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Off-Flavors in Brewing: When Good Beer Goes Bad

There are times in every homebrewers career that good beers go bad. This can leave you scratching your head wondering what went wrong. Becoming familiar with common off flavors and what causes them can help you pick up faults in your own beer and know how to fix them. Homebrew clubs and Certified Cicerones often offer off flavor tastings in which spikes are added to a light lager. These in person classes are extremely valuable. While signing up to taste a bunch of bad beer doesn’t sound that appealing, the firsthand knowledge you gain is definitely worth the price of admission.


Acetaldehyde is detected in aroma and flavor. It is often described as green apples, rotten apples, cidery, freshly cut pumpkin, and lawn clippings. Acetaldehyde is a naturally occurring compound produced during fermentation by the yeast. In a healthy complete fermentation this compound is cleaned up and turned into alcohol so that levels fall below the flavor threshold. This off flavor is mostly caused by brewers trying to get from grain to glass too quickly. Allow your beer to remain in contact with a healthy population of yeast for a few days after reaching final gravity before racking to secondary or packaging. Always pitch an appropriate amount of yeast and fully oxygenate wort at pitching.

Dimethyl Sulfide (DMS)

DMS is detected in aroma and flavor. It is often described as creamed corn, cooked vegetables, cabbage, spaghetti o’s, and shellfish. The precursor to DMS is a compound called S-methylmethionine (SMM). It is produced during the germination stage of grain malting. Most of this compound is removed during the kilning process. However, for the very lightest grain the kilning process is not long enough to completely remove this compound. When SMM remains and is heated during mashing or boiling it transforms to DMS. The good news is that DMS is very volatile and will be carried away by evaporation during the boil. Allow your wort to come to a good rolling boil and cool that wort as fast as you can, the longer it stays hot the more SMM is converted to DMS. Never cover the kettle during the boil or cooling as the condensation that falls back into the pot will be full of DMS. In grain bills with a lot of pilsner or other very light malt consider extending your boil to 90 minutes. It should be noted that DMS is acceptable in some styles, especially light lagers. Even in these styles it should come across as only a hint of corn and not overwhelm the beer.


Diacetyl is detected in flavor, aroma, and mouthfeel. It is often described as movie theater popcorn, buttered popcorn, buttery, toffee, buttermilk, butterscotch, or rancid butter. Oily, slick or creamy mouthfeel. Diacetyl is once again a by product of early fermentation. Generally, it is reabsorbed by yeast cells however it can be left behind by high flocculating yeast such as those originating from Britain, weak or mutated yeast, over or under oxygenating, and low fermentation temperature. Time and Temperature are the best ways to combat Diacetyl. A temperature rise near the end of fermentation, also called a diacetyl rest, provides a boost to yeast metabolism and encourages it to reabsorb diacetyl. Once again allowing your beer adequate time in contact with the yeast will allow it to clean up this fermentation byproduct.

Oxidation: Trans-2-nonenal

Oxidation is detected in aroma, flavor, and mouthfeel. It is described as stale, wet cardboard, wet newspaper, musty, soy sauce, and in dark beers, tomato sauce and sherry. It can also come across as a dulling of flavors.  Oxidation occurs when oxygen negatively reacts with the molecules in the wort or beer. Oxygen is a necessity for healthy yeast but after initial fermentation it needs to be avoided at all costs. When moving or transferring beer from one vessel to another try to minimize splashing and avoid pouring or dumping. When packaging be sure to purge kegs with CO2 and leave the appropriate head space when bottling. Since avoiding all oxygen is almost impossible it’s important to note what beers can be aged and what beers should be drank fresh. Beers with lower ABV, late addition hops, high levels of esters, and high levels of wheat, oats, or rye are more vulnerable to the effects of oxidation and should be consumed within 2-8 weeks. With more wheat focused styles such as Hefeweizens and Witbier peaking between 2-4 weeks. Darker beers with higher ABV can benefit from aging and can take on a sherry or wine character.


