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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 style guidelines and help judges learn how to judge sour and mixed fermentation beer.

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.

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 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.

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.

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.

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.