All foods change over time although these changes are not always harmful. Food eventually becomes harmful or unpleasant to eat, e.g milk goes sour. Some items even cause food poisoning if you eat them.
To preserve food or make it last longer and extend its shelf life, the enzymes and micro-organisms that cause this must be controlled.
Enzymes are chemical catalysts that are found in all cells. They break down plant and animal tissues casuing fruit to ripen for example.
If enzyme activity is allowed to continue food spoilage will occur.
Oxidisation or enzymic browning
Oxidistion or enzymic browning takes place when fruit and vegetables are exposed to air after being cut. This causes the flesh to turn brown,.
Ways to prevent oxidisation: Example
Adding acid. Add lemon juice to a fruit salad
Blanching Put carrots in boiling water
Preventing contact with air. Put peeled potatoes into cold water until needed