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Brettanomyces: friend or foe?
A masterclass with Matt Thomson, winemaker

Matt Thomson is a well-known New Zealand winemaker, who consults widely, with particular associations with Saint Clair and Delta (he’s part owner) in Marlborough . He also works in Italy (he’s been there every vintage since 1994), in association with David Gleave’s Liberty Wines.

The purpose of this masterclass was to look at Brettanomyces (abbreviated here as ‘brett’), that celebrated rogue yeast that’s responsible for making some red wines smell of animal sheds—it’s not usually encountered in whites.

Thomson brought over with him a range of barrel samples of three different wines, two Pinot Noirs and a Merlot, which had been taken from barrels that he and his winemaking team had identified as being bretty, to show us what brett tastes and smells like in its very earliest stages.

First, he began with a run-down on the facts about brett. It is a yeast, and the nomenclature used in the wine industry differs from the recently devised taxonomy, which brings the previous five ‘species’ of brett into a single species, Dekkera bruxellensis. It has been known for about a hundred years, first having been identified as an important component in British and Belgian beer styles.

Indeed, when the first single culture Saccharomyces cerevisiae yeasts (this is the species of yeast used for making wine) were used to make British beers, people noticed that something was missing: the imprint of brett, which in the context of a good bitter adds real interest. Interestingly, brewers commonly refer to brett character in beer as being ‘vinous’.

Brett is a slow-growing yeast that is pretty tough, and can survive hostile conditions such as high alcohol and low nutrient levels. As it grows, a small volume of CO2 is produced, which can often be a clue as to its presence in barrels that have already finished malolactic fermentation. Growth is stimulated by small amounts of oxygen, such as you might find in a barrel, and particularly one that isn’t topped up well.

The sensory effects of brett are many. The first sign is reduced varietal character, followed by the degradation of certain fruity aromas by esterases present in this yeast. Thus Pinot Noir is particularly badly hit: it loses its bright cherry and violet characters, and this loss of fruit is a good early cue for the presence of brett in barrels.

Specific aromatic compounds are produced by brett, and their imprint can vary significantly. Affected wines can taste medicinal, or earthy, or mousy, or smoky, or poopy. Compounds produced by brett include 4-ethylphenol (this is only ever produced by brett in wines, so it is used as an indicator of brett activity), 4-ethylguaiacol, isovaleric acid, 2-phenyethanol, guaiacol, ethyldecanoate, trans-2-nonenal, isoamyl alcohol and ethyl-2-methylbutyrate.

Brett likes oak. It particularly likes toasted new barrels, and has been found 8 mm deep in staves. It can feed off a compound, cellobiose, that is formed when barrels are toasted. It likes high pH, residual sugar, low SO2 and lees (it enjoys cloudy, turbid wine). It can go dormant, for example after an SO2 addition, so that culturing doesn’t reveal its presence, and then re-emerge later on to bloom in bottle. In short, it’s a bit of a bugger.

One misconception about brett is that it a hallmark of wineries with poor hygiene. ‘Brett can occur in the cleanest cellars’, says Thomson. He thinks that oak is largely to blame for many infections, because brett can live in the oak and is almost impossible to get out by cleaning. ‘If you use new oak, you will get brett: it is not something you can associate just with a dirty cellar’.

But Thomson goes further, suggesting that brett is not only associated with new oak, but also that he has identified specific coopers who have a problem with bretty barrels, although he won’t name them. He also thinks that brett is a growing problem. ‘I am convinced that in large numbers of wineries in both the new and old worlds, brett is a new thing.’ He has a theory that something happened to oak in the relatively recent past. ‘Something happened with the huge demand for new oak in the 1980s. Coopers had a boom period and started doing something different, and there was a change’. That’s an interesting idea.

What can be done to avoid brett, according to Thomson? This is where things get interesting, because many of the steps that need to be taken in order to ensure clean wines run counter to the sort of winemaking approach you’d want to take to make interesting wines.

