Background
Not long ago, the term fermentation was strictly confined to the context of anaerobic respiration However, the situation is complicated by the ability of many microorganisms, including many widely-used yeasts, to switch between aerobic and anaerobic respiration, according to the availability of oxygen. Further, where several species of microorganism are simultaneously involved in a process, aerobic and anaerobic pathways may well occur side by side. For these and other reasons, the term fermentation is nowadays widely used in a very much less precise sense, to describe any process, anaerobic or aerobic which includes the breakdown of food by microorganisms, more especially when there are products other than carbon dioxide and water. Thus, many commercial production activities utilising microorganisms are regarded as fermentation processes even though the microbial activity is largely or entirely aerobic. The commercial production processes for mycoprotein and many vaccines are aerobic and yet are described as fermentation. However, especially in the context of industry, many scientists now distinguish between 'aerobic fermentation' and 'anaerobic fermentation'.

A definition of fermentation considered appropriate to GCSE students and which avoids these complications is provided on page 27 of the biology syllabus. It states: fermentation is any process during which microorganisms use an external food source to obtain energy, chemically changing the medium. The term fermenter is defined there also as a vessel used to cultivate microorganisms for fermentation.

(The term bioreactor refers to any vessel in which a biological reaction or process takes place; this might be, for example, a straightforward enzymic hydrolysis in the absence of microorganisms. Thus, although a fermenter is a type of bioreactor, the two terms are not synonymous.)

The earliest fermenters, used in hot climates, included such 'vessels' as stomachs removed from goats, filled with milk and hung up to produce fermented foods such as kefir. A ball of dough can also be considered as a fermenter.

The requirements in modern industrial fermenters are of course very different and the discussion of such fermenters should emphasise the need to maintain a variety of factors within close limits; these may include oxygen, carbon dioxide, pH and the concentration of nutrients In this way, optimum conditions for metabolism and reproduction of a specific microorganism are provided.

Reasons for the paramount importance of aseptic precautions, including initial sterilisation of fermenters and of nutrients, should be appreciated in terms of avoiding both competition and contamination of the product Some form of agitation, either by mechanical stirring or by means of air jets, is also necessary (See the section on mycoprotein.)

Students will not be required to distinguish between open and closed fermenters (continuous and batch processes).