The conventional heating methods of milk did not cause any significant increase in the trans isomer content, with the exception of milk heated at 63 +/- 1.0 degrees C for 30 min and milk microwaved for 5 min, which were significantly increased by 19 and 31%, respectively. The chemical changes of lipids were generally accelerated with the severity of the heat treatment and duration of storage. The conjugated linoleic acid content of cheese heated in a microwave oven for 5 min decreased by 21%, and microwave heating for 10 min caused a decrease of 53% compared with that of freshly boiled cheese.