Brettanomyces: The Yeast Behind Brett Off-Flavors in Wine

You open a bottle of wine you've been looking forward to and pour it into your glass. As you bring your nose close to enjoy the aromatic bouquet rising from the wine, something seems off. Many of you have probably had this experience.

Wine that produces unpleasant odors that shouldn't normally be present is said to have "off-flavors." There are several types of off-flavors in wine, but one of the most notorious is "Brett." Brett is described as having odors reminiscent of barnyard, horse sweat, or animalistic smells, and is particularly common in red wines.

This is something you're likely to encounter at some point in your wine journey. Today, we'll explore this troublesome character in the wine world that causes Brett.

Brett is Caused by Yeast

Brett is an off-flavor with a clearly identified cause: it's produced by a yeast called "Brettanomyces bruxellensis" (also referred to as Brettanomyces).

While Brettanomyces is the well-known name in the wine world, this yeast is also called "Dekkera bruxellensis." This becomes somewhat technical, but the different names are used based on the yeast's method of reproduction. Dekkera refers to the form that reproduces by forming ascospores, while Brettanomyces bruxellensis refers to the form that reproduces only through cell division without sexual reproduction.

In wine and sherry production environments where Brettanomyces tends to be problematic, the environmental conditions for yeast proliferation are poor, and the yeast cannot afford the luxury of sexual reproduction. Therefore, it often proliferates as Brettanomyces strains rather than Dekkera strains, which is why it became primarily known as Brettanomyces.

The name "Brett" comes from the first part of the causative yeast's name, "Brett"anomyces. Generally, the term "Brett" refers to the off-flavor caused by Brettanomyces, and the yeast itself is not called "Brett."

As mentioned, Brettanomyces is a yeast. When we think of yeast, most people think of the yeast that plays an important role in alcoholic fermentation when grape juice becomes wine. Brettanomyces is indeed one such yeast, but it's a different species from those commonly used for alcoholic fermentation.

Wine, beer, and sake fermentation use a yeast species called "Saccharomyces cerevisiae." To use an analogy, it's like having Caucasian, African, and Asian populations within the broader category of humanity. While there aren't fundamentally large differences, there are slight variations in preferred food types, quantities, or physical capabilities depending on the specific type.

Brettanomyces can also produce alcohol, but many strains have been found to have lower production capacity than Saccharomyces cerevisiae. Additionally, alcohol production by Brettanomyces has disadvantages that far outweigh any benefits, so it's not commonly practiced.

Why Brettanomyces Isn't Used in Wine Production

The characteristics of Brettanomyces can be summarized as follows:

• Relatively low alcoholic fermentation capacity
• Broad sugar metabolism capability with low nutritional requirements
• Alcohol tolerance
• Production of acetic acid and esters
• Generation of undesirable fermentation byproducts
• Film-forming properties (ability to form films on liquid surfaces)

The role expected of yeast in wine production is clear. The expectations for yeast can be summarized in two points: "efficiently produce alcohol" and "not generate unwanted aromas." When we compare these requirements with Brettanomyces characteristics, it becomes clear why this yeast is not needed in winemaking.

Fermentation capacity here refers to alcohol production capability. As already mentioned, Brettanomyces has relatively lower alcohol production capacity compared to Saccharomyces cerevisiae, which is known as a common wine yeast. Simply put, using Brettanomyces for alcohol production is inefficient.

Furthermore, Brettanomyces produces acetic acid and esters during its metabolic processes, which are also the causes of Brett. This is exactly what we mean by "generation of unwanted aromas."

While Brett is sometimes considered a positive element for wine when present in low concentrations, "mousy taint," which is also caused by Brettanomyces like Brett, is clearly treated as an off-flavor. With poor efficiency in alcohol production and being a cause of off-flavors, Brettanomyces is normally not used in winemaking.

Where Does Brettanomyces Come From?

Brettanomyces is often said to inhabit the oak barrels used for wine fermentation and aging. This explains why Brett frequently occurs in red wines that use barrels extensively. But where did this Brettanomyces that settled in barrels originally come from?

Like other yeasts, Brettanomyces is a yeast that commonly exists in nature. It's said that warmer regions have higher populations, but this doesn't mean it doesn't exist in cooler regions.

Like other wild yeasts, Brettanomyces attaches to grape skin surfaces or gets mixed into the fruit pulp through insects and arrives at wineries.

During fermentation, dry yeast is added or yeasts with higher fermentation capacity become dominant, so it's rare for Brettanomyces to become dominant enough to contaminate wine. However, this doesn't mean Brettanomyces dies off. Brettanomyces continues to survive quietly in wine during and after fermentation.

