Heating Uniformity Study on Packaged Foods Processed with Radio Frequency Energy

Abstract

A second generation pilot-scale RF heating system (25 kW, 27.12 MHz) for processing multiple 2.7-kg food trays was developed at Washington State University. Heating experiments of mashed potato samples were conducted to study heating uniformity and heating rate as well as their influence factors and to improve the understanding on the RF heating technology. Salt content of sample, type of film for sealing trays, shape, size and positions of electrode plates, placement of RF feeding, tuning feeds and auxiliary coils were found to be the major factors influencing heating patterns. The RF applicator setup, with wide plates (390 x 1510 mm) being placed 35 mm above and 10 mm below (or 6 mm above and 10 mm below) the heating tube surfaces, and with threepoint RF feeding, three 4-turn coils, front tuning feeds to the end sides of bottom plate, and rear tuning feeds to the back side of top plate, provided most uniform heating for the mashed potato sample trays with 0.8% (or 1.3%) salt content, 85.9% water content, and aluminum lids. The average temperature difference of four trays heated at the same time and the temperature difference among the four trays were18oC (or 13oC) and 3.5oC (or 4oC) for the 0.8% (or 1.3%) salt-content sample, respectively, when the overall average temperature reached 55oC ( or 48oC). In addition to RF power, the electrical conductivity of circulating water and the salt content of sample were the two major factors affecting the heating rates of the samples. The heating rate increased with decreasing the water conductivity and/or the salt content.