Food decomposes like any other organic matter. Bacteria, fungi, oxygen and moisture all convert plant and animal matter back to basic organic soil nutrients--the lowest link on the food chain. Decomposition renders food unappetizing or even dangerous for human consumption. So we spend a lot of energy trying to understand food decomposition and how it can be prevented or delayed.
Our food in the food chain
Food decomposes like any other organic matter. Bacteria, fungi, oxygen and moisture all convert plant and animal matter back to basic organic soil nutrients -- the lowest link on the food chain. Decomposition renders food unappetising or even dangerous for human consumption. So we spend a lot of energy trying to understand food decomposition and how it can be prevented or delayed.
Bacteria and fungi colonise the food
Food would not decompose if not for bacteria and fungi. Collectively called saprophytes, these microbes are the primary decomposers of all dead organic matter. They are airborne or may be transferred to food by contact.
Surface damage allows microbes into food
Some foods, like fruit, have a smooth, dry skin that is a natural barrier to bacteria and fungi. Undamaged fruit may not decompose appreciably for several days, even at room temperature. But if the skin is broken by bruising or cutting, microbes have an entrance into the moist interior of the fruit. Decomposition then occurs rapidly.
Fungi are vital to decomposition of plant matter
Often the first microbes to colonise dead organic matter, fungi are able to decompose plant cell walls, or cellulose, making the nutrients in plant cells more accessible to bacteria. Certain fungi species inhabit the digestive systems of animals, like cattle and termites, that have diets high in cellulose, helping to digest grasses and wood for the animal. On our food, we notice fungus as mould on the exterior food surface.
Bacteria and fungi feed on the food
All dead organic matter is potential food for bacteria and fungi. But the molecules of plant and animal matter are too large to pass through the microbes' cell membranes. The molecules must be broken down first. Enzymes secreted by bacteria and fungi break large food molecules into smaller molecules that can easily be absorbed by the microbes. The fungi and bacteria use the food energy to divide and multiply.
Food decomposition requires oxygen and water
In addition to a food source, bacteria and fungi also require water and oxygen in order to live. A short supply of either will limit the number of microbes that can live on the food. This is the principle behind vacuum packing and freeze drying as methods of preserving food. However, some bacteria, like Clostridium botulinum (the bacteria that cause botulism), can decompose food without oxygen, so long as water is present. In this process, called anaerobic decomposition, dangerous toxins are produced. Dented canned goods are a prime breeding ground for the botulism bacteria.
Favourable temperatures speed decomposition
Temperatures between 4.4 and 60 degrees Celsius (40 and 140 degrees Fahrenheit) are ideal for food decomposition. At these temperatures microbe and enzyme activity is optimised, making food energy more available to the bacteria and fungi. Consequently they grow and multiply quickly. However, food does decompose at lower temperatures, even below freezing, though more slowly. This is why food does not keep indefinitely when stored in the refrigerator. In cooking, food-borne bacteria are killed at temperatures between 63 and 82 degrees Celsius (145 and 180 degrees Fahrenheit). The exact temperature depends on the particular bacteria species.
Chemical conditions inhibit decomposition
High sugar, salt or acid content creates a hostile environment for fungi and bacteria. Candying, salt curing and pickling are methods commonly used to preserve food.