Methods of food preservation

2. Refrigeration, freezing and freeze drying

This involves cooling food to low temperatures to slow down enzyme activity as well as the growth of microorganisms.

Refrigeration is cooling the environment artificially to bring the temperature below ambient temperature, usually ranging from 4 to 10°C. Foods commonly stored using cooling methods are usually highly perishable products, like meat, dairy, and fish, thus increasing their shelf-life.

Freezing means cooling food to temperatures of -18°C for longer storage periods. At temperatures below 0°C the water inside the food forms ice crystals, which means some foods cannot be frozen, because the melting ice will either cause structural breakdown of the food (like lettuce, cucumbers, soft herbs, dairy products, etc.) or be soaked up, leaving the food soogy, like crumb based cakes, etc.

Food type	Recommended freezing time frames (if your freezer runs at -18^oC)
Uncooked meat (roasts, steaks, chops etc)	4-12 months
Uncooked mince meat	3-4 months
Cooked meat	2-3 months
Cured meats (ham & bacon )	1-2 months
Uncooked poultry (chicken, turkey etc)	9-12 months
Cooked poultry	4 months
Fish-raw & cooked	2-4 months
Frozen dinners ( e.g. lasagne, pizza, shepherd’s pie)	3-4 months
Soups, stews and casseroles	2-3 months
Milk	Up to 1 month
Butter	Up to 3 months
Grated cheese	Up to 4 months
Vegetables	Up to 12 months
Fruit	Up to 6 months
Eggs – either separate the yolk and white, or beat the eggs before freezing	Up to 6 months
Cakes and baked goods without icing	6-8 months
Bread	Up to 3 months
Raw pastry	6 months

Freeze drying is also known as lyophilization. During this process, the moisture in the food is removed (sublimated) under frozen and vacuum conditions, which causes a direct transition of water from its solid (ice) to vapor, omitting the liquid state, and then desorbing the water from the “dry” layer. The process is used for heat-sensitive foods.