of the heated products must reach certain temperature. The temperature profile in the Advanced Computational Methods in Heat Transfer VI, C.A. Brebbia & B. Sunden (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-818-X 598 Advanced Computational Methods in Heat Transfer VI product depends on the absorption of microwave energy and on heat transfer by conduction, convention and evaporation. The absorption of microwave energy is controlled by a number of factors such as food and packaging composition and geometry as well as microwave field distribution in the oven and the resulting field in the food [11]. Research is concentrated on understanding these interacting factors. Ohlsoon and Risman [12] developed a finite difference numerical program to simulate the electromagnetic field distribution and the heat distribution in food pieces in a tubular microwave heater in order