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Improvement of heating uniformity in packaged acidified vegetables pasteurized with a 915MHz continuous microwave system

Koskiniemi, Craig B., Truong, Van-Den, Simunovic, Josip, McFeeters, Roger F.
Journal of food engineering 2011 v.105 no.1 pp. 149
blanching, broccoli, citric acid, cold, containers, continuous systems, conveyors, fiber optics, heat, pH, pasteurization, peppers, salt content, soaking, sodium chloride, temperature
Continuous microwave processing to produce shelf-stable acidified vegetables with moderate to high salt contents poses challenges in pasteurization due to reduced microwave penetration depths and non-uniform heating. Cups of sweetpotato, red bell pepper, and broccoli acidified to pH 3.8 with citric acid solution containing 0–1% NaCl were placed on a conveyor belt and passed through a microwave tunnel operating at 915MHz and 4kW with a 4min residence time. The time–temperature profiles of vegetable pieces at 5 locations in the package were measured using fiber optic temperature sensors. Addition of 1% NaCl to the cover solution lowered microwave penetration into vegetable pieces and decreased the mean temperature in cups of acidified vegetables from 84 to 73°C. Soaking blanched vegetables for 24h in a solution with NaCl and citric acid prior to processing improved microwave heating. Heating was non-uniform in all packages with a cold spot of approximately 60°C at a point in the container farthest from the incident microwaves. More uniform heating was achieved by implementation of a two-stage rotation apparatus to rotate vegetable cups 180° during processing. Rotating the cups resulted in more uniform heating and a temperature of 77°C at the cold spot. This is above the industrial standard of 74°C for in-pack pasteurization of acidified vegetables. The effective treatment involved blanching, soaking for 24h in a NaCl and citric acid solution, and 180° rotation. This work has contributed to a better understanding of the influence of salt addition and distribution during dielectric heating of acidified vegetables using a 915MHz continuous microwave system.