Freezing is a method of food preservation that involves the lowering of temperatures to delay food spoilage. The freezing method extends the shelf life of a food by inactivating enzyme activity and microorganism activity. When lowering temperatures, you can either chill or freeze foods. Chilling temperatures range between 0°C -4°C. Foods that you can chill include salads, dairy products, stock, cooked dishes, sauces etc. On the other hand, freezing temperatures are below −2 °C. Foods that can be frozen include: vegetables e.g. broccoli, beans etc, meat, poultry, fish etc
There are essentially 2 broad freezing processes constituting slow freezing and quick freezing. Slow freezing processes take more than 30 mins whilst quick freezing takes less than 30 mins to achieve.
Factors considered when choosing the type of freezing method:
- Product dimension
- Food product component
- Packaging
- Thickness of pieces
- Freezing rate
Methods of freezing
Plate freezing-This is a slow-freezing process. The foods whether packaged or fresh are put on frozen plates that are cooled by refrigerants. The food is then frozen when it comes into contact with the plates.
Air freezing-This is among the oldest methods of freezing that uses cool temperatures about -18 to -40C. Examples of air freezing include: fluidized bed freezing, air blast freezing and tunnel freezing
Liquid immersion freezing -With this method, certain liquids such as liquid Nitrogen, liquid Ammonia, etc. are utilized as refrigerants. They act as the freezing components in contact with the food. They emit heat to yield high heat transfer, and allow for rapid freezing.
Defrosting
When ready to eat, some foods such as vegetables do not need defrosting. Alternatively, others such as meat, fish etc require to be defrosted prior to cooking. Advisably, the defrosting should be done within the 0°C -4°C temperature range to prevent microbial growth. Room temperatures or temperatures above between 4°C -60°C are not recommended for defrosting as they favour the development of micro-organisms. Defrosting can be done in the refrigerator at temperatures between 0°C – 4°C to inhibit bacteria growth. Other alternatives of defrosting include the use of a microwave oven or cold water. Immediately you defrost a food, you should cook it to prevent it from being exposed to room temperature as this will again favour microbial growth. It is also best to ensure that the food is fully defrosted to cook uniformly.
Most importantly, freezing does not completely destroy all micro-organisms, some bacterial activity may actually just be slowed down and then restored after defrosting. Other micro-organisms i.e. viruses remain active or infective when a food is frozen. Proper measures should be used when defrosting and thawing to ensure food is safe for consumption.