Fermenting with Brettanomyces

Often referred to simply as ​“Brett,” Brettanomyces is a type of yeast known for its distinctive flavor profile and ability to continue consuming sugars over extended periods of time, typically during secondary fermentation. But what if a beer was made with 100% Brett? Let’s discuss some differences between Brett and traditional brewing yeast, and dig into what a full-Brett fermentation might look like.

While Brettanomyces and Saccharomyces are both strains of brewing yeast that we can use today, their lineages diverged more than 200 million years ago! In the time since, both families have evolved and adapted to work under certain conditions and ferment in wildly different ways.

In a traditional Saccharomyces fermentation, yeast consumes the simplest sugars available and uses them to create flavor, alcohol, and carbon dioxide, among other compounds. However, as Saccharomyces wraps up chewing on its favorite foods, there are typically still some complex sugars left behind — this is where Brett can step in and start to feast. Where classic brewing yeast strains tear through simple sugars quickly, Brett takes a bit longer and chews away at fermentation much longer. It’s also capable of metabolizing carbohydrates and polysaccharides that Sacch can’t, like lactose (some strains) and cellobiose, an important component of beta-glucan formation. The end result is a wide variety of flavors that wouldn’t result from a strictly Saccharomyces fermentation.

Brett has a few interesting flavor impacts in beer:

• In the presence of oxygen, Brett respires and produces acetic acid. 

• Brett converts hydroxycinnamic acids (p‑coumaric, caffeic, and ferulic acids) to 4‑vinyl and 4‑ethyl derivatives (like the phenols 4‑VG and 4‑EP) whereas Sacch is only able to produce 4‑vinyl derivatives (like 4‑VG).

• Brett can produce tetrahydropyridine (THP), a compound described as tasting like Cheerios® cereal or being mouse-like. Much of the research on THP has been carried out in the wine world, but it has been gaining prevalence in beer thanks to the rise of mixed-culture and wild fermentations. In fact, in the wine world Brett is a major contamination risk — winemakers go to great lengths to avoid Brett and some will even use sterile filters to ensure no Brett is present in their cellars. 

One fascinating characteristic of Brettanomyces bruxellensis is that it can survive in the package long after fermentation has ended, while Saccharomyces cells will eventually die out. So that Brett beer you forgot about for years in your cellar may still have some viable cells in it even though it’s long done fermenting — that is, if the bottle is still intact! There are instances of viable Brett cells being recovered from wine bottles dated back nearly a century. 

While Brett is commonly known for its potent phenolics, at least one non-phenolic Brett strain exists: researchers have found a strain that has lost its functionality to convert ferulic acid to 4‑ethylguaiacol (4‑EG), a smoky, spice-like phenol, and 4‑ethylphenol (4‑EP), a band-aid-like phenol. This strain, termed CRL-90, may be considered a good option for brewers to take advantage of Brett’s non-phenolic flavor characteristics, like rich fruit aromas and drying capabilities, without the added phenolics.

There are also strains out there that were once thought to be a Brettanomyces strain, with incredibly similar flavor profiles and attenuation levels, but after further research and genetic sequencing, turn out to be Saccharomyces. Omega’s Tropical IPA (OYL-200) is one of these examples, with rich tropical fruit aromatics and the potential to attenuate incredibly high thanks to the presence of the STA1 gene.

Brett’s ability to consume complex sugars doesn’t mean that it won’t also consume simple sugars — it is yeast, after all. Like Saccharomyces, some strains of Brett will consume simpler sugars and drop in gravity during primary fermentation, but likely won’t produce very interesting ​“Bretty” flavors in such a short time. Brett is sluggish to ferment and often co-pitched or used in secondary fermentation. Expect your primary fermentation to take a few weeks longer with Brett than it would with typical brewing yeast, and be patient while it works its way closer to your target gravity. Packaging too soon could result in excessive carbonation with the potential to become a dangerous mess.

