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Beer Fermentation
(selected excerpts)Fermentation is the process by which fermentable carbohydrates are converted by yeast into alcohol, carbon dioxide, and numerous byproducts. The byproducts have a considerable effect on the taste, aroma, and other characteristic properties of the beer. Fermentation is dependent upon the composition of the wort, the yeast, and fermentation conditions.
Wort composition, as discussed in previous chapters, influences fermentation by the presence and concentration of various nutrients, pH, and degree of aeration and temperature. These factors can affect the rate of fermentation, the extent of fermentation, the amount of yeast produced, and the quality of beer produced.
Traditionally, there are two types of beer yeast, based on their physical behavior: ale yeast (the "top-fermenting" type, Saccharomyces cerevisiae) and lager yeast (the "bottom-fermenting" type, Saccharomyces uvarum). Top-fermenting yeast flocculate and rise to the top of the fermenting wort producing a stable yeast head, while bottom-fermenting yeast flocculate and settle on the bottom of the fermenter. However, some modern ale strains are selected for use in cylindroconical fermenters because they are poor head formers, the yeast crop being harvested from the base of the fermenter. Furthermore, some ale strains work equally well in both traditional and cylindroconical fermenters, forming a yeast head in a traditional vessel and settling to the cone at the end of fermentation in the conical vessel.
The factors that affect fermentation conditions are time, wort temperature, volume, fermenter design, pressure, agitation, and currents in the wort.
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