Freezing is the process of subjecting foods to temperatures below 0°C or 32°F. Foods are generally frozen to create the final form (see Vanilla Ice Cream), to preserve for later use, or to freeze-dry a food. Depending on its molecular content, i.e. whether the item has a high concentration of salt, sugar, or alcohol, a food may not become solid at the freezing point of water. Home freezers should be kept at or below 0°F to compensate for the differing freezing points of various foods as well as temperature fluctuations that occur through use of the freezer.
Freezing for later use
To preserve a food by freezing, it should be well wrapped or stored in a freezer-safe container to prevent freezer burn. Generally, vegetables should be blanched or cooked before being frozen. Some fruits, meat and poultry can be frozen without being cooked beforehand. Other fruits, like apples or pears, benefit from being cooked before being frozen. Certain foods, such as potatoes and tofu, have dramatically altered textures after being frozen. The longer a food is to be stored, the more careful the cook needs to be in selecting and properly using the correct packaging. Vacuum bags are one option for the home cook who intends to store food for more than three months.
Storing dry foods inclined to become rancid, such as rarely used nuts, or whole-grain flour, in the freezer is recommended by some cooking authorities. Ingredients should be brought to room temperature, in their sealed packing, before using to prevent condensation from affecting the food's dryness.
Freezing for form
Freezing changes the food's texture into a solid mass, as can be seen when water turns into ice. Incorporating air into the mixture to be frozen can help create a lighter mass, as when cream is whipped before being used to create a frozen mousse.
Freeze-drying is used in commercial facilities. The food is brought to a temperature below the point at which the solid (frozen) and liquid states can occur, allowing moisture to sublime off into the atmosphere. Freeze-dried foods are shelf-stable at room temperature, and may retain better flavors and textures than conventionally dried foods.
National Center for Home Preservation University of Georgia Extension Service