It’s much more common to see Brett employed as a secondary fermentation organism thanks to its ability to slowly contribute complex flavors. After primary fermentation there will be some fruity esters like pineapple and stone fruit present, but given many weeks or even months, Brettanomyces will begin to build up a complex profile with notes of hay, dried flowers, dried tobacco, leather, and other spicy and peppery phenolic aromas. This continued fermentation will also create a much more dry finish in these beers with an exceedingly light body reminiscent of brut sparkling wines. 

Perhaps the most famous use of Brettanomyces in commercial brewing is the beer made at Orval, a Trappist brewery in southern Belgium, near the French border. Orval’s namesake beer defies style conventions, but is often referred to as a Belgian pale ale. It undergoes primary fermentation and dry hopping before being bottled with a bit of priming sugar and a blend of yeast that includes Brettanomyces. Because Brett works more slowly than standard brewing yeast, it can take about six months for Brett’s phenolic character to become apparent in the bottle, with further complexity developing as the beer ages. A great exercise is to shop for Orval periodically, keeping an eye out for the packaging dates and doing a side-by-side tasting of bottles of various ages. A relatively fresh bottle of Orval has a nice, fresh hop character with some light fruitiness, while a batch from many years ago will contain peppery and leathery aromas to complement the soft fruit character.

Within the Brettanomyces genus, there is a ton of diversity. Here’s a quick rundown of a few popular strains:

B. claussenii — mild Brett ​“funk,” with a bit of leathery phenolics and some pineapple esters. This strain is a low flocculator and is not a good candidate for a 100% Brett fermentation.

B. bruxellensis — classic Brett ​“funk,” with a characteristic horsey, barnyard-like aroma. The name translates to ​“British fungus from Brussels,” as the strain is thought to have come from the Senne Valley region near Brussels and is often found in lambics.

B. lambicus — intensely ​“funky” with strong horsey and spicy phenolics and baked cherry-like aroma. This strain gets its name from its historical connection with lambic production in Belgium, even though genetically it is another strain of B. Bruxellensis.

While all three of these would be excellent when used in secondary fermentations, B. Bruxellensis and B. Lambicus are suitable candidates for a 100% Brett fermentation with some planning and patience. 

It isn’t impossible to make a beer with 100% Brettanomyces fermentation, but there are a few areas where brewers should be cautious and aware of ways to mitigate these concerns:

1. Contamination: brewers take great care to avoid the introduction of Brett when making non-Brett beer, and the same goes for making an all-Brett beer — you want to avoid Saccharomyces getting into your beer, too. Saccharomyces will multiply quickly and outcompete the Brettanomyces. Brett will still be able to work over time and create typical Brett-like flavors, but the introduction of Saccharomyces will reduce the total amount of sugars available for it to continue contributing flavors. Contamination is also a concern for the rest of the brewery. Dedicate a set of hoses and maybe even a specific tank for Brett fermentations if you intend to do this regularly.

2. Mouthfeel: Unlike Saccharomyces, Brett does not produce glycerol, a sugar alcohol that lends body and the perception of smoothness to a beer. However, glycerol isn’t the sole enhancer of a beer’s body — beta-glucans and ethanol are two additional contributors. Some brewers may opt to include oats, a major beta-glucan source, to help increase the viscosity of the wort and counteract the lack of glycerol production. Watch out for Brett’s ability to continue reducing gravity and thinning out the beer’s body, basically counteracting any mouthfeel benefits ethanol may have contributed.

3. Flavor balance: Brett is a big phenol producer, so a 100% Brett fermentation can often taste out of balance due to the increased phenol presence. Also, with prolonged exposure to oxygen, Brett can produce acetic acid over time and lend an unpleasant vinegary note to the beer.

In short, yes — a 100% Brett beer is possible…as long as you’re aware of some of the ways an all-Brett fermentation will differ from a traditional Saccharomyces-only or mixed-culture fermentation. Plan ahead for the lack of glycerol production and build mouthfeel-enhancing ingredients into your grain bill, and figure out you’ll best harness Brett’s ability to chew through complex sugars over time.