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Yeast – agent of fermentation


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When top-fermenting yeast is fermenting vigorously, it creates a near-impenetrable blanket of foam atop the beer.

By: Stephen Beaumont


Fermentation is the process by which fermentable sugars are converted to alcohol and carbon dioxide by yeast, but these micro-organisms do much more than just that. Each of the literally hundreds, if not thousands of known brewing yeasts will impart different flavours and characters to beer.


Much of the effect of yeast depends on which of two broad families it belongs to – Saccharomyces cerevisiae, known as top-fermenting or ale yeasts, which generally yield fruity characteristics, or Saccharomyces carlsberensis (reclassified as Saccharomyces uvarum), known as bottom-fermenting or lager yeasts, which tend to cause a beer to develop greater crispness. (see Brewing Basics: Fermentation.)


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While yeast is still allowed to work in open fermenters in some breweries, most modern operations use closed conical fermenters such as this one.




Beyond the general family of yeast, individual strains will produce different flavours and aromas during fermentation. The group of species generally used to ferment Bavarian-style wheat beers, for example, will impart clove-like and/or banana-like aromas and flavours to the beers in which they are used, while other ale yeasts might instead produce spicy or plum-like aromas and flavours.

Certain beers, most famously Belgian lambics, will be fermented by wild yeasts and bacteria such as Brettanomyces and Lactobacillus, which ferment aggressively to produce very dry, sometimes tart-tasting beers.


See also: