Last week, Clapham Junction’s We Brought Beer served a group of beer geeks and experienced homebrewers cans of Coors Light. Don’t panic – this was all part of an off-flavours course, where the American pale lager offered the ideal blank canvas for tasting and identifying aromas that are undesirable in beer. Throughout the evening, we were guided through some of the most common off-flavours, spiking samples of the bland lager to comprehend how each one manifests itself in a pint.

Drinking bad beer won’t be for everyone, but this course proved to be an intensely insightful session. The off-flavour kits available on the market are often used for in-house training or studying for a Cicerone qualification – and they’re a costly investment for a single drinker, prices spiraling upwards into hundreds of pounds. An off-flavours course gives a collective group the chance to discern and identify that pungent taste or funky aroma without breaking the bank. While some have pointed out that the best training would stem from spiking a beer that you’d actually drink on a regular basis – i.e. a beer with more flavoursome or robust characteristics – using a bog-standard American pale lager is a ideal starting point for the uninitiated palate and nose.

Some off-flavours are vile, others are subtle. We learned that many can be tolerated in small amounts – this can be linked to the style of beer, where they might go overlooked in, say, a darker or hoppier drink. Some can be attributed to a single ingredient – and there are only four in beer, making it all the more apparent how each one plays a vital role in the final product. 

To keep our palates and noses sharply attuned to detect each nuance, we had a glass with a control beer, an untainted Coors Light, to remind us what characteristics are desirable and intentional in the beer. We experienced 9 spiked beers each, ranging from the most common off-flavours to some interesting style markers:

We experienced 9 spiked beers each, ranging from the most common off-flavours to some interesting style markers:

Dimethyl Sulphide (DMS)

On the nose, our group detected sweetness, mustiness and something vegetal. On the palate, an intense sweetcorn flavour. DMS is common in lagers and tends to be more pronounced in the aroma than in the taste. This is attributed to the malt and is a by-product of mashing and fermentation so will be present in a certain degree in all beers; it’s caused by S-Methyl Methionine (SMM), a chemical that is produced during the germination and kilning process of malt. High levels of DMS can be avoided with a longer boil, where it evaporates, or a quicker cooling period for the wort. DMS tends to be tolerated in Czech lagers, where it’s not necessarily considered undesirable in small amounts. It’s therefore important to ask whether that sweetcorn (or tomato sauce to some) taste should be there.


On the nose, we detected butter and butterscotch. On the palate, it was butter alongside slickness in the mouth. This is an undesirable off-flavour in lagers (or anything without a strong hop profile), but can be acceptable in bitters. Diacetyl is a natural by-product of the yeast reacting during fermentation, when the yeast is unable to reabsorb it, so it remains in the beer. There’s high flocculating yeast, where yeast clumps together and drops to the bottom of a fermentor tank too early, making the beer overly sweet. This can be avoided by using medium flocculating yeast of good quality to ensure that it behaves as it should.

Ethyl Hexanoate

On the nose, there was a whiff of apple cider or wine. On the palate were hints of aniseed and fennel. This is an ester flavour (usually fruity or sweet) and a by-product of fermentation, produced by yeast and therefore present in all beer to a certain extent. It’s deemed an off-flavour if detected in a high concentration. It can be mitigated with cooler fermentation temperatures.

Isoamyl Acetate

Very fruity on the nose with flavours of banana and bubblegum, and sweet on the palate, Isoamyl Acetate is another ester flavour produced by the yeast in fermentation. With higher fermentation temperatures and faster fermentation periods, this will be present in higher concentrations. This has qualities that are prevalent (and celebrated) in Belgian beers, where it is not considered a fault.

Isovaleric Acid

Pungent goats’ cheese and putrid feet are some of the descriptors that were used to identify the aromas of Isovaleric Acid, but it was not present in the taste. This is an organic acid attributed to stale, degraded hops, where the alpha acids have broken down. It can also be caused by wild yeast, notably Brettanomyces. Isovaleric Acid can be avoided by using fresh, high quality hops or improving sanitation throughout the brew.

Trans-2-nonenal (oxidation)

While not as detectable in the aroma, the spiked beer tasted intensely of wet cardboard. This is down to oxidation, making the beer stale. If beers are stored incorrectly or exposed to oxygen, the beer will eventually develop these characteristics. This can be avoided with proper storage.

In addition to these off-flavours, we spiked our beer with some flavour style markers, including hefeweizen, characterised by bubblegum and cloves. Microbial contamination can present certain spicy, peppery flavours to a the taste of beer – this can be found in aged beers – and we identified benzaldehyde, an almond or marzipan flavour that can be caused by oxidation or raw materials, such as fruit, that are introduced to beer.

As the above illustrates, there’s a lot of chemistry behind off-flavours; this shouldn’t be daunting, however. The main takeaway from this education is that every component of beer plays a pivotal role, whether it’s the alkalinity of water, the freshness of hops, malt-derived precursors or the many complications associated with working with a single-celled microorganism that is yeast. 

Despite having spent an evening swilling beer, we left miraculously sober. But more significantly, we left with a deepened respect for the list of minute complexities that can make good beer go bad.

We Brought Beer offer a schedule of fortnightly beer classes at their Clapham Junction site. For more information, see here.

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