Chapter 4

Brewers Yeast

(book excerpts)

Yeast is one of the most important ingredients in brewing beer responsible for metabolic processes that produce ethanol, carbon dioxide, and a whole range of other metabolic byproducts that contribute to the flavor and finish of beer. There are literally hundreds of varieties and strains of yeast. In the past, there were two types of beer yeast: ale yeast (the "top-fermenting" type, Saccharomyces cerevisiae) and lager yeast (the "bottom-fermenting" type, Saccharomyces pastorianus, formerly referred to as Saccharomyces carlsbergensis or Saccharomyces uvarum). Top fermenting yeasts produce beers that are more estery, fruity, and sometimes malty, whereas bottom-fermenting yeasts give beers a characteristic sulphurous aroma. Some other notable differences also include fermentation temperatures and flocculation characteristics. Top-fermenting yeasts are used for brewing ales, porters, stouts, Altbier, Kolsch while bottom-fermenting yeasts are used for brewing lagers such as Pilsners, Dortmunders, Marzen, and Bocks are fermented with bottom-fermenting yeasts.

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