G.B. Awuah

Radio Frequency Heating of Foods: Principles, Applications and Related Properties—A Review

ABSTRACT Radio frequency (RF) heating is a promising technology for food applications because of the associated rapid and uniform heat distribution, large penetration depth and lower energy consumption. Radio frequency heating has been successfully applied for drying, baking and thawing of frozen meat and in meat processing. However, its use in continuous pasteurization and sterilization of foods is rather limited. During RF heating, heat is generated within the product due to molecular friction resulting from oscillating molecules and ions caused by the applied alternating electric field. RF heating is influenced principally by the dielectric properties of the product when other conditions are kept constant. This review deals with the current status of RF heating applications in food processing, as well as product and system specific factors that influence the RF heating. It is evident that frequency level, temperature and properties of food, such as viscosity, water content and chemical composition affect the dielectric properties and thus the RF heating of foods. Therefore, these parameters should be taken into account when designing a radio frequency heating system for foods.

Heat transfer and lethality considerations in aseptic processing of liquid/particle mixtures : A review

Consumer awareness and demand for nutritious yet inexpensive food products call for innovative processing techniques that have both safety and quality as primary objectives. These challenges appear to have been met by aseptic processing techniques, especially for liquid and high-acid foods. However, the extension of aseptic processing principles to low-acid foods containing discrete particles in viscous sauces has not been approved by regulatory agencies, particularly in North America. This apparent limitation is due primarily to the lack of adequate temperature monitoring devices to keep track of particles in dynamic motion, as well as to the residence time distribution of particles flowing in the continuous heat-hold-cool sections of the aseptic processing system. These problems have prompted active research to describe the phenomenal behavior of particulates through sound mathematical modeling and computer simulators. The accuracy of mathematical models depends heavily on how accurate input parametric values are. These parameters include the thermophysical properties of the carrier fluid and particles, as well as the aseptic processing system characteristics in relation to residence time distribution and the fluid-to-particle interfacial heat transfer coefficient. Apparently, several contradictory findings have been reported in the literature with respect to the effect of various processing parameters on the above-mentioned input parametric values. The need therefore arises for more collaborative studies involving the industry and academia. This review brings to perspective, the current status on the aseptic processing of particulate foods with respect to the critical processing parameters which affect the fluid-to-particle convective heat transfer coefficient associated with particulate laden products.

Comparison of two methods for evaluating fluid-to-surface heat transfer coefficients

Abstract Fluid to surface heat transfer coefficients ( h fs ) were evaluated based on two analytical methods using transient time-temperature data obtained from regular objects. The first (rate method) was based on heating rate at any given location while the second (ratio method) was based on the ratio of temperature gradients at two locations. Regular shaped objects made from four test materials (Teflon, Lucite, polypropylene and Nylon) were used in the study under different experimental conditions. Depending on the ambient condition, test particle type, size and shape, ( h fs ) values ranged between 15 and 420 W/m 2 °C. The two methods generally compared well ( p > 0.05), especially in situations where the associated Biot numbers were low ( h fs ) than the rate method. An error analysis indicated that both methods were sensitive to variations in parametric values when the associated ( h fs ) values were high ( Bi > 20). Overall, the rate method gave more consistent and conservative ( h fs ) values.

Influence of particle characteristics on fluid-to-particle heat transfer coefficient in a pilot scale holding tube simulator

Abstract Fluid-to-particle convective heat transfer coefficients ( h fp ) were estimated for spherical and finite cylindrical particles resident in the holding tube of a pilot scale aseptic processing simulator. Experiments were conducted using both Newtonian and non-Newtonian carrier fluids at 100 and 110 °C. Average h fp values ranged from 60 to 1000 W/m 2 C, depending on operating conditions of fluid concentration, temperature, flow rate, particle size, shape and particle orientation in the holding tube. The corresponding Biot numbers ranged from 6 to 60. With the exception of particle length and its orientation, which had no significant impact ( P > 0.05) on the heat transfer coefficient, all other parameters had significant influences ( P h fp . Spherical particles had higher h fp values compared with finite cylinders of approximately the same diameter under similar experimental conditions. Contrary to reported trends, the heat transfer coefficient was found to increase with decreasing particle size under all experimental conditions studied. The effect of particle-to-tube diameter ratio on h fp was not clear cut.

Biological verification of fluid-to-particle interfacial heat transfer coefficients in a pilot scale holding tube simulator

Inactivation kinetics of immobilized bovine pancreas trypsin (type III) was used in conjunction with a finite difference model to verify fluid‐to‐particle heat transfer coefficients to particulates in a holding tube of a pilot scale aseptic processing simulator operated at temperatures ranging from 90 and 110 °C. The enzyme was sealed in a stainless steel capsule, which was pretested to be suitable for high‐temperature, short‐time applications, and embedded at the center of a finite cylindrical particle. The percentage retention of the enzyme activity was calculated by using transient temperatures and their respective D values in a finite difference program. Heat transfer coefficients estimated for the potato particles under similar experimental conditions were used as input data. Excellent comparison was observed between predicted and measured percentage retention of enzyme activity.