9.17B - 9.19B - Decomposition and decay rates

9.17B, 9.18B — What controls decomposition?

Decomposition, or decay, begins when dead biological material or waste is broken down. Microorganisms such as bacteria and fungi release digestive enzymes onto the material, absorb the small soluble products and use some of them in their cells. Faster enzyme action, growth and reproduction usually mean a faster rate of decomposition.

Three environmental factors control how active these decomposers can be.

Temperature. Decomposer reactions are enzyme-controlled. At a low temperature, molecules have less kinetic energy, so enzyme and substrate particles collide successfully less often and decomposition is slow. Warming increases the rate up to a favourable temperature, but the pattern cannot continue forever: at very high temperatures enzymes denature and many decomposer cells are damaged or killed, so the rate falls.

Water content. Water is needed for enzyme-controlled reactions and lets dissolved substances move into and within cells. If biological material is too dry, decomposers grow and act more slowly. In a compost heap, excessive water can also slow aerobic decomposition because water fills pore spaces that would otherwise contain air.

Oxygen availability. Oxygen allows aerobic respiration in decomposer cells. Respiration releases energy for cellular processes, including growth and reproduction, so a good oxygen supply usually supports rapid aerobic decomposition. Reducing oxygen slows aerobic decomposers, but it does not sterilise material: some microorganisms can respire anaerobically and decomposition may continue more slowly.

9.17B — Slowing decomposition in food

Food preservation aims to make conditions less favourable for decomposers. A method may change one factor, but preservation is often stronger when several factors are changed together.

Refrigeration and freezing lower temperature. Enzyme-controlled reactions and microbial reproduction therefore slow; freezing also leaves less liquid water available. These treatments do not necessarily kill every microorganism, so activity can increase again when food warms.

Fresh, moist food usually decomposes faster because water is available for microbial reactions, transport and growth. Drying lowers water content and slows these processes. Salt or sugar can also make water less available to microorganisms even when some water remains in the food.

Vacuum or reduced-oxygen packaging lowers oxygen availability. Less aerobic respiration can occur, so less energy is released for the growth and activity of aerobic decomposers. This can slow decay, but low oxygen alone does not stop anaerobic microorganisms; it should not be described as sterilisation.

9.18B — Speeding decomposition in compost

Composting uses the same three variables for the opposite purpose: it manages conditions so organic waste decomposes quickly. The fastest useful conditions are favourable rather than extreme.

A compost heap should be warm, not overheated. As microorganisms respire, energy is released and some is transferred to the surroundings as heat, warming the heap. This can increase enzyme activity and decomposition up to a favourable range. If the heap becomes too hot, decomposer enzymes denature and cells die, so decomposition slows.

The material should be moist, not waterlogged. Too little water limits enzyme-controlled reactions and microbial growth. Too much water fills spaces between pieces of material, reducing the oxygen that can diffuse through the heap.