Decomposition and food preservation

Ecosystems and material cycles · Indicator species and decomposition · note 2 of 4

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Decomposition and food preservationSpec 9.17B

In short

The rate of decomposition depends on temperature, water content and oxygen availability, because microorganisms need warmth, water and usually oxygen to grow and respire. Food preservation slows decomposition by changing these conditions: fridges and freezers keep food cold, drying or adding salt or sugar removes water, and vacuum packing or sealed cans keep oxygen out.

Food goes bad because microorganisms (bacteria and fungi) feed on it, grow and respire. The rate of decomposition depends on temperature, water content and oxygen availability. Food preservation slows decomposition by changing these conditions.

Factors affecting decomposition and how preservation uses them
Factor	Effect on decomposition	Preservation method
Temperature	Warm conditions speed up enzyme reactions in microorganisms, so decomposition is faster. Cold slows these reactions.	Fridges and freezers keep food cold. Heating (for example in canning) kills microorganisms.
Water content	Microorganisms need water to grow and reproduce. Less water means slower decomposition.	Drying, or adding salt or sugar, which draws water out of microorganisms by osmosis
Oxygen	Most decomposers need oxygen for aerobic respiration. Less oxygen means slower decomposition.	Vacuum packing removes oxygen, and sealed cans keep oxygen out

Exam tip:

Always link the factor to the microorganism. For example: low temperature, so enzymes in the microorganisms work more slowly, so they grow and reproduce more slowly.

Written and checked against the Edexcel GCSE Biology (1BI0) specification · Updated October 2026

Frequently asked questions

What are indicator species?

Higher tier Indicator species are organisms whose presence or absence gives evidence of the level of pollution in an area. In water, bloodworms and sludgeworms suggest pollution, while freshwater shrimps and stonefly nymphs suggest clean water with plenty of oxygen. For air, the species of lichen present and blackspot fungus on roses show the level of sulfur dioxide.

How does food preservation slow decay?

Food preservation slows decay by making conditions poor for microorganisms. Fridges and freezers keep food cold, so enzymes in the microorganisms work more slowly. Drying, or adding salt or sugar, removes water, which microorganisms need to grow. Vacuum packing and sealed cans keep out oxygen, which most decomposers need for aerobic respiration.

What conditions make compost decompose fastest?

Compost decomposes fastest when it is warm, moist and well aerated. Warmth speeds up the decomposers' enzymes up to an optimum, although very high temperatures denature them. Decomposers need water, but waterlogged material has little air. Turning the heap provides oxygen, so the decomposers can respire aerobically and decomposition is faster.

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