Milk should be heated gently and slowly. Excessive heat can result in a scorched flavour and/or a film of protein on the top of the milk. You can avoid these problems in one of two ways:
The double-boiler method: Place milk in the top of a double boiler or in a heat-proof bowl set over a saucepan of simmering water. Heat just until bubbles form around the edge and steam begins to rise from the milk. Stirring frequently will prevent a film layer from forming.
The microwave method: Pour milk into a microwave-safe container and microwave on medium-high (70%) power, stirring every 15 seconds, just until steam begins to rise from the milk. To scald milk for custards or yogurt, heat 250 mL (1 cup) on HIGH for 2 to 2 1/2 minutes.
Tip: Room-temperature milk is better in recipes than cold milk, so use the microwave to take the chill off milk that is fresh from the refrigerator. Microwave 250 mL (1 cup) for 40 to 45 seconds* on high (100%).
Note: Times are based on a 700-watt microwave oven. Adjust cooking times to suit your oven.
*Tip courtesy of Dairygoodness.ca