Wild Yeast is any species of yeast in fermentation other than the pitching yeast, often derived from the environment in or surrounding the brewery. The introduction of wild yeast into wort or beer can be intentional, as in the production of spontaneously fermented lambic, or unintentional, through contamination in the brewery. With the exception of lambic and other spontaneously fermented beers, wild yeasts are considered spoilage organisms in brewing and are avoided at all costs. See lambic.

Although the microbiology of beer is complex, the wild yeasts most often associated with brewing are natural strains of Saccharomyces and Brettanomyces (Dekkera) and, to a much lesser extent, the yeasts Candida and Pichia, as well as other oxidative yeasts. Wild Saccharomyces may include both natural strains of the ale, wine, and bread yeast Saccharomyces cerevisiae or the lager yeast Saccharomyces pastorianus. Although S. cerevisiae is known to exist independently of human activity and can be found on a variety of substrates including the surfaces of fruits and plants, as well as in soil and tree sap, there is a tendency for it to thrive in environments rich in simple sugars, such as ripe fruit. Wild Saccharomyces yeast contamination is only encouraged in spontaneously fermented beers. Brettanomyces, which is the bane of winemakers, is also found on the surfaces of fruits and is generally avoided, except in spontaneous fermentation or when intentionally pitched as a pure culture. Candida, Pichia, and other oxidative yeasts in beer are always unintentional because these yeasts can contribute high levels of acetic acid when exposed to oxygen.

During the crafting of lambic beers, which are spontaneously fermented in the Senne Valley region of Belgium, wild strains of both Saccharomyces and Brettanomyces are major determinants of the sensory profile and attenuation level of the beer. For production of lambic, the hot wort is cooled while being exposed to the ambient brewery environment, which invites inoculation of wild yeast from the air. The local flora of wild yeast and bacteria is thought to give the beers of each brewery their unique taste, and it is often said that the microbes for fermenting lambics take up residence in the “cobwebs” of breweries. The more likely scenario is that the microbes for lambic fermentation live in the porous wooden beams and fermentation vessels inside the brewery. These microbes are often local to the brewery producing the lambic, so certain lambics cannot be reproduced in another brewery because the brewery does not have the same natural flora. Such differences may be said to represent part of the terroir of these unique beers.

Although not technically “wild” yeast, many brewers are now experimenting with the controlled pitching of pure cultures of Brettanomyces, often in conjunction with acid-producing bacteria such as Lactobacillus and Pediococcus. A classic example of a beer that uses Brettanomyces in a controlled setting is Orval. Several North American craft breweries also use this method, some producing highly regarded, Brettanomyces-influenced sour beers. See sour beer.

In this new generation of sour beers, the introduction of wild yeast into cooled wort or beer is not always encouraged as it is with lambic brewing. Wild yeast can also be introduced unintentionally during fermentation or conditioning; this is termed wild yeast contamination. This contamination is undesirable because most brewers control the fermentation and/or conditioning process using the yeast(s) of their choice, which allows them to have control over the quality of the final product. Wild yeasts are frequently more robust fermenters than brewing strains and can survive and thrive under more adverse conditions, so these wild yeasts can outcompete the pitching yeast during fermentation. This presents a problem if the brewer repitches the yeast at the end of fermentation because the contaminating wild yeast will then be the majority yeast during the start of the next fermentation. Because of this phenomenon, brewers often employ rigorous tests to determine the presence of wild yeast before repitching yeast.

Wild yeast contamination can lead to unpredictable fermentation results because the characteristics of the wild yeast are unknown. They may differ from the pitching yeast in the level of sugar attenuation, production of esters, fusel alcohols, sulfur compounds, or other secondary metabolites that are important in beer flavor and aroma. The characteristic sign of wild Saccharomyces contamination is the presence of medicinal/phenolic or clove-like notes, which is not a characteristic of brewing yeasts, with the exception of German wheat beer yeasts. Flocculation is also generally lower in contaminating Saccharomyces, so contaminated beer that is unfiltered will tend to be turbid. Brettanomyces yeasts are able to metabolize higher sugars such as dextrins, so contamination with Brettanomyces can result in a highly attenuated, thin beer; if bottle conditioned and aged, these beers can become highly carbonated from the slow fermentation of dextrins that the Saccharomyces was unable to ferment. Brettanomyces is also responsible for the tastes and smells often colorfully described as “mousy,” “barnyard,” or “wet horse blanket.” As with some wines, these characteristics are considered “complexing agents” when found desirable, but ruinous when they are not. Wild yeast can be picked up during the brewing process any time that cooled wort or beer comes in contact with a nonsanitized surface or the air, such as during postboil wort transfer to the fermenter or during postfermentation conditioning or packaging. Along with spoilage bacteria, wild yeasts pose a biological threat to the intended quality of conventionally brewed beers.

See also brettanomyces.