Updated: Dec 15, 2020
Brewing is very time intensive, and so it is incredibly frustrating when a beer goes wrong and unexpected flavours arise. What’s worse, however, is not using these mistakes as opportunities to learn in order to avoid repeating the same results on another brew!
Cardboard / sherry
Buttery / slick
Sweetcorn / cabbage
Banana / Pear / Nail Polish
GREEN APPLE 🍏 - As yeast is converting glucose to ethanol it will produce acetaldehyde, which has a flavour akin to tart, green apples. A healthy fermentation will convert acetaldehyde to alcohols as the yeast cleans up after himself. However, struggling yeast won’t be able to do this as well so make sure to pitch enough yeast and avoid racking the beer off of the yeast too early. Oxidation can actually reverse this process and turn alcohols back into acetaldehyde, so do all you can to avoid oxygen pickup after fermentation starts.
ROTTEN VEGETABLES 🥦🍠 - One of the worst off flavours you can get, Mercaptan is the sulphury compound they use in natural gas to give it smell, so if your beer smells gassy or of an old bin then mercaptan is likely the culprit! Whilst it can be as a result of infection, mercaptan is easily absorbed from dead yeast if the beer has been on it for a long time, so make sure to rack your beer off of yeast it has been sat on for 4 weeks or more.
CARDBOARD / SHERRY 📦🥂- One of the most common off flavours when starting out brewing, oxygen will kill a beers flavour and aroma making it taste stale and papery. This is especially true with beers saturated with hops as they are more susceptible to oxygen spoilage. Keep oxygen out of beer as much as possible after fermentation starts, and if you can purge the headspace of the fermenter with CO2 after opening it to flush out oxygen then this will severely reduce the risk of oxygen pickup. Even better, if you can transfer beer from fermenters to kegs with CO2 in a closed system you shouldn’t have to worry about oxidation at all!
BUTTERY / SLICK 🧈 - Much like acetaldehyde, diacetyl is a natural product of fermentation that the yeast should, of healthy, reabsorb once it has finished fermenting. At lower levels diacetyl is less unpleasant with caramel-like flavours, but at higher concentrations will give a sickly butterscotch flavour and potentially a slick, oily mouthfeel which is very undesirable. The best way to tackle diacetyl is to raise the temperature at the end of fermentation to encourage the yeast to reabsorb it, and definitely don’t rack your brew off of the yeast before it has had time to do it’s thing!
SWEETCORN / CABBAGE 🌽- Flavours of sweetcorn, cabbage and cooked vegetables come from when the compound ‘SSM’ is converted to dimethyl sulfide (DMS) during the boil. Lager and Pilsner malts, as well as adjuncts like corn, have the highest levels of SSM so are most prone to overpowering DMS, as is grain with a high moisture content. Keeping grains dry and using less SSM-rich grains will lower your DMS levels, yet the easiest way to get rid of DMS is a vigorous boil, which you may want to consider lengthening to 90 minutes if you are using mainly lager malts.
BANANA / PEAR / NAIL POLISH 🍌🍐💅 - When yeast ferments it’s produced esters which are, much of time, desirable and give beers much of the fruity characters we know and love. However, stressed yeast that is overworked may produce an abundance of esters that lead to overpowering flavours which may be out of place for the beer style your brewing. Isoamyl Acetate, for example, can lead to a strong banana flavour which may be desirable in a Hefeweizen but not in a Pilsner. At very high levels it can even be perceived as nail polish! To limit ester production, pitch enough yeast so that it is not overworked, and ferment at lower temperatures to reduce fermentation speed. If you have a particularly strong ester character in a beer, often a decent time in the cellar will mellow it out and save your brew!
Whilst not all off flavours are as bad as each other, and some may even be desirable in small quantities in certain styles, it is important to be able to identify how a beer has gone wrong and understand the steps needed to stop it happening again! 🍻🔥