Hybrid Combination of Emerging Food Processing Technologies: Microwave and Pulsed Ohmic Heating

Abstract

Conventional thermal processing of foods containing particulates significantly rely on convective and conductive heat transfer and tend to be overly conservative in ensuring microbial safety, thus compromising quality. Temperature lags inside particles of solid-liquid mixture foods could lead to the danger of under-processing and therefore risking the food’s safety. Advanced food processing technologies such as microwave heating and ohmic heating have been developed in the last few decades as alternatives to conventional processing methods. These advanced technologies could contribute to shorten processing times, energy savings, and highly balanced safe food; however, they alone still cannot guarantee food safety without damaging the food’s quality. Therefore, a new concept to combine microwave and ohmic heating has been extensively evaluated. This combination technology would optimize each of the individual technology’s strengths and reduce each of their individual weaknesses.

In this study, a dual cylindrical microwave and ohmic combination continuous flow heater was designed and fabricated to heat treat solid-liquid mixture foods without under- processing the solid particulates. The electric field distribution under microwave and ohmic heating was numerically analyzed; the use of cylindrical microwave cavity was suitable to maximize the electric strength in the combination heater. Thermal profiles of solid-liquid mixtures consisted of chicken and potato particles, and sodium chloride solution (0.5, 1.25, and 2.0% concentration) at different solid fraction 10 and 15% were collected and compared. These profiles were recorded for both individual heating (either microwave or ohmic heating) and combination heating (microwave and ohmic together) until the exit temperature of either solid particles for solution reached 80°C.

In individual ohmic heating, particle size and slat concentration affected temperature differences between the particulates and the solution. In individual microwave heating, the solution temperature lagged behind the particle temperature with salt concentrations up to 1.25%, regardless of particle size and solid fraction; however, a different tendency was observed in the food mixtures including 2% salt concentration. The maximum temperature differences between solid and liquid phases obtained by individual microwave and ohmic heating were 7.1±1.7 and 11.9±2.9°C, respectively. In the combination heating, only small temperature gaps between solid particles and liquid (maximum difference < 3.08°C) at low salt concentrations (up to 1.25%) were observed. A 3D block diagram constructed using the controlled ranges of salt concentration, particle size, and solid fraction estimated by empirical polynomial equations was used to describe temperature similarities between solid and liquid phases when combination heating was applied. This unique continuous flow combination heater has the potential to thermally process multiphase foods with improvements in heat distribution, energy efficiency, and food quality.

Computational modeling for individual microwave and ohmic heating, and the combination heating in a continuous flow system were established. The modeling of the continuous flow system was important for future designing of a production-scale unit. Accurate prediction of temperature distribution in solid-liquid mixture foods was demonstrated using the computational modeling. Numerical modeling of the combination heating technique was a challenge due to simultaneous changes in heat and particulate flow pattern in the continuous flow system. This study employed the COMSOL software which enabled a numerical model to simulate the complicated simultaneous changes during the combination heating technique. The temperature profile of the combination heating was simulated during a lethality test of Escherichia coli K12 inoculated carrot balls. The carrot ball’s exit temperature reached 90°C in 56 seconds, as predicted by computational model. The reliable prediction of particulate temperature in this complicated system can be attributed to the integration of the two-dimensional moving mesh method and the arbitrary Lagrangian–Eulerian (ALE) equation in the COMSOL software.

Results indicated that the combination heating technique has potential to inactivate E. coli K12 in carrot balls with improved lethal activities. The simulated flow patterns and thermal profiles of multiphase foods showed the heating mechanism and the movement of particulates during combination heating. The experimental data were in good agreement with the simulated heating profile of particulates with a maximum prediction error of 5%.