Why can’t some plastic bags be microwaved? Composite packaging bags, aluminum foil bags, plastic packaging bags 1. Principle of microwave heating The principle of microwave heating food is to use the strong polar water molecules contained in the food as a medium, which can absorb the energy in the microwave, causing the molecules to vibrate and rub violently. thereby generating heat. Anyone who has used a microwave oven knows that metal or metal containers cannot be heated in a microwave oven. Because metals are also composed of strongly polar molecules. Microwave ovens are physical heating, and the cooking method of food is similar to that of cooking with an open flame. The heat of the food and the retention of nutrients are better than that of cooking with an open flame, and its heating can penetrate into the interior of the food about 5cm. 2. The material properties of the heating container are containers composed of non-polar molecular materials, which basically do not absorb or seldom absorb the energy in microwaves, such as plastic products (polypropylene, polyethylene, etc.) and insulators such as glass and ceramics. They can transmit microwaves without absorbing the energy in microwaves, and can be used as containers or supports for microwave heating, or as sealing materials. 3. The thermal stability of plastics Although plastics do not absorb the energy in microwaves, some plastics become less stable after heating food and are prone to release toxic substances or carcinogens. Only retort-grade plastic packaging bags can be used. Heating in a microwave oven ensures food safety. Generally, the heating temperature should not exceed 120 degrees to prevent the local temperature from being too high during the heating process, thereby releasing harmful substances and contaminating the food. Summary: There is no problem with putting plastic itself in a microwave oven. The problem is that the heat generated by the food will change the properties of ordinary plastic packaging bags and release toxic substances, resulting in food safety problems. Therefore, only bags that are marked for microwave heating can be heated in the microwave.