Yes, infected beer often forms a film (or pellicle) if left out, especially if it contains wild yeasts or bacteria that thrive in aerobic conditions. This film is a byproduct of the microorganisms consuming sugars and producing various compounds, including those that contribute to the characteristic look and sometimes altered flavor of infected beer. The type of film, its thickness, and its appearance depend heavily on the specific infecting organism(s) and the composition of the beer itself.
Understanding Beer Infection and Film Formation
Beer, a carefully crafted beverage, is vulnerable to various microorganisms besides the intended brewer’s yeast. These unwanted guests, collectively termed “beer spoilage organisms,” can significantly alter the beer’s flavor, aroma, and appearance. One prominent sign of infection is the formation of a film, also known as a pellicle, on the beer’s surface. This film is a visual indicator of microbial activity and is often a harbinger of off-flavors.
What Causes a Pellicle to Form?
Pellicles are typically formed by wild yeasts and bacteria that have gained access to the beer, often through improper sanitation practices during brewing or packaging. Some of the most common culprits include:
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Brettanomyces (Brett): This wild yeast is known for its ability to ferment complex sugars that brewer’s yeast cannot, leading to the production of unique, often funky, flavors. Brettanomyces pellicles are typically thin and wrinkly, resembling a sheet of tissue paper.
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Lactobacillus (Lacto): These bacteria produce lactic acid, which gives beer a sour taste. Lacto pellicles can vary in appearance, from thin and powdery to thick and bubbly.
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Pediococcus (Pedi): Another lactic acid-producing bacteria, Pediococcus can also create diacetyl, contributing a buttery flavor and aroma. Pediococcus pellicles are often thick and ropey.
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Acetobacter (Acetobacter): These bacteria convert ethanol into acetic acid, resulting in a vinegary flavor. Acetobacter requires oxygen to thrive, so pellicles are almost always present in infected beers.
The Aerobic Advantage
Most pellicle-forming microorganisms thrive in the presence of oxygen. When beer is left out, exposed to air, these organisms can access the oxygen they need to grow and reproduce. The pellicle itself often acts as a protective barrier, shielding the underlying beer from further oxygen exposure and creating a favorable microenvironment for the infecting organisms.
Identifying and Differentiating Pellicles
While any film on the surface of beer should raise suspicion, not all films are necessarily indicative of a serious infection. Factors like the beer’s original gravity, hop content, and alcohol percentage can influence the appearance of a pellicle. It’s essential to carefully observe and consider all the available information before concluding that a beer is infected.
Visual Clues and Flavor Profiles
- Thin, wrinkly film: Often associated with Brettanomyces, potentially contributing flavors described as funky, horse blanket, or barnyard.
- Thick, ropey film: Frequently linked to Pediococcus, potentially leading to buttery (diacetyl) flavors and a viscous texture.
- Powdery, white film: May be caused by Lactobacillus, typically producing a sour or tart flavor.
- Vinegary aroma: Strong indication of Acetobacter contamination.
Further Investigation
If you suspect your beer is infected, consider the following:
- Smell: Does the beer have any unusual or unpleasant aromas?
- Taste: Does the beer taste noticeably sour, vinegary, or otherwise off?
- Appearance: Is the beer hazy or cloudy? Are there any unusual sediments?
FAQs About Infected Beer and Pellicle Formation
Here are some frequently asked questions about infected beer and the formation of pellicles:
FAQ 1: Is a pellicle always a sign of infection?
Yes, a pellicle is almost always a sign of microbial contamination in beer beyond the intended brewing yeast. While some brewers intentionally introduce wild yeasts and bacteria to create sour or funky beers, the presence of a pellicle in a beer not intended to be sour or funky indicates an unwanted infection.
FAQ 2: Can I drink beer with a pellicle?
Whether you can safely drink beer with a pellicle depends on the infecting organism. While most beer spoilage organisms are not pathogenic (disease-causing), they can produce unpleasant flavors and aromas that make the beer undrinkable. Some infections may produce compounds that could cause digestive upset. It’s generally recommended to avoid drinking beer with a pellicle unless you are certain it was intentionally soured.
FAQ 3: How can I prevent beer infections?
Proper sanitation is crucial for preventing beer infections. This includes thoroughly cleaning and sanitizing all brewing equipment, including fermenters, bottles, and kegs. Using a no-rinse sanitizer like Star San is highly recommended.
FAQ 4: What’s the difference between a pellicle and krausen?
Krausen is a foamy layer that forms on top of beer during active fermentation, caused by the yeast releasing carbon dioxide. It’s a normal and desirable part of the brewing process. A pellicle, on the other hand, forms after fermentation is complete and indicates an infection. Krausen usually dissipates within a few days, while a pellicle tends to persist and even grow over time.
FAQ 5: Can high alcohol beers get infected?
Yes, even high-alcohol beers can get infected, although they are less susceptible to some spoilage organisms than lower-alcohol beers. Alcohol acts as an antiseptic, but some wild yeasts and bacteria are tolerant of high alcohol levels.
FAQ 6: Can I reuse brewing equipment that has been used for sour beer?
It is generally not recommended to use equipment used for sour beer brewing for regular beer brewing, as it can be difficult to completely eliminate the wild yeasts and bacteria. If you do, dedicate specific equipment for sour beer brewing only to prevent cross-contamination.
FAQ 7: What are the off-flavors commonly associated with infected beer?
Common off-flavors associated with infected beer include sourness (lactic acid), vinegary notes (acetic acid), buttery or butterscotch notes (diacetyl), medicinal or band-aid aromas (phenols), and funky or barnyard notes.
FAQ 8: How long does it take for a pellicle to form?
The time it takes for a pellicle to form depends on the type and concentration of the infecting organism, as well as the temperature and composition of the beer. A pellicle can start to form within a few days to several weeks after fermentation is complete.
FAQ 9: Can bottling or kegging equipment spread infection?
Yes, improperly sanitized bottling or kegging equipment can easily spread beer infections. Thoroughly cleaning and sanitizing these items is essential to prevent contamination.
FAQ 10: Does hop content affect the likelihood of infection?
Yes, hop content can inhibit the growth of some beer spoilage organisms, particularly some bacteria. Higher hopped beers are generally less susceptible to infection than lower hopped beers.
FAQ 11: Can I test my beer for infection?
Yes, several methods can be used to test beer for infection, including microscopic examination, plating on selective media, and PCR (polymerase chain reaction) testing. These methods can help identify the specific microorganisms present in the beer.
FAQ 12: What should I do if I think my beer is infected?
If you suspect your beer is infected, isolate it from other beers to prevent cross-contamination. Monitor the beer for changes in flavor and appearance. If the off-flavors are significant, it’s best to discard the beer. Consider reviewing your sanitation practices to identify and correct any potential sources of contamination.