If you put cold things, especially items with a high moisture content, in a microwave oven and then turn it on, several things can happen:
Uneven heating: Microwaves work by generating electromagnetic waves that excite water molecules in the food, causing them to vibrate and generate heat. Cold or frozen items have uneven water distribution, and some parts may heat up faster than others. This can result in uneven heating, where some areas of the food become hot while others remain cold.
Thermal shock: Placing very cold items, such as frozen foods or chilled liquids, directly in a microwave can cause thermal shock. This happens when the extreme temperature difference between the frozen/cold parts and the rapidly heating parts of the food leads to stress, which can result in cracking or even breaking of the item.
Steam buildup: When heating items with high water content, such as fruits or vegetables, steam can quickly build up within the food. If not vented properly, the steam pressure can cause the food to explode or burst, making a mess inside the microwave.
Delayed heating: When microwaving a cold or frozen item, it may take longer to reach the desired temperature than if it were at room temperature. This means you might have to increase the cooking time, which could lead to overcooking in some areas while other parts remain undercooked.
To mitigate these issues, it's best to allow cold or frozen items to thaw or reach room temperature before microwaving them. If you need to heat something quickly, you can use the microwave's defrost setting or set a lower power level to gradually heat the food without causing thermal shock or uneven cooking. Additionally, it's crucial to follow any specific guidelines provided in the food packaging or microwave oven's user manual to ensure safe and efficient heating.