The term "fermentation" does not apply only to alcoholic fermentation but to a wide range of reactions, brought about by enzymes or micro-organisms. The anaerobic respiration of glucose to alcohol or lactic acid was described as a form of fermentation.
Micro-organisms which bring about fermentation are using the chemical reaction to produce the energy which they need for their living processes. The reactions which are useful in fermentation biotechnology are mostly those that produce incompletely oxidized compounds.
The micro-organisms are encouraged to grow and multiply by providing nutrients such as glucose, with added salts and, possibly, vitamins. Oxygen or air is bubbled through the culture if the reaction is aerobic, or excluded if the process is anaerobic. An optimum pH and temperature are maintained for the species of microbe being cultured.
Yoghurt and cheese have been made by herdsmen for thousand of years. Both products result from the fermentation of milk by bacteria.
Initially these bacteria came from natural sources but since there would be a mixture of different bacteria in different conditions, the products were often unpredictable.
Today, the micro-organisms are carefully controlled so that the wanted product is assured. Milk from cows, sheep or goats may be used to prepare yoghurt. On a commercial scale the fat and protein content of the milk is adjusted and the milk is homogenized. This is a mechanical process which breaks up the fat droplets and prevents them from separating out. The milk is then pasteurized.
The pasteurized milk is then fermented by adding a "starter culture". Throughout the process, the materials and containers must be kept in a sterile condition which is free from micro-organisms.
The basic processes for cheese production are the same as for yoghurt. The composition of the milk is adjusted; it is homogenized and pasteurized.