Beer Sensory Skills 101 | Infections | Acetic Acid

This article is part of a series of lessons and exercises that focus on enhancing your sensory skills to better recognize and address brewing problems. By training your sense of smell, taste, mouthfeel, etc., you will have a better understanding of identifying off-flavors/aromas and their causes, which can greatly assist you when looking for solutions to these problems. This blog series will come in many parts dealing with Yeast, Hops, Malt, Water, Infections/Contaminations, and Miscellaneous issues. 
Each lesson includes an exercise in a separate tab to assist you in developing your palate and other senses to better detect these flavors, aromas, etc.

Lesson 5.2 - Infections: Acetic Acid - When Your Beer Turns Into Vinegar

Vinegar in my Beer?

Acetic acid, aka "Vinegar", can indeed be a problem in beer, as it is typically associated with sourness and vinegar-like flavors and aromas, which are undesirable in most beer styles. The presence of acetic acid in beer usually indicates contamination or spoilage, often caused by acetic acid bacteria (AAB) such as Acetobacter or Gluconobacter. These bacteria convert ethanol into acetic acid in the presence of oxygen, leading to off-flavors and aromas.

Acetic acid imparts a sharp, sour, and vinegar-like taste and aroma to beer. While a mild acidity might be acceptable or even desirable in certain sour beer styles (e.g., Berliner Weisse, Gose, Lambic), excessive acetic acid is generally unpleasant and overpowering, masking the beer's intended flavors and aromas.

The Chemistry:

The chemistry behind acetic acid contamination in beer involves certain bacteria, like Acetobacter or Gluconobacter, converting alcohol into vinegar when oxygen is present. Here's a simpler explanation: the alcohol in your beer, known as ethanol, can be transformed into acetic acid by bacteria like Acetobacter. This process requires oxygen. Initially, the bacteria use an enzyme to convert ethanol into acetaldehyde. Then, another enzyme turns the acetaldehyde into acetic acid.

This contamination typically happens when beer is exposed to air during fermentation, aging, or packaging, providing the necessary oxygen for the bacteria to thrive. If brewing equipment isn't properly sanitized, these bacteria can get into the beer and start converting alcohol into vinegar.

Acetic acid bacteria are aerobic microorganisms, meaning they require oxygen to thrive and carry out these oxidative reactions. The presence of oxygen in the brewing environment, especially during fermentation or storage, facilitates the growth of AAB and the production of acetic acid.


Avoiding Acetic Acid:

To avoid acetic acid contamination in beer, brewers need to focus on preventing the introduction and growth of acetic acid bacteria (AAB), which are responsible for converting alcohol into vinegar. Here are some key steps to avoid acetic acid contamination:


Sanitation: Ensure all brewing equipment, including fermenters, hoses, and packaging materials, is thoroughly cleaned and sanitized to eliminate any potential sources of contamination.

Oxygen Control: Minimize oxygen exposure throughout the brewing process, especially during fermentation and packaging. Use closed systems and purging techniques to reduce oxygen contact with the beer.

Yeast Health: Maintain healthy yeast populations during fermentation to outcompete any potential AAB and minimize the risk of contamination. Proper pitching rates, nutrient supplementation, and fermentation temperature control can support yeast health.

Temperature Control: Control fermentation temperatures to prevent the growth of AAB, which thrive in warmer conditions. Maintain appropriate temperature ranges for the specific yeast strains used in the beer.

Quality Ingredients: Use high-quality ingredients free from contamination to minimize the risk of introducing AAB into the brewing process.

Storage Conditions: Store packaged beer in cool, stable conditions to inhibit the growth of AAB and prevent the development of off-flavors over time.


By following these practices and maintaining strict quality control measures, brewers can minimize the risk of acetic acid contamination and ensure the production of high-quality beer with clean, desirable flavors.


Yeast autolysis stages

Bacteria on the left, yeast on the right.


Detecting Acetic Acid



Vinegar-like Sharpness: Acetic acid contamination in beer is often identified by a sharp, vinegar-like aroma. This distinct smell is reminiscent of household vinegar and is a clear indicator of acetic acid presence.

Solvent-like Notes: In some cases, acetic acid can produce solvent-like aromas, which might remind one of nail polish remover or paint thinner. These sharp, chemical-like smells are usually considered undesirable.

Sourness: A general sour aroma can also be present, often accompanied by the sharpness of vinegar. This sour smell can sometimes be mistaken for the natural sourness in certain beer styles, but it is typically more pungent and off-putting.

Green Apple: While primarily associated with acetaldehyde, in some instances, acetic acid presence can enhance or be accompanied by a green apple-like aroma, adding to the complexity of the off-flavor profile.



