Teachers Centre
More Links
science home
HOME
howdoesfoodfreeze.gif (10296 bytes)
 

Why do we freeze food?

No food will keep indefinitely in its natural form. All natural foods are 'alive' and subject to processes of decay and deterioration the same as any living material. Food deteriorates partly due to chemical changes in living protoplasm (e.g. cell enzymes) and partly due to changes caused by minute organisms which get into the food from outside (such as bacteria). When we preserve food we stop or slow down these processes.

 supermkt.jpg (18833 bytes)
Food Science and Technology

Food is made of living cells containing a great deal of water. Because of this high water content many foods will freeze solid at about -5ºC. If freezing is done slowly, ice crystals will actually form during the process and they will become so large that they will break through the cell walls and affect the texture of the food. When the food is returned to room temperature (thawed) much of the liquid will then drain out taking some of the minerals and vitamins that give the food its unique taste and texture. If items are frozen rapidly the ice crystals formed are so small that the damage to the cellular structure of the food is minimal and when thawed it will look and taste similar to its original form.
What alternatives to preserving by freezing or chilling can you think of?
What are the advantages and disadvantages of each?
Refrigeration and freezing are only temporary ways of preserving food. The micro organisms present in the food are not destroyed, their growth is merely slowed. Once the food has thawed the micro organisms will begin to grow again.
The table below gives an indication of how long different food items could be kept in a commercial freezer:
Vegetables peas, cauliflower, beans up to 1 year
Fruit oranges, blanched peaches, soft fruits in sugar up to 1 year
Meat joints and chicken portions up to 1 year
White Fish cod, plaice etc. 6 - 12 months
Oily Fish sardines, tuna up to 4 months
Bread   1 week
Pastry   3 - 6 months
Cooked Foods   up to 2 months

 
FACTS and FIGURES
  • For long term storage a consistent temperature of -25ºC to -30ºC must be maintained
  • The more rapidly food is frozen the better its condition on thawing will be
  • Domestic (home) freezer compartments are usually kept at about -18ºC, sufficient to store foods for up to 3 months
  • Commercial food freezers can reduce the temperature of an item from +25ºC to -18ºC in just 24 hours
If you want to find out more about food and freezing, try some of these sites:

 
Teachers Centre |Feedback| More Links | HOME