In microwave cooking, susceptors are built into paper packaging of certain foods, where they absorb microwaves which penetrate the packaging. This process raises the susceptor patch temperature to levels where it may then heat food by conduction or by infrared radiation.
Conduction heating occurs with good thermal contact between the susceptor and food. Because of the lower temperatures there is less browning, but more than if there were no susceptor at all.
If there is an air gap between the susceptor and food, the susceptor will heat to a much higher temperature, and at these higher temperatures, will radiate strongly in the infrared. This infrared radiation then shines onto the food below or next to the susceptor, causing a "broiling" type effect due to lower ability of infrared to penetrate foods, vs. microwaves. Conversion of some microwave energy to infrared is particularly useful for foods which require a large amount of crust-browning from infrared, such as frozen pies.
Design and use
Susceptors are usually made of metallised film, ceramics or metals. The susceptor is the reason products meant to be browned via susceptor-generated thermal radiation carry instructions to microwave the food while still inside its packaging. Susceptors meant to heat foods by direct conduction in places where less browning will occur may be seen in the gray lining of packaging directly holding the food, and in good contact with it. A typical example of the latter is the paper susceptor-lined dish directly holding a microwaveable pot pie or casserole. Susceptors built into packaging create high temperatures in a microwave oven. This is useful for crisping and browning foods, as well as concentrating heat on the oil in a microwave popcorn bag in order to melt it rapidly. Among the first microwave susceptors marketed were those from the mid-1980s in a product called McCain Micro Chips by McCain Foods. It consisted of a susceptor sheet which cooked French fries in a microwave oven. These sheets are currently used in several types of packaging for heating and cooking products in microwave ovens. Care in package design and use is required for proper food safety. A "crisping sleeve" is a device made of paperboard and affixed with a susceptor used both as a rigid container to support the food items within and to focus heat on the foodstuff. They are generally intended for a single use. Hot Pockets is an example of a product which uses crisping sleeves.
