Microbes and food 4. Food spoilage - preserving food
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4.4 Stopping the rot - heating and cooling
Milk on sale photo
Picture 4.4a All of this milk has been pasteurised.

Heating
Heating foods will kill all microbes – depending on the temperature. Most microbial cells will die at a temperature of 100 ºC. However, some bacterial spores will survive this and need temperatures around 130ºC to kill them. Cooking may kill all microbes in food but won’t get rid of any toxins they have already produced. The heat treatments used in food production include pasteurisation, sterilisation and canning.

Pasteurisation
Pasteurised liquids have been heated for a short time (to around 72ºC for about 15 seconds) and then cooled quickly. This kills most, but not all, of the harmful and spoilage microbes without affecting the flavour. The storage life is extended as a result. Fruit juices, beer, wine and milk are often pasteurised.

Sterilisation
This should kill all the microbes and their spores.

Some foods (like milks, soups and sauces) are heated to around 132ºC for a short time. The sterilised food is then put into sterilised containers (such as bottles or jars). These are then hermetically sealed so that no air or opportunistic microbes can get in.

Canning gives foods like meat, fish, fruit and vegetables a shelf life of many years. The product is sealed in a metal container and heated to an appropriate temperature and rapidly cooled. This process kills off the bacteria.

Freezer photo
Picture 4.4b A freezer will stop the growth of microbes. But make sure the temperature is set correctly.

Cooling
Refrigeration
Keeping foods at below 5ºC slows down the growth of most bacteria and fungi. However, some microbes have adapted to grow in chilled conditions. Refrigerating food doesn’t kill microbes or completely stop their action. So refrigerated foods have a limited shelf life and must be consumed fairly quickly.

The storage temperatures of many chilled foods are set by UK food safety laws.

Freezing
Freezing foods to below -18 ºC stops microbial growth but it does not kill the organisms. Once the food has thawed, the microbes swing back into action and the food must be treated as though it were fresh: it should be consumed quickly.


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