Development of methodology for assessing the heating performance of domestic microwave ovens C. James,* M. V. Swain, S. J. James & M. J. Swain MAFF Advanced Fellowship in Food Process Engineering, FRPERC, University of Bristol, Churchill Building, Langford, Bristol BS40 5DU, UK (Received 10 June 2001; Accepted in revised form 3 May 2002) Summary There is a need for standard methods of testing domestic microwave ovens that relate to their reheating performance with chilled convenience meals. The investigations reported here have produced a simple procedure for comparing the three most important reheating characteristics of a domestic microwave oven: its ÔtrueÕ power; heating variability; and repeatability. The tests are relatively simple to do, and require only a few additional items of equipment to those that a laboratory performing existing output power tests would already possess. Three identical tests are required: one with a liquid test material, one with a solid and one with a combination of the two components. One additional stage is done with the liquid test material. The data produced can be reduced to three numbers, which are measures of the true power, variability and repeatability of the oven. A further simple analysis, which weights the relative importance of each factor to the consumer, would produce a single value for the oven’s relative reheating performance. Ovens with unusual or extreme performance characteristics can therefore be identified easily. Keywords Characterization, chilled convenience meals, power output, temperature variability, testing. Introduction Investigations in the last 10 years have revealed considerable variability in the ability of different models and types of domestic microwave oven to reheat food (Burfoot et al., 1991). Studies have also shown that Listeria monocytogenes can sur- vive in some of the cooler areas of chilled foods after reheating in a microwave oven (Walker et al., 1991). They have also revealed a large degree of non-repeatability during reheating. A typical reheating instruction for a chilled food would specify Ôreheating for 3.5 min in a 750 W microwave ovenÕ. Traditionally, reheating times for ovens with different microwave powers were arrived at by modifying the time to produce a similar total energy input into the food. During investigations by Burfoot et al. (1991), it became clear that one of the reasons for the apparent differences in heating effect between microwave ovens was the lack of an agreed standard to define power output. Companies producing ovens have used up to ten different systems for defining power output. Since September 1990, microwave oven manufacturers have adopted a single standard for measuring the power output for microwave ovens to be sold in the UK. This standard, International Electrotechnical Commission (IEC), 1988, uses a 1 L water load and the test is made using a cold oven in defined ambient conditions. Studies have clearly shown that power output of a microwave oven is influenced by the ÔfoodÕ load in the cavity. Investigations in the USA (O’Meara, 1989) looked at power output into load sizes ranging from 50 to 2000 g. They showed that the heating rate of a water load in a 625 W microwave oven varied by a ratio of approximately 25:1, depending on the exact cavity loading. Studies by FRPERC (James et al., 1994) have shown that, in addition to the effect of load, its position within the cavity, the length of time the oven has been in *Correspondent: Fax: +44(0)117 928 9314; e-mail: chris.james@bristol.ac.uk International Journal of Food Science and Technology 2002, 37, 879–892 879 Ó 2002 Blackwell Science Ltd