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.