Micro-organisms that produce a single main by-product are termed homofermentative
whereas those that produce mixed products are heterofermentative. Fermentations can be
classified into those in which the main products are organic acids and those in which
ethanol and carbon dioxide are the primary products. Lactic acid and ethanolic
fermentations are among the most important commercial fermentations and details of the
metabolic pathways that are used to produce these products are readily available (for
example Stanier et al., 1976). Many fermentations involve complex mixtures of microorganisms
or sequences of microbial populations which develop as changes take place in
the pH, redox potential or substrate availability.