Honey should not be heated rapidly, over direct heat. Basically, the hotter you heat it, the more potential for reducing nutritional value. Excessive heat can have detrimental effects on the nutritional value of honey. Heating up to 37°C (98.6 F) causes loss of nearly 200 components, part of which are antibacterial. Heating up to 40°C (104 F) destroys invertase, an important enzyme. Heating up to 50°C (122 F) for more than 48 hrs. turns the honey into caramel (the most valuable honey sugars become analogous to sugar). Heating honey higher than 140 degrees F for more than 2 hours will cause rapid degradation. Heating honey higher than 160 for any time period will cause rapid degradation and caramelization. Generally any larger temperature fluctuation (10°C is ideal for preservation of ripe honey) causes decay.
-John Skinner, University of Tennessee