A method of operating a microwave oven to determine the correct cooking time for a food item placed within the oven. A food package having a thermochromic element which changes color over a predetermined temperature range is placed in the microwave oven. Microwave energy is supplied into the oven. The food package is rotated within the oven cavity and the color of the thermochromic element is monitored as it rotates past an opto-electronic device or detector capable of detecting the color changes of the thermochromic element such that a plurality of signal pulses are generated having a value corresponding to the color of the element. The supply of microwave energy into the oven is terminated when the rate of change between successive pulses decrease to a predetermined value.