ABSTRACT

Interest in brewing yeast centers around its different strains, and there are thousands of unique strains of Saccharomyces cerevisiae. These strains encompass brewing, baking, wine, distilling, and laboratory cultures. There is a problem classifying such strains in the brewing context; the minor differences among strains that the taxonomist dismisses can be of great technical importance to the brewer. Saccharomyces, Latin for sugar fungus, is the name first used for yeast in 1838 by Meyen, but it was the work of Hansen at the Carlsberg Laboratory in Denmark during the 1880s that gave us the species names of S. cerevisiae for head-forming yeast used in ale fermentations 227and S. carlsbergensis for nonhead-forming yeast associated with the lower temperature range of lager fermentations. Historically, lager yeast and ale yeast have been taxonomically distinguished on the basis of their ability to ferment the disaccharide melibiose. Strains of lager yeast possess the MEL genes, produce the extracellular enzyme α-galactosidase (melibiase), and are able to utilize melibiose, whereas ale strains that do not produce α-galactosidase are therefore unable to utilize melibiose. Traditionally, lager is produced by bottom-fermenting yeasts at fermentation temperatures between approximately 5°C and 15°C (maximum growth temperature is approximately 34°C) and, at the end of fermentation, these yeasts flocculate and collect at the bottom of the fermenter. Top-fermenting yeasts, used for the production of ale at fermentation temperatures between approximately 15°C and 26°C (maximum growth temperature is 37°C or higher), tend to be somewhat less flocculent, and loose clumps of cells adsorbed to carbon dioxide bubbles are carried to the surface of the fermenting wort. Consequently, top yeasts were traditionally collected by skimming from the surface of the fermenting wort, whereas bottom yeasts were collected, or cropped, from the fermenter bottom. The differentiation of lagers and ales on the basis of bottom and top cropping has become less distinct with the advent of vertical bottom fermenters and centrifuges. There is much greater diversity among ale strains than lager strains.