Low temperatures When the temperature is reduced, the activities of most micro-organisms are slowed down, until they become dormant (inactive), and growth and multiplication cease. Once the temperature is raised, growth and multiplication can start again. Some micro-organisms are resistant to cold temperatures and can continue to multiply and remain active, albeit at a slower rate.
There are three main ways of keeping food at low temperatures: • Chilling • Cook-chilling • Freezing
Chilling Chilling is used for foods such as sandwiches and cream cakes. • Food keeps for a short time • Food is stored between 1°C and 5°C • The low temperature slows down enzyme activity and bacterial growth.
Cook-chilling Cook-chilling is used for ready meals. • Food is cooked and cooled to 0°C to 3°C in 90 minutes or less. • It is stored in a refrigerator at 0°C to 3°C • Products have a shelf life of 5 days
Freezing Freezing is used for prepared dishes, fruit and vegetables. It can be done commercially or at home. • At home food should be kept at -18°C. • Blanching may be necessary for some fruit & vegetables before freezing • Commercially food is kept at a temperature between -18°C and -29°C before being sold. • Food can be kept for long periods of time • Frozen food is transported in special vehicles to keep it at the correct temperature. • Many foods can be frozen successfully
Principals involved in freezing • Ice crystals form inside food which is frozen • Fruits & veg are made up of many cells and these can be damaged by the ice crystals, if they are too large. • The cells rupture if the ice crystal exceeds the size of the cell. Normal cell Cell wall Ice crystal – cell ruptured
Once the food is thawed, its structure will collapse, releasing most of the liquid of which it is composed, because the cells no longer form the framework of the food. • Large ice crystals are formed in food if it is frozen too slowly resulting in a poor texture once thawed. cell on thawing
If food is quick-frozen, the ice crystals are formed rapidly and are much smaller in size. • Quick-freezing is defined as the drop in temp from 0°C to -4°C in 30 minutes in the centre of a pack of food.
Commercial freezing Food can be quick-frozen in several ways: • A blast of very cold air, which cools the food very rapidly. • Close contact with refrigerated plates. The food is passed into a cabinet called a multi-plate freezer, and is rapidly frozen to -18°C • Immersion in a very cold liquid, such as brine or liquid nitrogen.
When liquid nitrogen is used this is known as cryogenic freezing. This is used for foods such as strawberries which do not freeze very well using conventional quick-freezing methods. Liquid nitrogen is extremely cold (it boils at -196C). Food placed in or sprayed with it freezes instantaneously, with tiny ice crystals being formed, thus minimising cell damage.
Chillers & freezers To maintain the temperature of chillers & freezers…. • Regularly monitor the temperature with a probe or digital reader (data logging) • Record the temperature, so it can be used as evidence if required by an EHO • Use sensors & warning lights to warn of a rise in temperature • Keep doors shut • Check and maintain seals around doors • Defrost regularly • Do not overfill
Consumer use Most food poisoning is caused by poor food transportation, storage and use at home. Frozen, chilled and cook-chill foods should be … • Transported home quickly • Stored and cooked according to instructions • Cook-chill dishes must be eaten within two hours of cooking • Do not reheat leftovers • Do not refreeze frozen foods
Accelerated Freeze Drying Accelerated freeze drying (AFD) is a combination of freezing and drying. Food is quick frozen and then placed in a vacuum under reduced pressure. It is then heated and the ice is changed to vapour, leaving the food dry. AFD food is light and easily restored by adding water. E.g. dried soups. The nutritional value, colour and flavour are not significantly changed. These food products can be kept at ambient (room) temperature.