Off Flavours & Aromas


We are all familiar with the flavours and aromas we love from a beer; grapefruit, pineapple, melon, bubblegum, pine, coconut, biscuit, toffee, raisin, chocolate, coffee, the list goes on and on.  But what about those that when you get a whiff, or the liquid touches your tongue you think twice about finishing your pint (yes, unfortunately it does happen)?  Today we’re chatting the not-so-wonderful world of off flavours & aromas.
These come from a multitude of sources both in and out of the brewhouse.  Whether you’re brewing, working in a bar or bottleshop, or a consumer having a basic understanding of these little nasties allows you to both care for and asses a beer more fully.  We’ve chosen a few of our favourites to highlight here.
Probably the most common one you’re going to come across is Oxidation which, you guessed, is caused by oxygen! When fatty components of malt come in contact with O2 the results are both flavours and aromas of paper, cardboard, shoebox, a stale quality, waxy/lipstick, and honey which magnify over time and increased in warmer temperatures. Oxygen coming in contact with the beer is good at certain points in the brewing process, but more often than not oxygen should be kept to a minimum from wort through to glass.
Sometimes things have gone awry in the brewing, other times in the packaging where vessels - be them kegs, cans, or bottles – are not filled properly trapping too much oxygen in with the beer.  Now having said all that about it being a ‘fault’, there are many circumstances and many styles in which this characteristics are favourable for example in aged beers where the characteristics of Oxidation translate more like toffee, caramel and sherry-like.
The other off flavour & aroma we run into more often is Skunkiness.  This is caused by the liquid coming in contact with high energy light wavelengths (ie. Sunlight) converting the bittering compounds of hops into the same compound in skunk spray. It smells as you would expect, like a skunk…or for those who have never smelt a skunk, or skunk cabbage (a cabbage that smells like skunk)…it smells like harsh weed, or others describe it as rubbery.  Funnily enough if you want to identify this character grab yourself a Heineken, it is a classic descriptor for the beer likely because green bottles provide little protection from the light.  Another test is pour your beer into a clear glass, have a sip right away, now put it out into the sunlight, leave it for a few minutes (leave it for 10 if you want a super skunked beer), and have another sip. 
The most important takeaways from Oxidisation and Skunkiness are, if you’re cellaring beer, be sure to store it away from light and heat.
Yeasts are finicky little organisms which, if displeased with their environment (aka stressed) will kick up a fuss and not do their job correctly, and when they don’t do their job correctly, you can taste and smell it in the final beer.  Diacetyl and Acetaldehyde are both a result of this. 
The most common descriptor for Diacetyl is buttery popcorn, the type you’d get from a movie theatre, and butterscotch. While Acetaldehyde is commonly identified as green or overripe apple, wet grass, raw pumpkin, or latex paint (yuck!). Yeasts produce both Diacetyl and Acetaldehyde during fermentation (from precursors but we’re not going to go that scientific) but they clean up after themselves unless they are stressed due to improper temperatures or other variables. In English-style ales low levels of diacetyl are acceptable because the English yeast strains often flocculate (clump together and fall out of solution) before they can clean up the Diacetyl they’ve made.
You know how we said above we weren’t going to get into precursors?  Well, we lied a little.  Cause DMS or Dimethyl Sulfide, a sulfur compound which gives beer the aromas of creamed corn, cooked veg, cabbage, and in darker beers, tomato juice-like, is caused because of the precursor SMM (s-methyl methionine) which is found in grain that when heated converts to DMS.  It is often boiled off in…the boil…but there must be sufficient ventilation so the steam can release and the wort must be quickly chilled to prevent more DMS from forming.  SMM is found in higher quantities in paler malts, therefore a light presence of DMS is acceptable in Lagers.
The last off-flavour & aroma we are going to mention is one we’re confident you’ll never find at Carwyn Cellars and that is Infection.  It is a horrible combination of Diacetyl and Acetic Acid which makes for a buttery vinegar hit reminiscent of being sick. It is very unpleasant and a result of unclean beer lines and faucets at a venue. 
As a drinker, our ability to detect certain aromas and flavours differ person to person.  So while one person’s nose will barely touch the glass before they’re calling Diacetyl, they may struggle to pick up on DMS, while someone else may find that cooked veg quality obvious.  Its all so personal, which makes it easy in no way, but you are able to train yourself.  Sensory Off-Flavour kits are available to purchase online. So get your mates together, get some jugs of beer (Lager is best for first-timers), and drink some nasty stuff!