Chapter 12


(book excerpts)

Fermentation is the heart of the brewing process. The objectives of wort fermentation are to utilize yeast to metabolize wort constituents (sugars, amino acids, etc.) into ethanol, carbon dioxide and other fermentation byproducts (e.g., aromatic alcohols, esters, organic acids, sulfur-containing compounds) in order to produce beer with satisfactory quality and stability. It is these byproducts that have a considerable effect on the flavor, aroma, and other properties that characterize the style of beer. Manipulation of temperature, oxygen levels, and yeast pitching rate as well as yeast strain selection will all dramatically affect the production of aroma and flavor compounds produced during fermentation. For example, the banana and clove flavors common to Hefeweizen are due to by-products excreted from certain yeast strains during fermentation. Fermentation can take 2 to 14 days for ales, and lagers may take longer to complete the fermentation cycle. Another objective of fermentation is to produce yeast crops that can be confidently re-pitched into subsequent brews. After the fermentation cycle is complete, we have what brewers refer to as green beer.

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