Definitive Guide to Freezing Food
In the Food Industry, proper food preservation is essential to avoid food spoilage and damage to health. Therefore, preservation by freezing is often used, but why? The answer is simple, freezing is able to stop bacteriological and enzymatic processes that alter food products.
What is food freezing?
Food freezing is the solidification of water contained in food at a temperature below the melting point (0°C) where the proportion of water becomes ice.
In order to define the time that the freezing process should last in the different foods, the initial and final temperature, the temperature of the refrigerant and the coefficient of product transfer, among others, must be taken into account.
In the following table we show you the recommended temperature and time according to the food:
|FOOD PRODUCT||PERCENTAGE WATER||FREEZING TEMPERATURE||CONSERVATION TIME|
|Fruits||87-95%||-0,9 to -2,7 ºC||up to 8-10 months|
|milk products||87%||-0,5 ºC||up to 8 months|
|Vegetables||78-92%||-0,8 to -2,8 ºC||up to 10 months|
|fish||65-81%||-0,6 to -2 ºC||up to 3-6 months|
|Eggs||74%||-0,5 ºC||up to 6 months|
|meat||55-70%||-1,7 to -2,2 ºC||up to 12 months|
Why is it necessary to quick-freeze foods?
The food industry needs to subject a wide variety of foods to very rapid cooling so that macro ice crystals do not form and break up the structure and appearance of the product. If they are vacuum packed they can be kept for months in freezing chambers at temperatures between -18ºC and -20ºC.
It is important then, to know the difference between conserving and freezing. To freeze a food, a temperature of -30ºC is necessary and to preserve foods already frozen, a minimum temperature of -18ºC is required.
Types of freezing
There are three types of freezing depending on the equipment and speed with which the products are frozen:
• Slow Freezing: Used when large products are stored in large warehouses or when large capacity chambers are required to accommodate large quantities. Maintains quality and does not alter taste or smell. The freezing time is long.
It is the most common for freezing chambers in products such as lamb, beef, pork, etc. In Bernad Refrigeration we are specialists in this type of chambers that assure correct conservation.
• Medium freezing: It is done at a freezing speed of 1-5 cm/h in cold air tunnels at 20km/h and -40ºC
• Quick freezing: Used in professional kitchens and in the food industry to quickly freeze and stop the growth of bacteria after freezing or defrosting. It consists of an abrupt cooling where the maximum crystallization is achieved in less than four hours.
The process concludes after achieving thermal stabilization at -18ºC, then the product must be kept in cold storage at low temperatures.
Food freezing techniques
The techniques that are carried out for freezing food can be:
• By contact: A cold surface that extracts heat is in contact with the product. Plate freezers with thermoconducting metal sheets are used. Seafood and fish are usually frozen in this way.
• Cryogenic Methods: Cryogenic fluids such as Master Chef’s famous liquid nitrogen or carbon dioxide and Freon are used to replace cold air to achieve the freezing effect.
The food is introduced directly into these substances, which when evaporating take the heat with them. This process is also called ultra freezing as no more than two hours are required. The disadvantage is its high cost.
• By air: A current of cold air extracts the heat from the product until it reaches the final temperature. The use of air blasts at low temperatures and high speeds in a controlled space achieves rapid freezing. It is usually used for small products.
At Bernad we use the air freezing method in our freezing chambers and tunnels using ecological gases such as CO2, ammonia or glycol to produce this process.
In short, regardless of whether the food is frozen quickly or slowly, each food needs to be treated correctly and preserved according to its quality and cost.
Disadvantages of freezing food
• Although they do not lose their nutritional properties, it is true that frozen foods can lose some of their flavors if they have been in this state for a long time.
• You have to know that each type of food has its own characteristics, so you cannot proceed in the same way with all of them. Depending on their nutritional characteristics, the time and mode of freezing can be different.
• To store food in the freezer, it must be covered with plastic, never in a glass container as it could break with the ice.
• Be careful in the defrosting process because bacteria can be reactivated if it is not done correctly.