What are the main off-flavours in beer, and what are they caused by?

Without proper knowledge, the member of staff is left unsure whether to offer the customer a refund or a replacement pint, or take the beer off the bar altogether.

What’s more, unless the licensee can identify the specific off-flavour in question, they won’t know whether the problem is with their own handling and dispense of the beer, or whether the beer needs to be returned to the distributor or brewery and a refund demanded.

To help combat these woes, and to find out more about off-flavours, The Morning Advertiser​ headed to a tutored tasting, hosted by industry training body the Cicerone Certification Program​. Here’s what we learnt:

DMS (dimethyl sulphide)​

A compound created from malt, and usually the result of an insufficiently vigorous boil, DMS (or dimethyl sulphide) most commonly gives off-flavour descriptors such as sweet corn or cooked/rotten vegetables. DMS is accepted in some pale lagers, but because it has a very low perception threshold (we can detect it at very low levels), even small amounts can overwhelm lighter styles.

Luckily for pubs and bars, DMS is almost entirely a result of issues in the brewer, and hence if you’re getting high levels of DMS from a beer in your bar, it’s probably worth contacting the brewery and asking about a refund.

Diacetyl                              ​

Diacetyl can come about both during the brewing (fermentation) process, and in the pub, so it’s an important off-flavour for both brewers and licensees to be aware of. The most common descriptors for the flavour are butter, movie popcorn and a ‘slick’ mouthfeel on the beer.

In brewing, diacetyl is created by yeast during fermentation, but should reduce during maturation as the compounds are reabsorbed towards the end of the process. In the pub, diacetyl is likely to come about as a result of bacterial (such as pediococcus) infection in keg draught lines. 

Mild levels of diacetyl are acceptable in some traditional Czech pale lagers and English ales, but an excessive buttery flavour should be investigated to avoid customer complaints.

Acetaldehyde ​

Acetaldehyde is a naturally-occurring compound found in everything from ripe fruit to coffee. Common flavour descriptors for the compound include green or sour apple, while others report aromas of cut grass or even latex paint.

In brewing, the compound is created in the conversion of glucose to ethanol (alcohol) and is usually associated with young, immature beer that has been taken off yeast and racked too early.

Acetaldehyde is also produced by the oxidation of alcohol, such as when beer is exposed to oxygen during traditional cask dispense. Moreover, when aerobic bacteria (such as acetobacter) metabolise the compound into acetic acid, this produces a taste of vinegar and is considered its own off-flavour. This is why it is especially important to turn over cask beer quickly (ideally within three days).

Trans-2-nonenal (T2N)​

A fairly straightforward off-flavour, and one that nearly every consumer is likely to have experienced at some point, is Trans-2-nonenal (T2N). Scientifically speaking, T2N is the enzymatic or non-enzymatic oxidation of lipids and oxidised free fatty acids. If you’ve ever drunk a beer that tastes a little bit papery, or slightly like damp cardboard, it’s likely because of this compound.

T2N is usually simply the result of the impact of time on beer. Old beer that has not been properly handled or stored is especially susceptible to this compound. To reduce the speed at which beer deteriorates, it is important it is kept cold throughout the distribution cycle, right the way through to the point of consumption.

If you’re having issues with papery beer, it may be worth investigating how your beer gets from the brewery to your pub, and how fresh it is.

Lightstruck​

A hugely common off-flavour, and one that occurs almost universally within bottled beer products, is the chemical compound 3-Methyl-2-butine-1-thiol (3MBT). More colloquially this is referred to as when a beer becomes ‘skunked’ and is the result of specific wavelengths of light hitting a bottle, causing the beer within to become lightstruck.

This skunky, grassy and vegetal flavour is especially common among beers that are packaged in clear or green bottles, and can occur in a matter of minutes.

To avoid lightstruck beer, pubs should consider storing their product in a dark, cold place, avoiding displaying it in direct sunlight or near florescent bulbs, and switching to more canned, cask and keg products.

Infection (acetic acid + diacetyl) ​

Bacterial contamination in draught lines and improper cleaning methods are two causes of infected beer in pubs. Two of the most common compounds found in infected beer are acetic acid plus diacetyl, which contribute to common descriptors of butter, vinegar and gastric acid. Not a pleasant combination.

These contaminants can come about as a result of improper brewery practices and sanitation within the brewing and fermentation process, where wild yeasts and bacteria shift the flavour profile of the beer. However, if you’re ordering beer from reputable breweries, these companies should be testing and checking their beer before sending it out, and it is more likely to be a problem associated with serving and dispense.

Thankfully, infections are easily prevented through rigorous and regular cleaning of beer lines and good, sanitary conditions throughout your venue, especially in the cellar.