Lesson 4 - Fermentation and Beer Flavour

Yeast is responsible for the formation of green beer (beer at the end of fermentation). This process involves a huge number of metabolites, or beer-flavour compounds. This lesson explains what yeast is doing, how these compounds are produced, and what impact they have on flavour.

Yeast influences beer flavour in many ways, and the effects can be good or bad. We must therefore manage yeast activity to get the result we want. We can do this more effectively if we understand how and why yeast affects beer flavour.

To begin this lesson let's go through some of the effects that yeast has on beer flavour.

Direct effects:

• Removal of various aldehydes – such as 3-methylbutanal, 2-methylbutanal, and 3-methylthiopropionaldehyde – in early fermentation that collectively are undesirable and described as ‘worty’.

• Conversion of wort into green beer (discussed later).

Indirect effects:

• Viable cells are very good at scavenging oxygen and, where they are present, they will rapidly make conditions anaerobic, which is good for beer flavour stability. This is good for beers that contain live yeast cells such as traditional lager and cask and bottle-conditioned beers.

• Hop alpha acids bind to yeast cell walls and when the yeast is cropped after fermentation some bitterness is lost. Hopping rates need to be increased to allow for this loss.

• A lot of yeast cells die during fermentation; this is perfectly normal. When the cells lyse they release their contents into beer and, although this is not entirely proven, some of these compounds might have beneficial effects on flavour, especially on less obvious characteristics like beer ‘drinkability’.

• If the yeast death rate is a lot higher than normal, proteases are released that destroy beer foam proteins and stale ‘yeast bitten’ flavours are formed.

Although we need to keep all of these in mind, this lesson concentrates on the flavour effects that happen when wort is converted into green beer.