Lesson 4 - Fermentation and Beer Flavour
Yeast and fermentation have an enormous impact on beer flavour. The most obvious changes during fermentation are sugar utilisation and the resultant formation of alcohol and carbonation. This is only a part of the story. Many thousands of compounds disappear and similar numbers of new ones are formed in fermentation as a result of yeast growth. This lesson describes what these are, how and why they are formed, and what effect they have.
Direct Effects of Yeast on Beer Flavour
We will now look at the various metabolites produced by yeast in fermentation that have an impact on beer flavour.
These are the major groups of products of yeast metabolism when growing on wort that are important for beer flavour:
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Alcohol (ethanol)
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CO2
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Organic acids
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Aldehydes
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Fatty acids
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Higher alcohols (fusel alcohols)
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Esters
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Sulphur compounds
The most abundant compounds are alcohol and CO2, which give beers warming characters and mouth-tingle respectively. Typically, these concentrations are 3-10%, but usually 4-5%, alcohol by volume (abv) for alcohol and 2-3 volumes (4-6 g/L) CO2. A previous lesson (Yeast Biochemistry) explained why these compounds are formed by yeast when it grows on wort and we do not cover it again here.
The other compounds are formed at much lower concentrations but many are very flavour-active. They represent a huge group of individual compounds and the precise spectrum and concentration produced in fermentation varies with each individual yeast strain. The concentrations of many of these compounds are formed at concentrations close to their flavour thresholds, so choice of strain is very important.
The next section considers the pathways used by yeast to make these compounds. It also explains why yeast not only forms them but uses precious energy to release them into beer.