One Man's Banana Lollies is Another Man's Baby Vomit

There are some common faults in brewing beer and they result in identifiable 'off-flavours'. We had a session to try and isolate and identify them, which proved very interesting. You see, not all 'off flavours' are bad - for example; apples; honey; popcorn; aniseed might be pleasant aromas and flavours. Some, however - such as cardboard, sulphur, farts and baby vomit - will never be particularly palatable.

Most of these flavours are natural and occur during fermentation and other organic brewing processes. Some of them are present in all beers, but pass below the detection threshold, unless they are meant to be noticeable in certain styles, in which case they are perfectly acceptable.

The difficulty is that we don't all use the same vocabulary to describe these things, and we don't even all detect them in the same levels.Different people have different words to describe the same thing (green apple/ paint thinner).The ability to detect some tastes and aromas is genetic, and we all have different biological sensitivities. It is, naturally, of benefit to be able to identify one's own blind spots to assist in brewing.

To test these things, we took a control beer and added samples of off-flavours (chemical compounds) to it. The control beer was Mort's Gold, a crisp, hoppy, Czech-style pilsner brewed by BentSpoke. After we had all sampled it and made notes on how it was supposed to taste, our facilitator (and crazy chemist in the kitchen), Patrick Baggoley, added capsules of powder to different jugs of the beer, which were then passed around for our delectation. He did, first caution us with a service announcement - “If you see someone slipping powder in your drink in the pub, report it to the authorities.”

1. ACETALDEHYTE (CH3CHO) - I detected a sharp aroma of nail-varnish remover and an unpleasant finish which flattened both the hop and the malt profile. I am meant to detect green apples, cut grass,cut pumpkin and/or latex paint. This is naturally-occurring organic compound found in yeast is present in all beer in small quantities, and is part of the flavour profile of some American lagers (including Budweiser). High levels are generally due to poor yeast health or un-aged beer.
2. ETHYL HEXANOATE - I detected an aniseed/ fennel aroma and a saccharine flavour with detergent notes. I am meant to detect aniseed, red apple, strawberry and floral, sweet esters. It is produced by yeast early in fermentation and found in low quantities (0.07 - 0.5 mg/l). The perception threshold is 0.2 mg/l.
3. ETHYL BUTYRATE - I detected vomit and cooked creamed corn. I am meant to detect banana sweets and bubble-gum (deliberately encouraged in some Belgian beers), tropical fruits, mango, tinned pineapple, and a slightly cheesy fruity ester flavour, probably not encouraged in anything.
4. ISOAMYL ACETATE - I detected no aroma and an astringent, chemical flavour. I am meant to detect banana and pear-drops, which I don't even consider to be remotely alike! Apparently this is perfectly acceptable in Heferwizen
5. 4-VINY GUAIACOL - I detected root beer flavour, nutmeg, vanilla and clove,which was not unpleasant. I am meant to detect spicy, herbal, clove. Hurrah! This phenolic flavour is caused by/ found in wild yeast/ specialty yeast, and is present in all beers. It is perceptible in wheat beers, smoked beers and Specialty Belgian beers, but at higher concentrations it may taste medicinal.
6. CIS-3-HEXANOL - I detected a buttery aroma, and a flat, graphite, hay/corn flavour with a lingering astringency. I am meant to detect alfalfa (seriously? How many beer-drinkers know what that smells/ tastes like?), grass clippings, sagebush, hay and green leaves. This is a compound that arises naturally in vegetal matter when unsaturated fatty acids are degraded, and can be eliminated by reducing dry-hopping.
7. ISOBUTYRALDEHYDE - I detected not much; it stripped all flavour and left me with a tinny/ metallic impression. I am meant to detect a grainy taste. Clearly I don;t get this one at all. It is present in all grain husks, and is detectable in high quantities when using malt that hasn’t been stored long enough before use, excessively long mashing, over crushing or over sparging.
8. DIACETYL - I detected sweet buttery aromas and a flavour of plastic or rubber. I am meant to detect butter popcorn and butterscotch. This dulls the malt and the hops, and is a very common fault in homebrew. It can leave a slick mouthfeel, which gives the illusion of a richness, and the ability to detect it is easier in lower alcohol beers. It is acceptable in small quantities in ESB and Bohemian Pilsner. It is produced early in fermentation and is generally caused by wrong temperature, unclean beer lines and infection.
9. LIGHT-STRUCK - I detected skunky, wet dog, damp flats and soggy carpet aromas - it smells like a lager (Carlsberg/ Heineken/ Steinlager). I am meant to detect sulphur, which presents as a catty, skunky aroma - yay, me! It is caused by the photo-chemical reaction of exposing beer to light. Imported Euro lager (in green bottles) often tastes like this, to such an extent that many people now have accepted that is how it is supposed to taste. (Corona uses a hop additive/ chemically-modified hop to get around this problem and still be able to use clear glass bottles.) You can try this yourself at home (or at the pub if there is a nice sunny beer garden) by pouring an IPA and putting it in direct sunlight. Even after 30 seconds it will start to taste differently. The solution is simply not to expose beer to sunlight after hops have been added – don’t use clear or green bottles.
10. DIMETHYL SULFIDE (DMS) - I detected rotten veg and boiled cabbage. I am meant to detect cooked broccoli, corn, and/or parsnips. This is a sulphur compound produced during fermentation of malt and is present to some degree in all beers (the highest concentrations to be found in German lagers). Fortunately it often boils off naturally in the brewing process, so can be eliminated by the use of a long, rolling boil.
11. 2-NONENAL - I detected a spent fireworks and peated whisky aroma, and a strong cardboard flavour. I am meant to detect wet cardboard, paper, ball-point pen, and tomato juice. This indicates that the beer has oxidised, and can be avoided by purging bottles with CO² prior to filling, storing beer cool, and drinking beer fresh.
12. MERCAPTAN (3-METHYL2-BUTENE-THIOL) - I detected sulphur and rotten eggs. I am meant to detect drains, polecats, rotten veg; and a farty catty sulphur. This is usually caused by light-strike (see 9).
13. HYDROGEN SULPHIDE - I detected burning, lighter fuel, and an astringent mouthfeel. I am meant to detect rotten eggs and burning matches. This is produced by yeast during fermentation, and, whereas in small quantities it can make beer taste ‘fresh’, at higher concentrations it is an ‘off-flavour’. It is hard to detect because olfactory senses quickly adapt to this flavour. “The more you look for it; the less likely you are to find it.”

This seemed like a good note on which to end the session and go and have a decent beer! For more information on off-flavours in beer check out these links:
BJCP Guide to Beer Faults
FlavorAvtiV
The Complete Beer Fault Guide by Thomas Barnes