System and method for detecting changes in food load characteristics using coefficient of variation of efficiency

A method of controlling cooking in an electromagnetic cooking device having an enclosed cavity in which a food load is placed and a plurality of RF feeds configured to introduce electromagnetic radiation into the enclosed cavity to heat up and prepare the food load, the plurality of RF feeds configured to allow measurement of forward and backward power at the plurality of RF feeds, the method comprising:

• (a) causing the generation of RF excitations at a specified frequency and phase shifts from each of the plurality of RF feeds for a predetermined time period;
• (b) during the predetermined time period:
• measuring and analyzing the backward power at the plurality of RF feeds to calculate efficiency, determine a coefficient of variation in the efficiency, and monitor the coefficient of variation to identify possible changes in a volume of the food load;
• and
• (c) repeatedly performing steps (a) and (b) until such time that a change is identified in the volume of the food load based on changes in the coefficient of variation over a cooking cycle, wherein the RF excitations are at different frequencies or phase shifts in accordance with a heating strategy for other predetermined time periods, wherein the coefficient of variation is determined by:
• determining a mean and a standard deviation of the efficiency during the predetermined time period;
• and computing the coefficient of variation in the efficiency based on the mean and standard deviation.