Metallic off flavors can be detected in appearance, flavor, and mouthfeel. It is often described like sucking on a penny, iron, aluminum foil, blood, and rust. It can cause haze, affect foam, and leave a tingly sensation in your mouth. Usually caused by wort being boiled in unprocessed metals including iron, aluminum, and steel (excluding stainless) Can also be extracted from metal brewing equipment, kegs, chrome plated beer faucets, and bottle caps. Improperly stored grain can also cause metallic off flavors. To avoid this uncommon off flavor, go with stainless when you can, replace chrome plated beer faucets when the brass begins to show through, use milled grain promptly and store unmilled grain in airtight containers in a cool dry area, and always store bottles upright.

Light Struck (Skunked)

Light Struck flavors can be detected in flavor and aroma. Light Struck is often appropriately described as skunked. It evokes memories of roadkill or an unfortunate mishap. Light is the enemy of beer. It can turn a good beer bad in seconds but avoiding this off flavor is rather simple. Bottling in brown bottle blocks 98% of the sun’s UV Rays. While watching fermentation might be tempting, UV rays can pass through windows in your house so keep those carboys in a dark place or wrap a blanket around them to keep out the light.

Acetic & Lactic (Sour)

Acetic is detectable in flavor and aroma. It is often described as Acidic, vinegary, sour, tangy, and tart. This off flavor is almost always the result of a bacterial or wild yeast infection. Great if you’re brewing a Lambic style beer not so great if you’re brewing a pilsner or stout.

Lactic is detectable in flavor but rarely in aroma. It is described as citric, lemony, crisp clean sour, sour cream, sour milk, tart, and yogurt. Mostly caused by Lactobacillus and/or Pediococcus. Can be created with Acidulated Malt and Lactic Acid.

Sour on purpose can be a great thing but when it shows up where it hasn’t been invited, it's time to look at your sanitation practices. It’s said don’t get into brewing if you don’t like to clean. Don’t skimp on cleaning materials either. Is it worth possibly having to dump a batch of beer and replace equipment to save a couple of bucks? Wild yeast is all around us so clean, clean, clean and sanitize afterward. Anything that comes in contact with your beer after the boil needs to be sanitized. This includes any fruit and adjuncts. Don’t simply wash and toss. If you encounter an infection you will need to replace any and all plastic brewing equipment such as Fermentation buckets, bottling bucket, auto siphon, bottling wand, etc. The infection can live in any scratch (even ones you don’t see) and no amount of cleaning and sanitation can clean it out after it sets in.

Contamination/Infection (Draft Issues)

Contamination/Infection in your home draft lines is detectable in flavor and aroma. It comes across as a combination of sour and butter. This comes from dirty draft lines and dirty faucets. In a commercial setting draft lines are cleaned out every two weeks, but home draft lines can go a little longer. Once a month or every time you kick a keg is plenty (provided it doesn’t take several months to kick). This is a relatively simple process. Add beer line cleaner and a few quarts of water to an empty keg, pressurize the keg run it through the lines and allow to sit 15 mins, then flush with a keg of plain water. If you don’t have empty kegs, a draft line cleaner pump can be used as well. Replace draft lines about once a year. Pouring beer properly from the faucet will also keep contamination/infection away. Keep the faucet out of the beer while pouring. Learn how to take a faucet apart and put it back together and clean those quarterly.

Avoiding off flavors comes down to three main keys:

1. Clean and Sanitary always.

2. Patience is a Virtue. Allow your beer time to finish completely and the yeast time to clean up fermentation by products.

3. Avoid introducing oxygen after fermentation begins.

I’ve heard it said in the shop, your first few beers will be fine it's when you think you have it down and can rush the process that things will go wrong. Hopefully you never find yourself with a glass of bad beer in your hand, but should you, identifying the problem will be the key to solving and preventing it in the future.

--Sandra DiPretore
Cicerone Certified Beer Server, Homebrewer, and Tireless Recipe Researcher


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