Off-flavors can really ruin your beer and make you question how things could have gone so wrong. You tell yourself you did everything right but somewhere along the line your beer took a turn for the worse. Sometimes this can be due to stress yeast, sometimes it’s due to improper techniques, and sometimes it just happens without you doing anything wrong.

The key to mastering off flavors is being able to identify them and what typically causes them. That way you can backtrack and look at your notes to see if you can pinpoint areas that need to be fixed for the next brew. But luckily some of these off-flavors can be fixed even after you detect them.

Oxidation

Oxidation is most commonly identified as wet cardboard, a musty, dull flavor, but as hoppy beer brewers know it also greatly impacts the hop characteristics in the beer. Not only does it dull the hop aroma and flavor, but it can also turn the beer sickly sweet. It can also impact the color of your beer making it turn darker or even have a purplish heel which is more noticeable in hazy beers.

Oxidation can happen anytime once fermentation has begun. Oxygen helps support yeast growth at the beginning of fermentation but after it’s in full swing any additional oxygen exposure can really ruin your brew. That means avoiding splashing, roughly transferring, or opening your fermenter needlessly. And when packaging purge your keg with CO2 before transferring it.

Luckily for bottlers, you will do a second fermentation in the bottle. So when you add priming sugar the fermentation kicks back up and the yeast will consume any oxygen left in the bottle. But still, try to minimize oxygen as much as you can. Unfortunately, once your beer is oxidized there’s no going back. Depending on how badly you oxidized your beer will reflect on how quickly you see changes in your beer. But there’s really no way to fully prevent oxidation.

Sulfur

Sulfur is a unique off flavor because for some beer styles a little sulfur is actually accepted. But when it’s too much it can make it hard to drink. Sulfur is commonly identified by a strong aroma that’s reminiscent of rotten eggs. For the most part, it only impacts the aroma of beer, but scent and taste are closely tied so it can be hard to get past that initial shock of smell.

Sulfur can come about a few ways but most simply it comes from stress yeast. When the yeasts are working hard to convert sugars into alcohol they can get overwhelmed if they don’t have enough nutrients or there is not enough of them to do all the work. So to combat this it’s always smart to pitch a good amount of healthy yeast by making a starter.

Thankfully sulfur is one of those off-flavors that can be actually cured. Not always but in some less extreme cases, all you need to do is to give it some time. Leave the beer in the fermenter for a few extra days or weeks until the sulfur smell dies down. During that time as the yeast wrap up fermentation, they will do a little cleanup before going to sleep. In the process reducing off flavors. That clean-up time can actually help with other off-flavors as well.

Acetaldehyde

When Acetaldehyde is present in large amounts you notice it can be described as a strong green apple flavor or even like cracking open a ripe squash. But don’t let the idea of green apples confuse you. It doesn’t necessarily taste sour. It just has that flavor and aroma. This off-flavor can be increased for several reasons, stress yeast, bad sanitation practices, and oxidation during fermentation.

Most notably this can come about when you package or consume beer when it’s too young. This can happen with a lot of off flavors, especially sulfur but if you keg up or bottle your beer while still at the tail end of fermentation it can lock in those off flavors and there’s nowhere for them to get off-gassed or cleaned up by the yeast.

Diacetyl

Diacetyl is another fermentation-based off flavor. Meaning its naturally created during fermentation and can be reabsorbed by the yeast. But if not it’s one of the more unique off flavors. It can be identified as buttered popcorn, or butterscotch and even give an oily, slick mouthfeel. In fact, they actually use diacetyl as the compound in microwave popcorn to give that movie theater smell.

You can encourage the reabsorption of diacetyl near the end of fermentation by doing what’s called a diacetyl rest. This is where you raise your fermenter’s temperature by 3 to 4 degrees for a few days to speed up the yeast’s job of taking back the off flavor. But diacetyl can also occur when there’s a bacterial infection. Lactobacillus is one bacteria known to promote it so keep your fermenters clean and sanitized.

Dimethyl Sulfide DMS

DMS is said to be perceived as cooked cabbage, creamed corn, or even tomato paste, or ketchup flavor. None of which you really want in your beer, Especially a light body lager. To understand where DMS comes from we first need to look at the SMM which is a precursor to DMS.

SMM is produced during the germination of grains and then is converted to DMS during the boiling on brew day. It’s most notably found in pilsner malts and some adjuncts like corn so you can already guess that light lagers can be tough. Thankfully the best way to remove DMS is pretty simple. Just have a vigorous boil. The experts say at least 60 minutes.

Malting has come a long way over the last few decades so we think this is not one you need to worry about too much. But if you notice that ketchup flavor in your beer it might be a good idea to boil a little bit longer or have a more intense boil next time.