Vinegar-like Tang: The most prominent flavor from acetic acid contamination is a vinegar-like tang, which imparts a sour, acidic taste to the beer, similar to that found in vinegar or pickled products.

Harsh Acidity: The presence of acetic acid can result in a harsh, puckering acidity that dominates the palate, often overwhelming the beer's other flavors and leading to an imbalanced taste experience.

Chemical or Solvent-like Taste: Acetic acid can also produce chemical or solvent-like flavors, reminiscent of nail polish remover or industrial solvents, which are usually unpleasant and undesirable in beer.

Overripe Fruit: The sourness from acetic acid can be accompanied by flavors reminiscent of overripe or fermented fruit, contributing to a cloying, off-putting taste.

Cider-like Sharpness: In some instances, the sourness imparted by acetic acid can give the beer a cider-like sharpness, which might be mistaken for a pleasant tartness in very small amounts but becomes undesirable at higher concentrations.

Off-Flavors: In some instances, the sourness imparted by acetic acid can give the beer a cider-like sharpness, which might be mistaken for a pleasant tartness in very small amounts but becomes undesirable at higher concentrations.


Acetic acid contamination in beer is characterized by distinctive vinegar-like, sour, and solvent-like aromas and flavors. The intensity of these sensory attributes can vary, but they generally lead to an imbalanced and unpleasant drinking experience. Proper sanitation and oxygen control are essential in preventing acetic acid contamination and ensuring the production of high-quality beer.


Sensory Training:

Taste Different Beers: Begin by sampling a range of beers across different styles, paying close attention to their flavor profiles. Seek out beers known to exhibit acetic acid characteristics, such as certain sour ales, lambics, or beers that have been improperly stored or infected.

Conduct Comparative Tastings: Compare beers that have different levels of acetic acid. Try tasting a fresh beer alongside one that has been known to develop acetic acid to understand the differences in flavor and aroma resulting from acetic acid contamination. Add a small amount of white vinegar to some light beers and try detecting it at different levels.

Taste Foods Containing Acetic Acid: Familiarize yourself with the taste of acetic acid by consuming foods and beverages that naturally contain it. Vinegar, especially apple cider vinegar or balsamic vinegar, is a direct source of acetic acid. Other fermented foods like kombucha and certain pickles can also help you identify the sour, sharp flavor profile associated with acetic acid.

Experiment With Food Pairings: Pair beers suspected of having acetic acid with various foods to see how the flavors interact and affect your perception of the beer. This can help in distinguishing the specific sourness and sharpness of acetic acid from other sour flavors. Here are some examples:

     ° Sour Ales: These beers often contain intentional sourness from lactic or acetic acid. Pair them with rich, fatty foods like cheese or charcuterie to see how the acidity cuts through the fat.

     ° Lambics: Traditional lambics can have a balanced acetic character. Try them with fruit desserts, where the tartness of the beer complements the sweetness of the fruit.

     ° Improperly Stored Beers: Beers that have been exposed to oxygen may develop acetic acid. Compare these with fresh counterparts to understand how storage conditions affect flavor.


By actively engaging in tasting exercises, expanding your beer knowledge, and seeking feedback from others, you can gradually train your palate to detect acetic acid flavors in beer with greater precision and confidence.

Recommended Beers For This Lesson

Acetic acid is generally considered an off-flavor in most beer styles due to its sharp, vinegar-like characteristics that can overpower the beer's intended flavors. However, there are certain beer styles where a controlled and balanced presence of acetic acid can be desirable, contributing to the complexity and depth of the flavor profile.

In traditional Belgian Lambics and Gueuzes, acetic acid is often present in small amounts, complementing the lactic acid and other souring compounds produced during the spontaneous fermentation process. These beers are known for their complex, tart, and funky flavors, and a touch of acetic acid can add to the overall sensory experience without becoming unpleasant.

Similarly, in Flanders Red Ales and Oud Bruins, acetic acid is a natural byproduct of the extended aging process in wooden barrels, where acetic acid bacteria can thrive. When balanced correctly, the acetic acid contributes to the beer's rich, fruity, and sour character, enhancing its depth and complexity.

In these styles, the presence of acetic acid is carefully managed through controlled fermentation and aging processes to ensure it adds to the beer's overall profile rather than detracting from it. The key is balance; too much acetic acid can lead to an overwhelmingly vinegary taste, while the right amount can enhance the beer's unique and desirable characteristics.

Unfortunately, we do not have any recipes for these styles here at Mr. Beer due to how difficult and time-consuming they can be to make. A typical gueuze, for example, can take up to 7 years to make due the aging and blending of the lambics that make up this rare style.