Microwaves cause the molecules present in the food to vibrate. The intense motion results in increased friction of the particles, which is associated with heat generation. Microwaves can be used to either cook or reheat food. Microwaves are electromagnetic waves whose frequency range is between infrared waves and broadcast waves. They are much lower in energy than X-rays. For this reason, microwaves themselves do not cause chemical changes in food. However, if specific safety and cooking instructions for microwave ovens are not followed, both overheating and insufficient heating can lead to changes in food that are harmful to health. If the food is exposed to high heat for a long time – due to longer defrosting, cooking or reheating times, as well as keeping it warm for longer periods – and the microwave is set to a high power level, it is quite possible that nutrients and vital substances (macro- and micronutrients) will be lost [1, 3, 4, 5, 6 ]. In particular, the fat-soluble vitamins A, D, and E in meat and fish, as well as essential fatty acids, are affected because meat and large pieces of fish require a longer time than vegetables or dairy products to be cooked or fully heated. Furthermore, the texture of the food suffers under these circumstances. Pieces of meat become tough and leach out as the high temperature removes liquid from them. Carbohydrate-rich foods with high water content also suffer from fluid removal and leaching. The food shrivels up and tastes bland, as loss of flavor is also to be expected. Consequently, high levels of water-soluble and heat-sensitive vitamins disappear. Vitamin C, as the most susceptible of the vitamins to light, oxygen and heating, is largely destroyed.
The microwave is often used to reheat and prepare food. Inadequate heating – by heating too quickly and unevenly – increases the risk that pathogens such as Salmonella or Listeria are not sufficiently killed in the less heated zones or may even multiply there.
To avoid both insufficient heating and overheating, the following should be observed:
- Basically, observe the cooking times specified in the operating instructions.
- For the heating of the food use special microwave-suitable dishes.
- If possible, use food with the same thickness or chop large pieces.
- Cover the food with loose-fitting lid or cover hood or microwave-suitable foil.
- Heat food in all areas to 70 °C for several minutes.
- Prepare food such as poultry or dishes containing fresh eggs for a long time at a lower power level, perform several temperature checks.
- Stir or turn over all dishes in between, let stand for a short time for even heat distribution.
For smaller quantities of food, the use of the microwave is useful because the food can be prepared with little addition of water or fat and with little energy.