When wine containing living Brettanomyces is placed in oak barrels, the yeast "infects" the barrel from the wine and subsequently contaminates wines that are continuously placed in that barrel.

What's troublesome here is that barrels contaminated with Brettanomyces are not limited to those that have previously contained wine.

As already mentioned, Brettanomyces is a ubiquitous microorganism that commonly exists in nature. Therefore, this yeast's attachment targets are not limited to grape skins.

Wood materials used for barrels like oak are stored outdoors for drying for several years after harvesting. This means Brettanomyces can attach to the wood during storage.

Barrels made from such wood may already be contaminated with Brettanomyces even when they're unused and freshly shipped. It has been pointed out that Brett can develop in wine when such contaminated barrels are used.

It's relatively easy to kill or remove Brettanomyces present in wine. However, it's virtually impossible to completely remove Brettanomyces that has contaminated wooden barrels.

This is a commonly misunderstood point: Brett doesn't occur simply because Brettanomyces exists in wine. Brett occurs as a result of Brettanomyces present in wine or in areas that wine contacts performing metabolism. In other words, even if Brettanomyces exists in wine, if it's inactivated or removed before sufficient metabolism occurs, no damage will occur to the wine.

In cases where Brettanomyces enters through grape skin attachment, it's possible to address this before damage spreads. However, when storage locations like barrels that hold wine for extended periods become contaminated, such measures suddenly become difficult. This is why barrel contamination is so troublesome.

Common Misconceptions About Brettanomyces

Despite being widely known in the wine world, Brettanomyces is surprisingly subject to many misconceptions. One such misconception is that Brettanomyces is some kind of bacteria that develops during fermentation, and Brett is spoilage caused by this bacteria.

As we've seen, Brettanomyces is a yeast. Since yeast is a type of microorganism, it doesn't "develop" during the fermentation process. In this regard, Brettanomyces is a contaminant in the winemaking process, and Brett is the result of such contamination.

Another misconception is that Brettanomyces only exists in red wine, and therefore Brett never occurs in white wine. This is also incorrect.

Brettanomyces has high survival capability and exists regardless of whether it's in red wine or white wine. When attaching in vineyards, it doesn't choose between white grapes or black grapes.

It's simply more commonly observed in red wines due to differences in the original wine's pH value, timing of adding antioxidants (sulfur dioxide, SO2, also called sulfites), management temperature, and frequency of wooden barrel use. The metabolism that causes Brett begins with types of acids commonly contained in grapes. Since these acids are contained in both white and black grapes without difference, Brett generated from them doesn't occur "only in red wines and not in white wines."

How to Deal with Brettanomyces

Damage from Brettanomyces is often described as "contamination." As this expression suggests, sanitation management is crucial in dealing with Brettanomyces.

Sulfur dioxide use is effective for suppressing Brettanomyces. It can also be removed relatively easily through wine filtration. Conversely, unfiltered wines with no SO2 addition, as seen in some natural wines that have become popular recently, have higher contamination risks. Additionally, Brettanomyces proliferation and further Brett contamination can occur not only in wineries but also in bottles after bottling.

From a prevention standpoint, it's important to promptly discontinue use and dispose of contaminated barrels, and avoid unlimited intentional propagation.

Summary: Positive Use of Brett Requires Serious Commitment

Brettanomyces contamination spreads much more easily than most people imagine. The reason damage doesn't become too severe is solely because its spread is suppressed by routine cleaning activities.

However, routine cleaning activities are rarely performed with conscious awareness of Brettanomyces prevention. In such cases, successful Brettanomyces prevention was simply due to good fortune. The same applies to wine filtration and sulfite use.

What has worked well unconsciously until now can stop working with just slight changes in behavior. This represents an extremely large risk.

In this context, there's a movement to use Brett positively to give wines character and differentiate them from other wines. To use Brett positively, it's necessary to intentionally introduce Brettanomyces into wineries and allow it to proliferate. While such attempts are permissible, the scope of subsequent effects is likely to be much larger than most people expect, which is often not considered.

Attempting such things without properly understanding Brettanomyces ecology leads to contamination in unwanted areas. When wines resulting from such contamination reach the market, the scope of "contamination" extends to the consciousness of consumers who obtain them.

If you want to experiment with Brettanomyces, you must approach it with appropriate commitment and preparation. Attempting this without established methodologies will only harm yourself without bringing any benefits.

More detailed information about Brettanomyces and possibilities for "experimental approaches" with this yeast will be published as member-exclusive articles in the online circle "Peering into the World Winemakers See." Partially re-edited versions of exclusive articles are also published on note, so please check those out if you're interested.