Effects of packaging, equipment, and storage time on energy used for reheating beef stew

Energy used to reheat 3 kg of a standard beef stew to 74 degrees C was measured to determine (a) the benefits of a retort pouch packaging processing system that keeps food microbially safe at room temperature compared with a system that packages food in a plastic bag that requires refrigerated storage; (b) the most economical form for reheating (in bulk in bags, in bulk out of bags, or in portions); (c) the most economical equipment for reheating (convection oven, infrared oven, microwave oven, compartment steamer, or steam-jacketed kettle); and (d) the influence of storage time (7, 28, or 85 days). Energy used for reheating the retort product was 18,883.7 British thermal units (BTU) compared with 31,035.6 BTU for the plastic bag product. Reheating in portions used 6,857 BTU; reheating in bulk out of bag used 23,419 BTU; and reheating in bulk in bag used 64,247 BTU. The order of least to greatest energy use for equipment was microwave oven, 324 BTU; infrared oven, 5,406 BTU; convection oven, 11,399 BTU; steam-jacketed kettle, 30,713 BTU; and steamer, 51,412 BTU. Storage time in the plastic bag significantly (P less than .05) affected initial product temperature and the energy required for reheating; this was not true for the retort product. Our findings indicate that microwave heating, heating in portions rather than in larger quantities, refrigerated storage of 7 days instead of 28 days, and use of retort pouch products achieve the least energy cost in reheating a product such as beef stew.