Know Your Off Flavors and What Causes Them

Off flavors in home brew can be disheartening. If you have the unfortunate opportunity to peruse these off flavors it best if you use the unwanted experience to analyze what has gone wrong with your brew, so that it does not happen again. Using a brew journal or software program to log your successes and failures can be useful in keeping track of your problems and knowing something about the microbiology of it all. Getting more familiar with off flavors and what causes them can help eliminate potential threats and maintain a sterile brewing environment.

Click to enlarge the flavor wheel

I will go over four of the most common and important beer-related flavor compounds that have to do with off flavors that might creep into your homebrew, including diacetyl, acetates, DMS or dimethyl sulfide, and phenols. Many of these factors can be controlled to produce desired flavor effects, but if present in too great of quantities, can cause various forms of unwanted and unpleasant off-flavors.

Diacetyl
Diacetyl is one such factor – known to the scientifically minded as C4H6O2. Diacetyl is a compound that occurs naturally as part of the fermentation process, but can also be increased through bacterial contamination from lactobacillus and pediococcus. One sign that this is the case is if the beer has an overly sour flavor.

The amount of aeration, the type of yeast strain used, and the fermentation level can all have an effect on the diacetyl level as well. The threshold at which diacetyl is discernable in beer is 0.1 PPM (Parts per Million) according to White Labs' Dr. Chris White, a well-respected yeast rancher. Homebrewed beers, in general, have a diacetyl level of 0.05 PPM to 1.0 PPM or greater.

Diacetyl is a tricky flavor to diagnose as being an “off flavor” because it will always be in beer at some level. Often, the question is whether the beer is suited for diacetyl or not. Diacetyl can add a buttery or butterscotch flavor to beer, and is considered an acceptable flavor in beers such as English Pale Ales – in the right quantity. If there is too much diacetyl, the beer can develop a somewhat rancid flavor fairly quick. Keep in mind that diacetyl is a compound which is used in margarine and similar oil based butter substitutes.

Diacetyl has been getting some press lately linking the chemical, which is used as an artificial butter flavoring in popcorn, to Alzheimer's. [Read the Study]

One thing you can do to eliminate some of the diacetyl production in your home brew is to force a “Diacetyl Rest”. This is a process which activates the yeast in the brew to re-absorb diacetyl and break it down into other compounds that have little or no flavor effect. If the beer is contaminated with bacteria, the diacetyl rest can indicate this, as the off flavors will become more pronounced. The process of the diacetyl rest relies upon activating the yeast through a slight raise in temperature near the end of the fermentation.

This usually takes place for two days, starting when the specific gravity of the beer is 2-5 points away from its terminal gravity. For lager beers, this means raising the temperature from 50-55 degrees fahrenheit to 65-68 degrees fahrenheit. For ale production, no increase in temperature should be necessary, but it is wise to give the beer some extra time to rest the diacetyl for a day or so even after the terminal gravity is reached.

Acetates
The production of acetates in home brew can also be controlled by modifying the fermentation temperature and the amount of yeast that is pitched into the wort. The question is, which acetates do you want to increase and which do you want to decrease?

Ethyl acetate is the most common acetate that is produced by brewing yeast, which has a flavor threshold of around 2PPM, and starts out with a fruity or pear-like flavor, but can end up tasting solvent-like at around 30 PPM plus. Once again, the type of beer being made has a great effect on the concentration and types of acetates that are desired in the resultant beer. Increasing the fermentation temperature will allow more fruity flavors to emerge from the beer, while decreasing the temperature will prevent these flavors, of which acetates are a factor.

Likewise, a healthy yeast starter will decrease the resultant acetates, while a weak starting yeast will produce more. This is why it is important that the timing of your yeast starter be good. If the yeast is not ready in time for pitching, and some other yeast gets colonized into the beer, it can have an additional acetate lacing effect which is undesirable.

Sulfides
Sulfides, especially DMS (Dimethyl Sulfide), can produce “creamed corn” or “celery” -like flavors in home brew at high levels. At lower levels, the DMS compound is an important aspect of the body and flavor of the beer.

DMS is a volatile, sulfur-based compound. The flavor threshold of DMS is 10-150 PPM, depending on what else is going on in the beer, and in some beers, it is as high as 175 PPM. DMS is the product of the breakdown of other compounds which occurs at a temperature of 140 degrees fahrenheit. This means that there are two factors which can contribute to increasing levels of this compound in beer:

1. The wort is chilled too slowly.
2. The brew kettle is covered too long during the boil, preventing the compound from escaping.

Phenols
Phenols are another group of compounds that can throw off the taste of beer. There are only a few beers where the taste or smell of a medicinal, spicy, or clove-like factor can be considered desirable. For most beers, an over-abundance of these flavors is undesirable. This flavor is more easily brought out of wheat based beers and beers brewed with certain Belgian and English yeast types. Hence, the flavors can also be brought out by wild yeasts that also have this quality.

These flavors can also be enhanced by the presence of chlorine in brewing water. This is an important reason why it is suggested to use filtered tap water or natural spring water for brewing beer, since it is very difficult to truly ascertain what is in the tap water. Algae traces can also contribute more phenols to the beer through the water supply.

The Siebel Institute offers some very useful sensory training kits for brewers to learn how to detect certain compounds in beer. These training kits sharpen your awareness of certain off flavors and compounds and are a great tool for homebrewers looking to get BJCP or Cicerone certifications. The training kits contain 24 vials of pre-measured “standards” representing some of the most important flavors and aromatics found in beer. Kits include the following compounds:

These kits cost a few hundred dollars, so gather some brewing compadres and all pitch in to do the training together.

Christian Lavender is a homebrewer in Austin, TX and founder of Kegerators.com and HomeBrewing.com.

Related Articles:
How to Protect and Store Grain, Hops & Yeast - Storage and protection techniques for your grain, hops and yeast to take some of the off flavor factors out of your recipe creation.
Throw a Brew Tasting Party - Highlight the most important aspects of beer with other brewers.
Keeping Homebrew Chill - Control your homebrew fermentation temperatures for better tasting brews.