Some bacteria and fungi cause rotting. This is very useful since it results in nutrients being returned to the soil. These nutrients can then be reabsorbed by plants. Saprophytes gets their energy by breaking down organic substances in dead animal or plant material. This can be quite impressive when the heat generated in a pile of rotting horse manure causes “steam” to rise out of the dung heap.
Here are some examples of saprophytic decay:
- The compost heap:
Bacteria and fungi rot kitchen waste (eg potato peelings)in the compost heap. This releases nutrients that will be used by vegetables growing in the kitchen garden. If the compost heap is too wet, the fungi do not get enough oxygen and the bacteria take over: you will end up with an evil smelling slimy heap that is no good for the kitchen garden. If it is too dry, the bacteria and fungi are unable to do anything at all. To complete the production of compost, worms and other small animals eat the rotted material releasing nutrients such as nitrates and phosphates.
Yeast is a fungus that can rot fruit even when there is no oxygen available. The anoxic respiration of yeast produces alcohol: wine is the result of rotting grapes, beer is produced by rotting grain, and cider comes from rotting apples.
- Bread making:
Yeast will produce lots of carbon dioxide when it rots (ferments) organic materials. So as yeast ferments the sugar mixed in with flour, it produces carbon dioxide. This is the gas that makes the dough rise.
Various bacteria and fungi are used in the process of cheese making. These saprophytes give the cheese its flavour. The acids produced during cheese making help to preserve it. Moulds may grow on the surface of the cheese, but these can be easily scraped off. The right fungi and bacteria give cheese the flavours we like, and the wrong ones give it an unpleasant taste.
The bacteria in yoghurt produce lactic acid, hence its sour taste. This acid preserves the yoghurt.
Saprophytic decay (fermentation) can be useful because it produces something we need: nutrients for the kitchen garden, alcohol to drink, carbon dioxide to make bread rise or beer frothy, lactic acid to preserve cheese and yoghurt, and to produce tastes that we like.
Saprophytic decay (rotting) can be a problem by making things mouldy. If you leave some damp shoes or clothes in a heap on the floor, they will soon go mouldy.