Microwaves are one of the great inventions of the 20th century. They are popular because they save time and energy in the kitchen by cooking food more quickly and using less power. But how often have you tucked into that plate of leftovers only to find the middle still cold?
The trouble with microwaves is that they don’t heat food evenly and cold spots can occur. These cold spots allow any pathogenic bacteria present to survive, which may cause foodborne illness. There have been several cases overseas of foodborne illness caused by inadequate heating of food in microwaves and it’s not hard to see why there might be a problem.
Microwaves don’t cook food from the inside out – most of the energy is absorbed just below the surface of food to about 30mm. Thicker foods are cooked by conduction, when the heat moves from the outside in. To cook food thoroughly without overcooking the outside layers, it needs to be cooked longer at lower power.
Uneven heating is the reason why manufacturers recommend standing times – to allow the food to heat all the way through. Many microwaveable meal packs carry the instruction to stir the food part way through the cooking process. Items that can’t be stirred, such as lasagne, should be given standing time to allow the whole product to reach a uniform temperature.
Some foods, such as meat and poultry, are more at risk of harbouring pathogens and special care needs to be taken when cooking or reheating these foods. In particular, cooking whole, stuffed poultry in a microwave oven is not recommended. While the outside may be cooked, the inner parts of the bird and the stuffing might not have enough time to reach the temperature needed to destroy the pathogenic bacteria.
For these foods and others where it is hard to check the internal temperature, it is important to use a food thermometer. Test the food in several places to be sure it has reached the recommended safe temperature to destroy bacteria and viruses that could cause foodborne illness.
How evenly the food will heat also depends on the thickness of portions and on the composition and moisture content of the food. Differences in the food, such as density and thickness, or the amount of fat or sugar, will affect how well the heat is transferred within the food.
Cold spots in food can also be caused by interference of the microwaves as they bounce around in the oven. This is why microwave ovens now have turntables, to move the food around and away from any interference zones.
Research has also demonstrated that frozen food does not heat as well as defrosted food in microwave ovens. The water molecules in frozen food are immobilised by ice crystals, which prevents them from rotating freely and hence heating. So frozen foods need much more time to heat in a microwave.
If you don’t know the wattage of your microwave oven, and can’t find the information anywhere, try the ‘Time-to-Boil’ test to estimate the wattage.
Measure a cup of water in a 2-cup glass measure. Add ice cubes; stir until water is ice cold. Discard ice cubes and pour out any water more than 1 cup. Set the microwave on high for 4 minutes, but watch the water through the window to see when it boils (adding a pinch of coffee can make it easier to see).
If water boils in:
• less than 2 minutes, it is a very high wattage oven 1000 watts or more
• 2½ minutes, it is a high wattage oven about 800 watts or more
• 3 minutes, it is an average wattage oven 650 to 700 watts or more
• more than 3 minutes or not by 4 minutes, it is a slow oven 300 to 500 watts.
Do not move the container until the water has cooled for 30 seconds. Liquids can ‘superheat’ and may ‘erupt’ when the container is moved.
The following practices are recommended for good microwave cooking:
1. Rotate food to help prevent cold spots. If your microwave oven doesn’t have a turntable, stop the oven during cooking and rotate the food item by 90 degrees every few minutes.
2. Stir food frequently (if possible) to help distribute heat.
3. Let food sit for at least two minutes after microwaving to allow more time for the heat to spread throughout the food.
4. Follow the manufacturer’s instructions for cooking times – don’t shorten cooking times. Models have different power levels and their power may decrease as ovens age.
5. Use a food thermometer, especially with meat, and check the temperature in several places to be sure there are no cold spots.
6. Cook large pieces of meat on a lower power for a longer period of time to allow more time for heat to reach the centre.
7. Don’t continue eating if the product seems cool – stop and reheat it.
8. Cover food to keep it moist; the steam generated helps distribute heat.
9. Cook meat, poultry, egg casseroles and fish immediately after defrosting in the microwave oven because some areas of the frozen food may begin to cook during the defrosting time. Do not hold partially cooked food to use later.
10. Use a container suitable for use in microwave ovens.