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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.