Cooling of hot food

Cooling of hot food

Rapid cooling of cooked foods to be chilled or frozen is extremely important to prevent spore germination or the multiplication of any surviving pathogens or those introduced. It is usually recommended the food is cooled below Cooling 10°C in less than 1.5 hours to minimise time in the danger zone. Practically, only blast chillers can achieve this. A more pragmatic standards adopted by the Food and Drugs Authority in America is to cool joints from 60°C to 21°C in two hours and from 21°C to 7°C within a further four hours. Ice water baths cool faster than cold air.

Large quantities of liquids such as gravies and stews which are intended for reheating, create ideal conditions for anaerobic spore-forming bacteria to cause problems. Cooking activates the spores which germinate during long, slow cooling. Large number of vegetative bacteria may be produced and mild rewarming will not destroy them. Food poisoning is likely. Cool spots may occur during cooking or reheating because of the volume of liquid, the use of large, tall pans, absence of lids, failure to stir and the base of the pan being bigger than the heat source. Even though the surface of the liquid may be boiling uneven heat distributions sets up currents which generate cool spots where bacteria may survive and even multiply.

When cooling food, the following basic rules should be applied:

  • minimise bulk – the smaller the size of the food, the faster it will cool. The maximum joint size recommended is usually 2.5kg;
  • maximise surface area – square or rectangular containers present greater surface areas than round containers. Shallow containers should be used instead of deep containers;
  • maximise differential – the greater the difference in temperature between the product and its surroundings, the faster it will cool; and
  • air flow from a fan about 10 cm from the food will increase heat transfer by a factor of 3 (food should be covered).

Although there is a potential hazard if joints of meat are cooled too slowly, the risk may be smaller than that from post-cooking contamination, e.g. slicing with a dirty knife. Hot food must be completely cooled before wrapping in impermeable materials, such as cling film, to avoid condensation and mould growth. Wrapping also slows down the cooling process.

Blast chillers 

This is equipment designed specifically for the rapid chilling of food. Usually, chilled air at 2°C to 7°C is circulated in a cabinet around the product. Some blast chillers use the vapour from liquid nitrogen and solid carbon dioxide. Some equipment can be set to hold foods after chilling, acting as a normal refrigerator.

Alternatively, if suitable blast-chilling equipment is not available, hot food can be cooled rapidly in an other wise empty freezer and taken out when safe temperatures have been reached, for storage in a refrigerator.

For cooling liquids, ice cream freezers can be used and, for very large quantities, vacuum chillers are available.