Beer Judging Supplemental Guide - Understanding Flavors and Aromas in Sour and Mixed Fermentation Beer

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This guide is aimed at helping to teach new beer judges how to judge wild and sour beer. Understanding and judging sour and mixed fermentation beer can be a challenge for anyone new to these styles. The best way to learn how to judge these types of beers is to drink commercial examples. There are a wide array of beers that fall into this category, from beers that exemplify the fruity character of Brettanomyces yeasts, to beers that focus mainly on sourness from lactic acid. This guide aims to accompany the BJCP/BA style guidelines and help judges learn how to judge sour and mixed fermentation beer.

Note on Styles

While there are a seemingly unlimited number of different types of beers that can blur the lines between styles in the BJCP guidelines, understanding sour and mixed-fermentation beer styles begins with the understanding that these styles were originally inspired by two European beer styles: lambic and Flanders Red/Brown. These European styles can sometimes have clearer definitions than modern craft examples, which tend to be more experimental and less well defined. In general though, the flavors and aromas of sour and mixed-fermentation beers are usually microbiologically driven. Malt and hops may also influence flavor and aroma in sour and mixed-fermentation beers, but the microbes used to ferment these beers may alter the character of these familiar ingredients. For example, dry hopping a mixed-fermentation sour beer with citra hops will make the hop character come across differently than it might in an IPA dry hopped with citra.

However, the microbes that are used in a sour or mixed fermentation beer do not define the beer within the context of judging. A judge should never assume which fermentation processes created a beer’s flavors and aromas. For example, a beer fermented with a lactic acid-producing yeast strain, such as some strains of Lachancea thermotolerans, might taste like a kettle-soured beer. It is up to the brewer to enter their beer into the category that the beer’s flavor and aroma profile best fit, not the category that best matches the beer’s fermentation profile. Brewers should provide as much information about their beer, including the microbes and special ingredients that were used, so that judges can better judge their beers.

Common Flavors and Aromas

Flavors and aromas in sour and mixed fermentation beers can be broken down into three major categories: fruity, sour, and what brewers collectively call “funky.”

Fruit flavors and aromas, whether imparted by microbes or actual fruit, are dominant characteristics of many types of sour and mixed fermentation beers. It is useful for brewers and judges to characterize the types of fruit flavors and aromas in sour or mixed fermentation beers, and the extent to which they are present. In general, when fruity flavors are not derived from actual fruit, they come from ethyl esters and other types of esters produced by microbes (often by wild Brettanomyces yeast, but also by wild Saccharomyces yeast and lactic acid bacteria). Lactic acid or low amounts of acetic acid can also lend their own fruity character to a beer, which often synergizes with any esters present. These types of fruity flavors can include pineapple, pear, cherry, apricot, overly ripened stone fruit, citrus, banana, mango, tropical fruit, apple, black currants, floral/roses. Fruit character can also come from actual fruit, the use of which brewers should declare, as appropriate, for the style category. Aging may minimize, change, or even eliminate fruit-derived flavor and aroma, but generally, flavor and aroma from fruit additions should be vibrant and noticeable.

The “sour” flavor and aroma of beers categorized as “sour beers” is driven by organic acids such as lactic acid and acetic acid, and in the case of fruit sour beers, citric acid and other organic acids derived from fruit. Judges should seek to describe both the flavor characteristics from the acid present in a sour beer, the sourness level, and any mouthfeel sensations such as a pleasant light puckering, or unpleasant burning in the back of the throat. Lactic acid generally comes across as a citrusy and sour flavor. Acetic acid, at low levels, comes across as a tart, sometimes green apple fruitiness. In high levels, acetic acid smells and tastes like vinegar and is considered an off-flavor (see Common Off-Flavors For Sour and Mixed Fermentation Beer below). Lactic acid tends to be less harsh on the throat than acetic acid, and is generally more desired than acetic acid for sour beer styles. However, low levels of acetic acid are often desired in some styles. In general, anywhere from low to high levels of lactic acid can be pleasant or desired in sour beers (consult the individual styles in the guidelines for appropriate level ranges), with higher levels leading to a more sour taste and lower levels leading to a light tartness. Organic acids derived from fruit can increase the overall sourness, or the perception of sourness, and should be taken into consideration by judges for fruit beer styles. While pH is useful for brewing processes, it is generally regarded as an inaccurate way to measure how sour a beer tastes. Instead of using pH to measure perceivable acidity, Titratable Acidity is considered to be more accurate.

The term “funky” is often used as a catchall for different combinations of individual flavors, and it means different things to different people. Judges should seek to pinpoint the individual flavors from this section’s generic “funky” umbrella, call out those flavors, and quantify the level at which they are present (e.g. “none” to “very high”). For the purposes of this guide and judging sour and mixed fermentation beers, “funky” refers to a range of flavors including medicinal, horse blanket, leather, barnyard animal, dried hay, plastic, smokey, band-aid, clove, goaty, cheesy, waxy, sweaty, solvent, rancid, soapy, oily, white glue, minty, dried rose, eucalyptus, vomit/bile, foot odor, coconut, and vanilla. These flavors are generally derived from phenols and/or fatty acids produced during fermentation by yeasts such as Brettanomyces, Pichia and other wild yeasts, or bacteria including enteric bacteria (in the case of spontaneous fermentation), lactic acid bacteria, and acetic acid bacteria. Defining whether these individual flavors are “off-flavors” or “desired flavors” can be tricky, but a good rule of thumb is that if a particular “funky” flavor is low enough to not distract the drinker and adds to the complexity and overall enjoyment of the beer, then it should be considered desirable. If, however, the level of a specific flavor is so high that it is off-putting, or distracting from the overall enjoyment of the beer, then it could be considered an off-flavor. Some styles, such as mixed fermentation sour beers and lambic/gueuze, welcome some level of many of these characters. Other styles, such as kettle sours, should have few or none of these flavors.

