By Greig McGill
Has anything been more talked about in brewing circles than off-flavours? There must be thousands of articles, videos, and brewing conference presentations about the subject. I’m sure you’ve all read, seen, or listened to at least one of them. So what can I add to that vast body of knowledge? Well, probably not a lot, but I can perhaps help put it all in perspective, and help you decide where to focus your diagnostic processes to help you make better beer! In doing so, I’m going to have to retread some tired old ground just in case anyone has been living under a rock, and to ensure we’re all on the same page. I apologise if anyone is reduced to groan-inducing boredom by those bits. In this article, I will just focus on the three most common, and easy to avoid, off flavours in beer. If there’s enough interest, I may write a follow-up with some of the lesser known, or rarely encountered off flavours. Let me know if that’s something you’d like to see.
So what’s an off flavour anyway? Simply put, it’s something in beer that you can taste, smell, or even feel, that’s not supposed to be there. That beer you don’t like, because Sabro hops really aren’t your jam? No, that’s not an off flavour. Perhaps if that same beer has an astringent, puckering quality to it also? Yep, that’s an off flavour. Off flavours can be the result of any and every part of the brewing process, from the ingredients themselves, right through to how the beer is packaged, or the materials it comes into contact with before or during consumption. I’ll talk about what you might smell, taste, or feel, and work backwards to tell you what it (probably) is, and what to do differently to avoid it.
One final and important point to note before we get down to specifics; not everyone perceives the same compounds in the same way. Some have different levels of sensitivity so some of the chemical compounds which are responsible for off flavours, and some may not perceive them at all. Also, some might identify a particular compound as one specific flavour or aroma, while others may associate it with something else entirely. It’s also important to remember that some compounds are very similar to others, and may not be an off flavour. The most common example of that is diacetyl and high levels of caramel malt, but there are many more. If you have a chance, it’s good to hone your senses by attending a fault tasting session, where beer is dosed with the compounds which cause off flavours, and you can tune your palate accordingly. This also helps with finding your blind spots, or areas where you are particularly sensitive. It’s rare for any individual to be equally sensitive to all possible off flavours, so you almost certainly will be blind to some faults and hyper sensitive to others.
OK! That’s plenty of preamble, and perchance it’s putting you in a persnickety presence of perception. So let’s push on!
Vicinal Diketones (VDKs) are a family of compounds which are responsible for the most common off flavours in beer. Diacetyl is the most common VDK and is responsible for buttery, butterscotch, or excessive caramel notes. At higher levels, it will form a buttery slickness on the palate. It also suppresses hop perception, particularly aroma, so if your hoppy beer doesn’t seem as hoppy as it should, yet you’re not sensitive to diacetyl, this could be a good spot to begin diagnosis. 2,3-pentanedione is the other common VDK found in beer, and is often confused with oxidation, as it produces a sickly sweet, honey-like flavour. VDKs are produced (indirectly) by all yeast to some degree as a natural part of fermentation, as the yeast produces precursors that naturally break down into VDKs. Luckily, healthy yeast in a well managed fermentation will consume most of those waste products during the last part of fermentation. If you are having VDK problems, you should ensure your yeast is healthy, your pitch rate is sufficient, and your fermentation temperature never falls during fermentation, rising a little towards the end to keep the yeast active as long as possible. Because lagers ferment cooler, they also take a lot longer to clean up VDK, so a good lagering phase should never be skipped or shortened. VDK can also be a byproduct of bacterial contamination at any stage of the beer’s life, including packaging in contaminated bottles or cans, or being served through contaminated draught lines or from contaminated kegs/keg fittings. While less common, if you’re doing everything else correctly around fermentation, this is a possible source of VDK.
Acetaldehyde is a precursor to ethanol which, like VDKs, is produced as a natural part of fermentation. Fully fermented beer should have almost zero acetaldehyde remaining, certainly nothing above the human flavour threshold, so if it’s present, it can be an indicator that your fermentation regime isn’t as good as it could be. The most common causes are unhealthy or stressed yeast, and cutting off fermentation too soon. Most people perceive acetaldehyde as green apple, though I’ve heard some describe it as pumpkin seeds/pumpkin guts, as well as wet paint! Excess acetaldehyde may also give the perception of thin, cidery beer. To avoid perceptible acetaldehyde, simply check that you are pitching the correct amount of healthy yeast, that there is enough nutrient and oxygen present for the yeast in the wort, and that you allow fermentation to proceed at the optimal temperature for the yeast, and let it complete fully - following the same process as for VDKs above.
Oxidation is a tricky beast, and there’s very little you can do to remove it entirely, but there is a lot you can do to minimise it. The flavour impact of oxidation can vary dramatically depending on the underlying beer, the amount of TPO (total packaged oxygen), the storage conditions of the beer, and many other factors we’re still coming to grips with! Classic oxidative flavours are a “tired” or “flabby” character in the beer, where the hops don’t pop, the malt tastes indistinct or overly sweet, sometimes reminiscent of honey, or the beer takes on a hard-boiled sweet note, akin to barley-sugar sweets. This is one area where fault tasting sessions aren’t overly helpful, as the chemical used to dose the beer with is trans-2-nonenal, which is that classic “wet paper/cardboard” descriptor often used to describe oxidation in beer. If only it were that simple! I’ve found this specific compound most commonly present in oxidised lager, while in other, often more complex styles, the effects of oxidation are themselves more complex. The simplest way to put it is that oxidised beer just doesn’t taste fresh! To combat it as best you can, think of every point after fermentation that oxygen could contact your beer, and take steps to minimise it. Dry hopping post ferment? Consider purging the headspace of the beer with CO2 while you introduce the hops. Transferring to secondary? Try to eliminate that step, unless you need to age on an adjunct such as fruit. Kegging your beer? Ensure the keg is properly purged with CO2 first, and you have positive CO2 pressure on your fermenter while transferring so as not to suck in any oxygen. Crash cooling? Hook up a low pressure regulator so that the beer doesn’t suck in any oxygen as the headspace gas contracts under cooling. Bottling? Purge your bottles, or use a counter pressure filler. Remember that while CO2 is heavier than air, gases want to mix and will find equilibrium, so a “CO2 blanket” isn’t enough to protect your beer from oxygen. Hot side aeration? Well, while hot side aeration of beer is a potential cause of higher levels of packaged oxygen in commercial beers, it’s not worth concentrating on this at a homebrewing level.
So, there’s the three faults I see most commonly in beer. These are the faults that almost all new brewers have, and can even bite an experienced brewer on occasion. If you’re interested, and in Hamilton, I occasionally run a fault tasting session at the brewery (Brewaucracy, 34 Mahana Road, Te Rapa) so you can always get in touch and see when the next one is coming up.
Until next time, may the gods of beer smile upon you and keep you fault free!