It is very important that food containers which have been designed to package frozen or chilled foods such as ice cream or margarine, are not exposed to high temperatures in a microwave oven. The low melt temperatures of these plastics may result in migration of undesirable contaminants into the food or in physical disintegration of the containers themselves. As migration is more likely to occur into hot fatty foods, glass containers are preferred to plastic for heating them. Container shape may also influence the way a food reacts to reheating. Circular or oval containers help prevent edges of the food burning because energy absorption occurs evenly around the edges.
Square containers tend to encourage burning on the edges of a product.
Shallow containers, because they provide a large surface area, are a good choice for heating foods. Packaging for microwavable meals has been especially designed for use at high temperatures. Susceptors consist of a plastic film metallised usually with aluminium and laminated to paper or paperboard to hold the required shape. They are designed to enhance browning and crisping of a product and to improve its texture. For example without the use of susceptors, pizzas heated in a microwave oven would be soggy.