Food processing

Processing method

Principle of preservation and effect on micro-organisms

Preparation methods:
Washing
Sorting
Slicing, chopping, mixing, mincing, etc.

No preservative action. No destruction of micro-organisms (some processes such as chopping or mixing may actually promote growth of micro-organisms by making the food more available for them to grow on)

Heating:
Blanching
Boiling
Pasteurizing
Canning/bottling
Concentrating*
Extruding*
Baking/roasting
Frying*

Heat kills all types of micro-organisms and destroys the ability of naturally occurring enzymes to act.
Mild heating (such as blanching and pasteurization) kills some but not all micro-organisms. Different types of micro-organisms have different degrees of resistance to heat. Other types of more severe heating, such as canning, kill most micro-organisms.

Removing heat:
Freezing*
Chilling

These processes slow down the rate at which micro-organisms can grow and enzymes can act, but processing does not destroy them. Freezing also turns water to ice so that it is not available for micro-organisms to use. Freezing therefore has a greater preservative effect than chilling.

Removing water:
Drying
Distilling
Other processes*

Water is needed for micro-organisms to grow and for enzymes to act and drying therefore prevents this. However, many types of micro-organisms are not killed by lack of water and they can grow again when food is dehydrated.
Water is also removed by other processes (*) which has a combined effect in preserving the food.

Chemical:
Fermenting to produce acids or alcohol
Adding acids (pickling), sugar*, salt*(curing) or preservatives.

Micro-organisms are killed by high concentrations of alcohol, salt or sugar.
Different types of micro-organisms have different levels of resistance to these chemicals and in general moulds and yeasts are able to grow whereas bacteria cannot.