No satisfactory definition exists and features of commonly encountered yeast such as alcoholic fermentation or growth by budding are absent from a substantial minority of yeasts. Yeast is generally accepted as fungi, which are predominantly unicellular, there are various borderline “yeast-like fungi” which are difficult to classify.
The current classification is still basically that by Kreger-van Rij (1984), although subsequently updated to recognize the discovery of newly discovered species or changes of attitude to of Taxonomic tests.
Classification of Yeasts
Classification of filamentous fungi (molds) is largely in the form of vegetative growth and the nature of the spores, if formed. Physiological properties, as widely used in bacteriology, are not used in identification of molds, but are used for yeast identification, mainly to distinguish species. Identification of yeast genera is by the shape of the cells, and also spores, if formed. Identification of species is mainly by biochemical properties: growth on various carbon or nitrogen compounds, fermentation of sugars, etc. At one time growth tests for identification included over 30 carbon or nitrogen sources, but this has now been reduced to 18.
The Genus Saccharomyces
There are at least 1,000 separate strains of the species.Saccharomyces cerevisiae. These strains include:
5. Laboratory cultures
There is a problem classifying such strains in the brewing context; the minor differences between strains that the taxonomist consider as inconsequential are of great technical importance to the brewer. Yeast taxonomists have assigned all strains employed in brewing to the species Saccharomyces cerevisiae. They are referred to in the scientific/technical literature as S. Cerevisiae (ale type) and S. Cerevisiae (lager type)
These strains have been distinguished on the basis of their ability to ferment the disaccharide melibiose (glucose-galactose).
Strains of S. cerevisiae (lager type) produce the extracellularenzyme a-galactosidase (melibiase) and are therefore able to utilise melibiose.
Strains of S. cerevisiae (ale type) do not produce a-galactosidase and are therefore unable to utilise melibiose.
Ale strains can grow at 37oC, whereas lager strains can not.
Difference between ale & lager yeast
In practice the principal differences involve the way the yeast is cropped:
1. Ale yeast is skimmed from the top of a fermenting vessel
2. Lager yeast settles to the bottom of the vessel and is cropped from
The other major difference is in the fermentation temperatures used:
Ales tend to have a warmer, short fermentation followed by a short conditioning or maturation time while Lagers ferment cooler and generally take longer to ferment; they are traditionally followed by a long maturation time.
Ale and lager yeast belong to the species Saccharomyces cerevisiae – but in brewing they are referred to as S. uvarum (lager) and S. cerevisiae (ale).
Industrial Uses of Yeast
Yeasts are the most important group of microorganisms commercially exploited. This importance is based largely on the capacity of certain yeasts to effect a rapid and efficient conversion of sugars into ethanol and carbon dioxide or biomass. Saccharomyces species and Candida utilis are of major importance. New genetic improvement techniques have permitted the development of novel strains with improved fermentation properties.
Yeasts can also be used for:
a.) Potable ethanol (beer, wine and spirits) production;
b.) Industrial ethanol (solvent, sterilant and fuel) production;
c.) Baker’s yeast biomass;
d.) Carbon dioxide production;
e.) The production of heterologous proteins