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United States Department of Agriculture

Agricultural Research Service


Location: Food Science Research

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

item Koskiniemi, Craig
item Truong, Van-den
item Simunovic, Josip
item Mcfeeters, Roger

Submitted to: Journal of Food Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/9/2011
Publication Date: 3/24/2011
Publication URL:
Citation: Koskiniemi, C.B., Truong, V.D., Simunovic, J., McFeeters, R.F. 2011. Improvement of heating uniformity in packaged acidified vegetables pasteurized with a 915 MHz continuous microwave system. Journal of Food Engineering. 105:149-160.

Interpretive Summary: Microwave technology has the potential to preserve foods with higher quality and with more efficient use of energy than conventional heating technologies. This occurs because food can be heated rapidly and because, with proper engineering, microwave systems convert a high percentage of the input energy into heat within the food itself. This research shows the distribution of heating in acidified vegetables covered with a brine solution in single serving plastic cups using a continuous flow commercial microwave heating unit. A method to improve the uniformity of microwave heating to quickly attain the required pasteurization temperature when the product contains up to 1% salt is demonstrated. This work provides an initial step toward the use of microwave technology to pasteurize acidified vegetables to make them shelf stable at room temperature.

Technical Abstract: 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 915 MHz and 4 kW with a 4 min 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 °C to 73 °C. Soaking blanched vegetables for 24 h 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 24 h 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 915 MHz continuous microwave system.

Last Modified: 06/26/2017
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