Common Off-Flavors For Sour and Mixed Fermentation Beer

As with all off-flavors, everyone has a sensory threshold for each of these. Some people may be more sensitive to some off-flavors and less sensitive to others. Judges should seek to understand how these individual off-flavors taste to them, and should seek to understand their strengths and weaknesses in detecting different off-flavors.

Acetic Acid

Among the most common off-flavors in mixed fermentation beers is an abundance of acetic acid. Often desirable in these beers at low levels, a high level of acetic acid smells and tastes like vinegar. At these levels, acetic acid is harsher than lactic acid and may cause an unpleasant burn in the back of the throat. Acetic bacteria (such as Acetobacteria, Gluconobacteria and others) produce acetic acid. Brettanomyces yeasts can also produce large amounts of acetic acid. In most cases, high amounts of acetic acid are only produced when an abundance of oxygen is available to the acetic acid-producing microbe(s). Too much acetic acid indicates the brewer may have exposed the beer to too much oxygen during the fermentation and/or aging processes. Low levels of acetic acid can impart a pleasant fruity character and are acceptable in sour beers, and/or beers brewed with Brettanomyces. However, this character should be restrained. Flanders Red style beers are an exception to this, where low to moderate amounts of vinegar flavor can be to style if they are balanced with maltiness, and resulting in a pleasant ‘balsamic vinegar’ character. Excessive acetic acid character and excessive ethyl acetate character can often be present together.


Ethyl Acetate

Ethyl acetate is a yeast-derived ester produced from acetic acid and ethanol. In small amounts, it smells and tastes fruity, often like pineapple, and can be beneficial to a mixed-fermentation beer. However, a high amount of ethyl acetate smells and tastes like a solvent, or “nail polish remover.” Solvent character is always considered a flaw in mixed fermentation beers (Flanders Red is an exception to this rule since classic examples often have a solvent character). A high amount of ethyl acetate indicates too much oxygen exposure during the fermentation and/or aging of a beer. Excessive acetic acid character and excessive ethyl acetate character can often be present together.



An acronym for tetrahydropyridine, THP is a microbe-derived ketone (like diacetyl) and is generally perceived in the aftertaste of a beer. Different forms can have slightly different flavor descriptors, but it is generally described as grainy at light levels, and like a mouse cage at higher levels. It is often compared with the flavor of the American breakfast cereal, Cheerios. THP is believed to be derived from certain strains of Lactobacillus (heterofermentative strains) and/or Brettanomyces. Its production has been associated with the beer’s exposure to oxygen. THP generally ages out of mixed-fermentation beers if they are kept at room temperature for an extended period. Small amounts of THP may not be offensive to some, but to many, any level of THP is considered to be an off-flavor. As with diacetyl, around 30% of the population cannot taste some forms of THP.


Butyric Acid

Butyric acid is microbially produced in sour beers and is considered an off-flavor. It is characterized as smelling and tasting like bile or vomit, or sometimes as “baby vomit,” with sweetness. This is a common off-flavor in kettle-soured beers and its presence generally indicates an unwanted contaminant.


Isovaleric Acid

Isovaleric acid can be microbially derived, or derived from hops that have been exposed to oxygen. It is generally characterized as tasting and smelling like pungent cheese, or foot odor. Very small amounts may be acceptable in some styles such as lambic-style beers, but any amount of this flavor is generally considered an off-flavor amount in most sour and/or mixed-fermentation beer styles. This character dissipates over a period of months as the beer ages.


Burnt Plastic/Rubber

Burnt rubber and/or burnt plastic aroma and/or flavor is generally considered an off-flavor in mixed-fermentation beers. However, very small levels can contribute to the complexity of the beer in a positive way, as long as they do not distract from the overall pleasantness of the beer. These compounds are generally thought to be caused by sulfur-based compounds such as mercaptans or phenols, and are indicative of unwanted microbes or a stressed fermentation.


Described as “fecal” or “pigs on a farm,” this character is considered an off-flavor in mixed-fermentation beer at any level. Its presence is indicative of unwanted microbes at some point during fermentation.



A mouthfeel character rather than a flavor, ropiness refers to increased viscosity. The increase may be merely threshold on the palate, or visibly pouring as if thick, like a vegetable oil or a syrup. Microbes can cause ropiness, specifically Pediococcus, but also genera such as Pichia, Debaryomyces, Candida, and Lactobacillus. Ropiness is actually the microbe metabolism byproduct, exopolysaccharides (EPS). Moderate to high ropiness in a mixed fermentation beer may indicate the beer needs more aging. In the presence of Brettanomyces, ropiness generally disappears after 3-9 months of aging at room temperature, although matured beers can still display a light viscosity that is considered desirable.


Additional Resources

Judges who wish to hone their understanding of sour and mixed-fermentation beers, and learn more about the production processes that produce desired (and sometimes undesired) flavors and aromas, can consult the following resources:


Authors: Dan Pixley, Dan Leone. Contributions and edits by: Fabian Kovacs, Fredrick Ekstedt, Debbie Cerda, Lana Svitankova, Gail Ann Williams, Scott Christoffel, Khristopher Johnson, Jon Stanley, Mike Lentz, Bob Givens.

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