9.17B Explain the effects of temperature, water content and oxygen…

The idea

Decomposition is the breakdown of dead organic material by microorganisms such as bacteria and fungi. These microorganisms release enzymes that digest the food, then respire the products, so the rate of decomposition depends on how well their conditions support growth. In food preservation we deliberately slow decomposition. Temperature: warm conditions speed up enzyme activity and microbial growth, so decomposition is fastest in the warm range; refrigeration slows it and freezing stops growth almost entirely, while very high temperatures (cooking, canning) kill microorganisms. Water content: microorganisms need water, so drying, salting or adding sugar removes available water and slows decomposition. Oxygen availability: many microorganisms are aerobic and need oxygen, so sealing or vacuum-packing limits their respiration and slows decay.

What earns the marks

• Temperature: higher temperatures (within the warm range) increase enzyme activity and microbial growth, raising the rate of decomposition; cooling or freezing slows or stops it.
• Water content: reducing available water (drying, salting, sugaring) slows microbial growth and enzyme action, lowering the rate of decomposition.
• Oxygen availability: removing oxygen (sealing, vacuum-packing) limits aerobic microorganisms, slowing decomposition.
• Each factor must be linked to the needs of decomposing microorganisms and to a named food-preservation method.
• Recognising that preservation works by making conditions unfavourable for microorganisms, so decomposition is slowed.

Common mistakes and how to put them right

• Thinking that freezing kills all microorganisms.

  Put it right: Freezing mainly stops or greatly slows their growth and enzyme activity; many survive and resume activity on thawing.

• Assuming all microorganisms need oxygen.

  Put it right: Some are anaerobic, so removing oxygen slows aerobic decomposers but does not stop all decomposition.

• Confusing 'water content' with 'adding water'.

  Put it right: Preservation reduces available water, for example by drying, salting or adding sugar, which draws water away from microorganisms.

Exam tips

• For each factor, state the direction of effect (faster or slower) and give the reason linked to microbial growth or enzyme activity.
• Name a specific preservation method for each factor rather than describing factors in the abstract.
• Check the question asks about food preservation, not composting, before choosing an option.