Brettanomyces is a genus of yeast traditionally associated with old stock ale from 19th-century Britain and well-recognized as being responsible for tertiary fermentation in Lambic and Flanders red ales. Considered an integral part of terroir in a few select, barrel-aged red wines, Brettanomyces has historically been considered a “wild yeast” because of its spoilage capabilities and the characteristically funky flavors and aromas it can produce. Descriptions range from “floral” even “earthy” to “horse-blanket,” and where the winemaker blanches, the craft brewer rushes in. Today, Brettanomyces in the brewery is increasingly anything but wild; many craft brewers are culturing this fickle organism and purposefully using it to gain complex characteristics in their beers, like a new paint on an ever-broader canvas. Craft brewers casually refer to the yeast as “Brett,” a name that sounds appropriately like a new friend.

Since the first literature was published on this yeast in the early 20th century, nomenclature of the genus has changed multiple times, leading to confusion between the terms “Brettanomyces” and “Dekkera.” Brettanomyces is the asexual budding form known as an anamorph and Dekkera the sexual reproducing form known as a teleomorph. These are the same organism in different forms, which have not been observed simultaneously. The genus of Brettanomyces has five species, of which two are currently used in brewing, Brettanomyces bruxellensis and Brettanomyces anomalus. A third strain, Brettanomyces custersianus, has possible application in brewing but is yet to be used in a commercial beer. Brewers show little regard for scientific nomenclature and instead brewers will often refer to a species by its strain name, which confusingly is usually the old nomenclature that yeast scientists no longer use. Strains such Brettanomyces lambicus and Brettanomyces claussenii are actually Brettanomyces bruxellensis and Brettanomyces anomalus, respectively.

While modest amounts of research concentrated on Brettanomyces during the 20th century, its natural use went on unaffected. The Trappist beer Orval shows the iconic use of Brettanomyces as a secondary fermentation yeast, and even the spontaneous Lambic brewers producing one of the oldest styles of beer, tip their hat to this complex yeast. See lambic and orval brewery. Although frequently considered a Belgian yeast due to its previous lack of use in beers of other origins, one would be incorrect to make this assumption, as Brettanomyces can be found all over the world. Indeed, the name “Brettanomyces” literally means “the British yeast.” Increasingly, however, it is becoming the American yeast.

Craft brewers, particularly in the United States, have embraced the use of Brettanomyces, especially in conjunction with oak barrels. Brewers hoping to add complexity to their beers have turned to old traditions and started putting beer into oak barrels for aging. Modern-day brewers have been inoculating barrels with different strains and observing the changes in ester profiles, leading to the addition of unique flavors and aromas. Not much research has concentrated on the beneficial effects of Brettanomyces in brewing, and thus brewers who are inoculating barrels are just learning how versatile this yeast can be. One such use of Brettanomyces is in conjunction with bottle conditioning. See bottle conditioning. Beer styles that traditionally would have naturally contained Brettanomyces are now being recreated in the new world, and the addition of Brettanomyces at bottling has been carried out with notable success. Unlike more traditional brewers’ yeasts, Brettanomyces is hard to control, and harder yet to predict. Adventurous brewers take this as a challenge, one that can sacrifice strict control for the reward of exciting results.

One feature of Brettanomyces is its ability to produce certain acids. In the presence of large amounts of oxygen, acetic acid production can be high, as Brettanomyces oxidizes ethanol and residual sugars into acetic acid. See acetic acid. During barrel aging, small amounts of oxygen diffuse through the oak quickly being scavenged by Brettanomyces cells, and producing a crisp acidity. When using Brettanomyces for bottle conditioning oxygen is minimal so less acidity is produced leaving clean Brettanomyces flavors.

A common misconception is that beers produced with Brettanomyces are sour. Brettanomyces is not a souring organism; lactic acid bacteria are needed to create truly “sour” beers. Brettanomyces will not give more then a small tartness when used as the sole secondary or primary fermenting yeast. More important effects include the production of new esters and the reduction of other esters. Isoamyl acetate, a compound responsible for a banana-like aroma, is greatly reduced during Brettanomyces secondary fermentations while ethyl acetate and ethyl lactate appear in greater quantities. Recently it has been shown that when Brettanomyces is used as the primary fermentation yeast, two flavor-active esters are produced at detectable levels. Ethyl caproate and ethyl caprylate produce fruity, pineapple aromas and floral, apricot, tropical fruit aromas, respectively. Depending on the strain and technique used, these two esters can be produced at levels three to five times what trained tasters can smell, and so they appear to be important characteristics of an all-Brettanomyces fermentation. Volatile phenols are another group of compounds characteristic to Brettanomyces fermentations. The amounts produced vary depending on the strain and can leave crisp clove aromatics or horsey, medicinal aromas. Balanced with other aromatics, these phenols can become part of a unique and even beguiling beer aroma.

Modern Saccharomyces yeasts give today’s brewers much to work with, but inevitably some brewers want more, and many see Brettanomyces as one way to remove the leash of obvious commercial acceptability. Primary fermentations using 100% Brettanomyces yeast are true examples of the art and creativity seen in craft brewing around the world. Only a handful of brewers have produced Brettanomyces primary-fermented beers, and as no traditional style exists, each of the beers so produced is unique, leaving interpretation open to the consumer.