Contamination

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Contamination (or "infection") refers to the presence of unintentional microorganisms in a fermentation. A variety of different yeast, bacteria, and molds are known to contaminate beer and wine at various points in the process.

Possible signs of a contamination:

What to do

First, relax and take a deep breath so we can deal with this rationally.

Unexpected pellicle

A pellicle is a definite sign of wild microbes. (See Pellicle) If a pellicle is the only sign of contamination, anecdotally most of the time the wild microbes will have no noticable effect on the flavor.

Follow these simple steps:

  1. Verify that fermentation has completed (e.g. take a hydrometer reading). This is to make sure it is safe to taste, since alcohol kills pathogens.
  2. Smell and then taste the product.
  3. If there are off flavors, see the appropriate section below.
  4. If it smells and tastes normal, we recommend to treat it as normal.
    • Beer: go ahead and package it.
    • Wine, cider, mead, etc: consider using sulfite after fermentation.


Important caveats if bottling a contaminated beer:
Some wild yeast can consume sugar that brewers yeast leave behind, which may result in unexpected additional fermentation in the bottle. The likelihood of over-carbonation and eventual explosions is small, but some simple precautions are recommended just in case... Monitor the carbonation level every few days. If possible, move the bottles into the fridge once carbonated (this helps to slow down any wild microbe activity). Don't distribute bottles to friends/family/judges.

Unexpected off flavors

Wild microbes may cause a variety of flavors. If the unexpected flavor(s) are unpleasant, the beer/wine will most likely need to be wasted, with couple possible exceptions:

  • If a beer/wine has hydrogen sulfide (rotten egg aroma), you should be able to remove it. See hydrogen sulfide for methods to remove hydrogen sulfide and certain other volatile sulfur compounds.
  • If a beer is sour, it may be worth a try lowering the sour taste with sodium bicarbonate. However it likely won't taste good, and it may continue to sour after packaging. Use a bench trial to determine whether sodium bicarbonate will help.


Unexpected phenolic flavors in beer are a sure sign of contamination. These include clove, peppery, spicy, smoky, and medicinal flavors. There's no way to remove these flavors from the beer once they're present.

Unexpected mold growth

Mold growth is able to be visually identified.

Mold is allergenic and can produce toxins and/or carcinogens. Therefore if your beverage has mold growing on it, it should be dumped, even if it smells or tastes normal.

See Mold for more information.

High attenuation

Some wild yeast can eat sugars that pitched yeast leaves behind (in beer), or can tolerate higher levels of alcohol than pitched yeast (in wine).

Usually if a contaminating microbe is increasing the attenuation, it will do so slowly. However many factors can affect attenuation and fermentation rate, so diagnosing a contamination solely by the attenuation is not always definitive.

Generally the best way to prevent over-attenuation in a contaminated beverage is to keep it cold during storage such that the wild microbes are inhibited and stop any fermentation activity.

Cleaning

Equipment should be properly cleaned after every fermentation. Radical methods are generally not needed after a known contamination.

See Cleaning for more information.

Preventing contaminations

We don't brew in a sterile environment. Wild microbes are everywhere — on surfaces and floating around in the air. Invariably some unwanted microbes make it into our beverages at some point during preparation, fermentation, handling, or packaging. Fortunately, the anti-microbial effects of our processes and pitching strategies tend to prevent effects of contamination most of the time, but only when our processes are solid. Preventing contamination is one of the most important aspects of brewing beer, and also important in wine production to a much lesser degree.

  • Clean your equipment thoroughly after use. See Cleaning.
  • Store the equipment where it won't get dirty again.
  • Use a quality no-rinse sanitizer on the equipment before use and just as importantly don't contaminate the sanitized equipment by touching it or touching it against anything unsanitary. See Sanitation.
  • Sanitize ingredients. If you don't want wild microbes, don't use ingredients that aren't heat-pasteurized (besides yeast).
  • Minimize open air exposure. Air is full of microbes.
  • Minimize time in the fermenter. This reduces the likelihood that a biofilm will form.
  • Consider avoiding risky processes, such as brewing with "no-chill" or no-boil recipes, repitching yeast, kettle souring, open fermentation, excessive numbers of transfers, secondary fermentation vessels, etc.

Terminology

"Infection" is not the best word for unwanted microbes in a fermentation, because an infection is a microbial invasion of a host organism. Beer/wine/etc. is not an organism.

Generally the term "contamination" should be reserved for unintentional introduction of microbes. Many times wild microbes are introduced intentionally, and the brewer should expect them to have some kind of effect. For example a wine made from unpasteurized fruit is expected to contain wild microbes, and therefore the appearance of a pellicle does not reflect "contamination".

Science

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