Food starts to go off or spoil from the time that it is harvested or slaughtered. The bad flavours and odours and a breakdown in texture show that the food is no longer fit to eat.

In order to slow down the process of going off and to keep foods for longer, a wide range of preservation methods is applied to foods. Most methods of preserving work by removing or controlling the things which help bacteria to grow.

These include:

• temperature control - the use of low or high temperatures
• moisture control - dehydration
• use of chemicals
• physical methods.

Some preservation methods affect the taste, smell, appearance and nutritional value of the food. Sometimes different methods are used together as this can work better.

Low temperatures
Low temperatures slow down the growth of most pathogens and bacteria which spoil food.

Preservation methods using low temperatures include:

• Refrigeration - storage between 0° and 5°C.
• Freezing - storage at -18°C.

High temperatures
Heat is used to destroy both pathogenic and food spoilage bacteria; some heat processes will also destroy spores.

Preservation methods using high temperatures include:

• Pasteurisation - heating at the right temperature for the right length of time can destroy active pathogens and some spoilage bacteria in the food, e.g. for milk, heat at 72°C for 15 seconds.
• Sterilisation - destroys all micro-organisms with temperatures above 100°C.
• Ultra-heat treated (UHT) - destroys micro-organisms without changing the flavour, as it uses high temperature for a very short time.

Preservation methods using high temperatures include:

• Cooking - usually used to improve taste rather than storage time.
• Ohmic heating - a high-voltage electric current through foods destroys micro-organisms.
• Canning - food is heated to the right temperature for the right amount of time to destroy micro-organisms. Oxygen is removed and containers are specially sealed to stop bacteria getting in.

Removing the moisture from food prevents micro-organisms from growing in it. Preservation methods like this include:

• Traditional methods such as sun-drying.
• Commercial methods using hot air.
• Accelerated freeze-drying which uses warm air after food has been quickly frozen.

Many chemical can be used for food preservation because they stop the growth of micro-organisms in different ways. Use of chemicals is controlled by law.

Chemicals commonly used for food preservation include:

• Salt -makes water unavailable for use by micro-organisms.
• Sugar -acts in a similar manner to salt.
• Acetic and lactic acids - lower the pH below the normal growth range of most micro-organisms.
• Benzoic acid and sorbic acid -inhibit mould and yeast growth in acid foods.

Different physical methods of preservation are used, including:

• Modified atmosphere packaging - air around the food product has a reduced oxygen content and increased level of nitrogen and carbon dioxide which will slow down growth of many micro-organisms.
• Vacuum packing - removes oxygen to stop some bacteria growing. Often combined with refrigeration.
• Food irradiation - uses ionising radiation to destroy micro-organisms but not spores.