Lesson 4 - Fermentation and Beer Flavour

This section explores aldehydes and ketones. These form a big group of diverse compounds which share the common feature of having a carbonyl group (carbon and oxygen atoms joined by a double bond).

Aldehydes

Aldehydes can arise in beer via the action of yeast but many are also formed during wort production. At least two hundred are found in beer and as a group their presence at high concentration is not desirable. Many are highly flavour-active and are associated with beer staling.

High levels in beer can be related to sulphur metabolism; sulphur dioxide (SO₂) binds to aldehydes to form adducts, and in this form they cannot be attacked by the enzymes that would normally break them down. One aldehyde, E-2-nonenal, is a potent beer-staling compound which produces a ‘cardboard’ character. It is formed by the oxidation of unsaturated fatty acids, usually in mashing. In adduct form with SO₂ it is not volatile enough to exhibit this stale flavour. Under the right conditions, yeast produces SO₂ from sulphate in the wort, which improves beer flavour stability.

Wort aldehydes include 3-methylbutanal, 2-methylbutanal, and 3-methylthiopropanaldehyde. These contribute to undesirable ‘worty’ flavours and are removed by yeast in early fermentation via the actions of a number of NAD- or NADP-linked reductases.

Aldehydes are the last intermediate before the formation of ethanol and higher alcohols and so can arise via amino acid metabolism and from pyruvate via glycolysis.

Acetaldehyde

Acetaldehyde has a ‘grassy’ flavour and in beer has been reported to have a flavour threshold of 1.1-33 mg/L. Concentrations should fall to levels below the flavour threshold in late fermentation but occasionally do not. The reasons are poor fermentation management, high levels of wort oxygenation, high pitching rates with poor quality yeast, too much oxygen, and high fermentation temperatures. All of these can adversely affect the conversion of acetaldehyde to ethanol (alcohol dehydrogenase) or to acetate (acetaldehyde dehydrogenase).

The following table shows the concentrations of some aldehydes found in beers. We can see that the favour threshold values decrease with an increase in chain length.

Table: Concentrations of aldehydes found in typical beers together with their flavour-threshold values.

As a group, ketones are not major fermentation products and many are formed from breakdown products of hops. VDKs are the exception, especially diacetyl; it is formed by yeast at relatively low concentrations but it has a massive impact on beer flavour because of its low flavour threshold.