Evaluation of Pilot-Scale Radio Frequency Heating Uniformity for Beef Sausage Pasteurization Process

Abstract

Radio frequency (RF) heating has the advantages of a much faster heating rate as well as the great potential for sterilization of food compared to traditional thermal sterilization. A new kettle was designed for sterilization experiments applying RF energy (27.12 MHz, 6 kW). In this research, beef sausages were pasteurized by RF heating alone, the dielectric properties (DPs) of which were determined, and heating uniformity and heating rate were evaluated under different conditions. The results indicate that the DPs of samples were significantly influenced (p < 0.01) by the temperature and frequency. The electrode gap, sample height and NaCl content had significant effects (p < 0.01) on the heating uniformity when using RF energy alone. The best heating uniformity was obtained under an electrode gap of 180 mm, a sample height of 80 mm and NaCl content of 3%. The cold points and hot spots were located at the edge of the upper section and geometric center of the sample, respectively. This study reveals the great potential in solid food for pasteurization using RF energy alone. Future studies should focus on sterilization applying RF energy and SW simultaneously using the newly designed kettle.

Keywords: RF sterilization; beef sausage; dielectric properties; heating uniformity.

Figure 1

Simplified schematic diagram of sterilization kettle in the RF heating system.

Figure 2

Detailed cross-section view of the sterilization kettle with beef sausage placed in the center and fiber optic sensor inserted in the sample.

Figure 3

The location of fiber optic sensors (1, 2, 3) and four layers (upper, A; middle, B; lower, C; longitudinal, D) in the beef sausage sample for temperature distribution measurement (all measured in millimeters). (a) Detailed diagram of sample. (b) Longitudinal section of sample.

Figure 4

DPs related with frequency of beef sausage at different temperatures (25, 30, 40, 50, 60, 70, 80 and 90 °C): (a) dielectric constant (ε′) as dependent variable; (b) dielectric loss factor (ε″) as dependent variable.

Figure 5

Temperature-dependent DPs of samples at five selected frequencies: (a) dielectric constant (ε′) as dependent variable; (b) dielectric loss factor as dependent variable. RF band included 13.56, 27.12 and 40.68 MHz; microwave frequency included 915 and 2450 MHz.

Figure 6

Time–temperature heating curves of beef sausages (a) and the contour plots of temperature distribution (b) at different electrode gaps (175, 180 and 185 mm) at a sample height of 80 mm and when the temperature was initialized at 25 °C.

Figure 7

Time–temperature heating curves of beef sausages (a) and the contour plots of temperature distribution (b) at different sample heights (75, 80 and 85 mm) under an electrode gap of 180 mm and when the temperature was initialized at 25 °C.

Figure 8

Time–temperature heating curves of samples (a) and the contour plots of temperature distribution (b) with different NaCl contents (1%, 2% and 3%) at an electrode gap of 180 mm, a sample height of 80 mm and when the temperature was initialized at 25 °C.