The first is to avoid barrels. Stainless steel can be cleaned properly, and ‘then you can pretty much eliminate it’, says Thomson, ‘although you have to be careful with ball valves’. Second, you need to avoid cross-contamination. When taking barrel samples Thomson uses plastic barrel thiefs that are used just once, and then sterilized. He also avoids doing rack and return where the wine would go from several barrels to be mixed up in one tank: instead, each barrel is racked separately to tank and returned, and the tank cleaned before the procedure is repeated with the next barrel.

The next stage is to keep pH low, either by acidifying or harvesting earlier. Brett doesn’t like low pH, and at low pH SO2 additions are much more effective. Other steps include avoiding lees ageing, keeping barrels topped up, and aggressively cleaning new and used barrels (if you decided to use them in the first place). Thomson says that high pressure water washing can be effective, but it takes many of the beneficial components from the oak away. He’s looking at steam cleaning barrels, and is currently working on how long this has to be done for to eliminate all yeasts.  

Two more approaches to brett control are the use of a chemical called DMDC, and filtration. DMDC stands for dimethyl dicarbamate (also known by its trade name of Velcorin). This is extremely toxic to microbes, but breaks down into harmless products once it has done its job. It is now legal in the EU. Thomson describes it as ‘quite promising’.

Filtration is another way of dealing with brett: Thomson agrees that it is a hot topic and each wine needs to be treated on a case-by-case basis. ‘You can sterile filter some wines and you can’t tell the difference; in other cases the wine can be absolutely stuffed’, he maintains. ‘You need to understand when you can get away with filtering and when you can’t’.

It strikes me that there are two different philosophical approaches to the issue of brett. One is the inoculum concept: that you want to avoid any presence of brett in your wines in much the way that a surgeon will scrub up and try to keep the operating theatre sterile. The other is the habitat concept: assuming that brett is pretty much everywhere, you want to make your developing wines the sorts of places where brett isn’t going to thrive. This latter approach seems to me to be more appropriate.

Then we looked at some wines. There were two flights of Pinot Noir, and one of Merlot. Each flight consisted of barrel samples from the same wine which Thomson and his team felt were showing signs of brett. He outlined the sorts of effects of brett on Pinot Noir as the bloom developed:  

Progression of effects in Pinot Noir

  • Loss of primary fruit, particularly lifted violet and sweet cherry characters. Loss of new oak characters.

  • Hints of smoke and spice appear (4-ethylguaiacol)

  • The wine begins to smell and taste medicinal (4-ethylphenol)

  • Any silky characters disappear as the wines lose flesh

  • The bones of the wine are exposed, leaving acidity and rustic, drying tannins

  • Aromas of horse and bandaid appear (4-ethylphenol)

Tasting the wines was instructive. They varied from being pretty bright and expressive to rather muted and earthy. But if I’m honest, it was only because this was a brett seminar that I was able to say with any degree of certainty that I was discerning brett in these wines: the effects at this early stage were relatively subtle. ‘These wines still have a lot to lose’, says Thomson, although he reckons that as the brett bloom develops, they will become quite unpleasant.

One final thought: if you are a winemaker, you want to be able to spot brett at a very early stage, but an acute sensitivity to it is not something a critic necessarily wants to develop. The risk is that you end up with ‘cellar palate’, being able to spot ‘faults’ at a thousand miles. ‘One regret I have is that as a winemaker, you become less tolerant’, says Thomson. This means that there are some wine styles he just can’t enjoy because of the presence of ‘faults’.

Having said this, brett is something that the trade needs to become more aware of. ‘Lots of winemakers still haven’t grasped the complexity of brettanomyces’, says Thomson, ‘and there’s still a bit of denial out there’. It’s also something that he thinks everyone can get. ‘All decent tasters can pick the nuances of brett once you tune into it.’ In conclusion, I’d suggest that we need to become more aware of brett, and better at spotting it in its early stages, while keeping an open mind about wine styles where it adds complexity, and not becoming brett policemen, always trying to spot it in whatever wine we’re tasting.

Article published November 2007

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