Improving Your Beer: Part 2 – Common Off Flavors

Last month we discussed how to determine if your beer has any flaws. This month, we’re going to begin looking at what some of those flaws might taste like, how they are caused and what we can do to avoid them in the future. We won’t get into all of them, as many are not overly common and honestly this article would be so long you probably wouldn’t finish, so for today I think we’ll just focus on the top 5.

  • Diacetyl
  • Skunked
  • Dimethyl Sulfide (DMS)
  • Acetaldehyde
  • Oxidation



What is it?
By far one of the most common off-flavors you will come across in beer. Diacetyl creates a buttery or butterscotch aftertaste, similar to that of popcorn. I will also create a slick feeling in your mouth that isn’t unpleasant, but also isn’t very appealing. It is extremely easy to detect in lighter styled lagers or ales, while dark, roasty beers can often hide smaller amounts of it.

What causes it?
Diacetyl is a natural by-product of fermentation. Usually, with ales it is reabsorbed due to the higher fermentation temperature, however if the levels are high it can still be present in the final product. Lagers that are not given a proper diacetyl rest will often exhibit noticeable amounts of this trait as well.

How do we get rid of it?
Don’t rush your beer. Many new homebrewers are eager to get their new beer in the bottle so they can drink it as fast as possible. It also doesn’t help that most kits give you instructions to simply wait 1 or 2 weeks.  Unless I really need the beer done, I will usually give it 3-4 weeks before considering it done with fermentation. Not only does not give the yeast plenty of time to clean up after itself, but it also allows the beer to bulk age a little. If you’re brewing a lager, ramp the temperature up to the mid to high 60’s for about a week, a process known as a diacetyl rest. While it’s true yeast can still remove diacetyl at cold temps, it will do so much more efficiently at higher temps.



What is it?
Anyone who has ever had a Heineken knows exactly what this one tastes like. It tastes, well, as described, like a skunk. It never really quite gets to that level of offense, but the aroma and flavor are unmistakable.

What causes it?
We’ve all been told at one point or another, “I bought this beer cold, so I’m going to put it in the fridge so it doesn’t get skunked.” I don’t know how this rumor began, but I’m here to tell you it’s simply wrong. Skunked, or lightstruck beer, has nothing to do with the temperature. Instead, it has to do with a chemical reaction between the alpha acids in the hops, riboflavin in the beer and light.

The reason you taste this more in beers like Heineken, is due to their green bottles. Green bottles only block 50-80% of light, while brown bottles used by most craft breweries block as much as 90%. This is also the main reason many breweries lately, especially those with excessively hoppy IPA’s are switching back to aluminum cans.

How do we get rid of it?
First, only use dark brown bottles to bottle your homebrew or if it’s an option, keg as that will block 100% of all light. Secondly, make sure that if you are using a clear fermentation vessel, such as a carboy, you store it in a dark place like a closet, or cover it with a t-shirt or blanket. Your beer can’t tell the difference between sunlight or artificial light, so best to protect it from both.

Dimethyl Sulfide (DMS)

What is it?
Dimethyl Sulfide, or DMS, is derived from a compound known as SMM, that is produced during the malting process. 6-row and pilsner malts typically have higher levels of this compound, than other base malts such as 2-row. It tastes of creamed corn, and while it is acceptable in some styles such as pilsners and cream ales, it is mostly considered an off flavor.

What causes it?
High moisture content malts will release larger amounts of SMM into the wort, which will convert to DMS during the boil. DMS is created during the warm, non-boil stage of brewing.

How do we get rid of it?
There are few things you can do to ensure you remove as much of the DMS as possible. Starting with a vigorous, and sometimes longer boil. It’s often suggested to increase boil time for beers containing a lot of Pilsner Malt to 90 minutes, however many believe the malts are modified to the point of this being an unnecessary step. Over-sparging your mash can also cause additional SMM to be pulled out, which will increase your chances of DMS.

It is also important to achieve a fast boil and fast cool-down. As DMS is created when the wort is warm, but not boiling, so you want to reduce the time spent in these stages as much as possible.



What is it?
A naturally occurring compound found in everything from fruit to coffee. Typically associated with tasting like green apple, latex paint or jolly ranchers. All beers have acetaldehyde to a certain degree.

What causes it?
Acetaldehyde is produced during fermentation by the oxidation of ethanol. Some yeasts such as White Labs 840 (American Lager yeast) produce higher levels, due to it being an acceptable flavor small amounts in lighter American lagers.

How do we get rid of it?
The easiest way to avoid this is to limit exposure of your beer to oxygen once fermentation has begun, and even more so once it has completed. Pitch a healthy, strong yeast starter to ensure fermentation begins as quickly as possible to provide a blanket of CO2 on your beer as it ferments. It is also important to maintain sound sanitation as aerobic bacteria can metabolize the compound into acetic acid, which can give your beer vinegar off flavors.



What is it?
Sherry wine, cardboard, stale or possibly even musty off flavors in beer.  More apparent in the final product as the beer ages. It can also cause lighter colored beers to appear darker.

What causes it?
Oxidation is caused by exposure to oxygen after the fermentation process has completed, usually introduced during transfer or bottling.  The more the beer is exposed to oxygen, the faster it will develop.

How do we get rid of it?
Once fermentation has begun, resist the urge to not open the fermenter and constantly check the process. Doing this will remove the blanket of CO2 that has built up and allow oxygen an opportunity to get into the fermenter. If possible, use closed CO2 transfers when moving from one vessel to another (some fermenters such as stainless steel conicals or the plastic Fermentasaurus have methods to do closed transfer).

When transferring with an auto-siphon, make sure your siphon is prepared and expanded prior to inserting the end into the beer, this will ensure when you begin the process of siphoning, you are not forcing air into the liquid. Also, when transferring, take care to avoid splashing on the bottom of your bottling bucket or keg. When bottling, use oxygen resistant caps and be sure to fill your bottles to the proper level. At the homebrew level, it is difficult to remove 100% of oxygen, however bottle conditioning will usually scrub most oxygen from the bottle. If you notice bubbles in your line, there is a good chance oxygen is being introduced. Make sure all of your connections are tight and see if that removes them.

Note on kegging: A good practice when kegging is to purge the keg of oxygen prior to filling with a low level of CO2. Bleed the pressure off a few times and rack the beer into the CO2 filled keg. This can also work with bottling. Prior to filling the bottles, using a wand or bottling gun, purge the bottles of any oxygen by providing a small amount of CO2 as a layer of protection.

We may discuss more off flavors in this series, however these are the 5 I see arise most commonly. As always, check with the staff at Jay’s Brewing to inquire about any future Off Flavor Seminars. They have hosted many over the past few months to very positive reviews. Also feel free to speak with anyone at Jay’s if you feel something in your process could be causing flaws. Very often it’s not a problem with the brewer, but rather equipment. I was beginning to have oxidation problems about 2 years ago and eventually determined the culprit to be a cracked auto-siphon pulling in large amounts of oxygen as I kegged my beer. What eventually ruined a few batches of beer, was easily remedied with a new $20 accessory.

Until next time, happy brewing!

Stephen Boyajian has been an avid homebrewer for 4 years. A fan of many styles, with a particular love for IPA’s and Stouts. He lives in Gainesville, VA with his wife, 3 kids and dog. When not brewing, he enjoys golfing or playing guitar.

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