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Brettanomyces is a common defect in wine, but controversy surrounds the subject. Jamie Goode sifts the available data and canvasses the views of world-famous winemakers to determine how widespread the problem is, and whether brett can ever be a good thing. (Originally published in Harpers Wine and Spirit Weekly, 18 April 2003, p 42-46)

When Brian Fletcher, chief winemaker at Calatrasi in Sicily, found out I was writing this feature, he couriered me a bottle of red wine. Labelled simply as ‘Brettanomyces’, it was a sample from Puglia that Brian had recently been sent by a producer there. So I opened it and poured a glass. Immediately, I got a whiff of animal sheds with some savoury, cheesy character. The palate was similarly animal-like, with a thin metallic edge. Very rustic. Not undrinkable, but getting there, and a textbook example of a Brettanomyces-infected wine.

For those scratching their head wondering what on earth Brettanomyces is, let me explain. It’s a yeast – that is a unicellular type of fungus, not a bacterium – that is a common spoilage organism in winemaking. The goal of this article is to assess how much of a problem it is, what its effects are and how it can be prevented. Finally, I’ll look at the controversial issue of whether low levels of ‘brett’, as it is widely known, can ever be a good thing, adding complexity to certain sorts of wines. 

First, a dull but necessary paragraph to clear up a potential confusion. The name Dekkera is often used interchangeably with Brettanomyces. They are actually the same genus (this is the taxonomic group just above ‘species’), with Dekkera being used for the ascospore-forming (sporogenous) form of this yeast, and Brettanomyces used for the non-spore forming type. There are currently five recognized species of Brettanomyces/Dekkera: B. nanus, B. bruxellensis, B. anomalus, B. custersianus and B. naardenensis, with a range of synonyms in common use. Of these, research indicates that B. bruxellensis is the most relevant to wine.

The microbiology of wine production is a complex business, and it is beyond the scope of this feature to go into too much detail. But let me try to give you a feel for the concepts involved. Think of the plants growing on the slopes up the side of a mountain. At the bottom there are hundreds of different types, with the pattern of vegetation changing and progressively decreasing in diversity with altitude (and a corresponding drop in temperature). It’s a bit like that with fermenting wine, except that here the variation is temporal and not spatial -- it is a gradually changing environment for yeasts. In freshly crushed grape must there are many different yeast species present, including those normally found on grapes. These rapidly disappear as fermentation starts and alcohol rises. The environment becomes more and more inhospitable, and after a while the only significant yeast species present is Saccharomyces cerevisiae. As alcoholic fermentation finishes, the S. cerevisiae population decreases significantly. If by this stage the sugar and nutrient supplies are exhausted, that’s the end of things and the wine is stable. But if they aren’t, this leaves the way open for spoilage bugs to develop; brett is one of the worst culprits here.

What does bretty wine taste and smell like?
Volatile phenols and fatty acids are the key molecules responsible for the olfactory defects in wines affected by brettanomyces. According to Peter Godden, of the Australian Wine Research Institute, ‘The anecdotal dogma in this area is that 4-ethyl-phenol, isovaleric acid and 4-ethyl-guiacol are the key molecules, in order of sensory importance’. But he adds that he has seen variations in brett character in different bottles of the same wine. 4-ethyl-phenol is the most prominent molecule in bretty wines, giving aromas of stables, barnyards and sweaty saddles (apparently, but I must admit to never having smelled one). Its presence in wine is an almost certain indicator of a brett infection, and this is what most diagnostic labs test for to indicate the presence of brett. 4-ethyl-guiacol is a little more appealing, known for its smoky, spicy aromas. Isovaleric acid, a volatile fatty acid, is known for its rancid, horsey aroma, and as yet there is no analytical technique that picks it out: in gas chromatography/mass spectrometry (GCMS) another compound elutes at the same time, which masks it. Godden emphasizes that this is a complex area of study: ‘There is not much of a relationship between overall brett character and 4-ethyl-phenol levels, and there are synergistic effects between the three most important sensory compounds.’

As with other volatile odorants, people differ widely in their sensitivity to these molecules, and each individual shows a range of different thresholds (for example, the threshold for detecting an odorant differs from the threshold for recognition of the same odorant). Godden suggests that a useful sensory threshold to use for 4-ethyl-phenol is 420 micrograms/litre. At this concentration and beyond, a wine will typically be noticeably bretty. Below this concentration, the character of the wine may be changed but people won’t, on average, recognize that this is due to 4-ethyl-phenol. Because the threshold for 4-ethyl-phenol drops when 4-ethyl guiacol is also present -- and in brett-infected wine they always occur together in a ratio of about 10:1 -- this threshold is calculated for a 10:1 mixture of 4-ethyl-phenol and 4-ethyl-guiacol.

How common is brett?
The short answer is that brett is highly prevalent, and represents an increasing problem, even in new world countries such as Australia. ‘We first started raising this as an issue four years ago’, says Peter Godden of the Australian Wine Research Institute, ‘ and on 1st July we’re planning to start a major project looking at Brettanomyces’.  As a scientist, he feels that for such an important issue, this is a relatively under-researched area. ‘There is a lot of conjecture: anecdotal observations are very important but we have to be careful with them because they can skew people’s opinions.’

Although brett can and does occur with whites, it is predominantly a red wine problem. This is because red wines are far higher in polyphenol content, and generally have a higher pH, both factors which encourage brett development for reasons which outlined below.

With rising standards of winemaking worldwide, I was a little surprised to hear that brett is on the increase. There seem to be two contributing factors to this rise. First, there is the current trend for ‘natural’ wines. ‘Minimalist winemaking is a perfect recipe for bretty wine’, says Godden. ‘It’s probable that the increase in brett in the 1990s can be traced back to the winemaking fad to stop adding sulphur at crushing’. Indeed, the most effective way of preventing brett is to maintain an adequate concentration of free sulphur dioxide (SO2). Randall Grahm of California’s Bonny Doon comments, ‘If one is ideologically committed to no sulfitage at the crusher, this increases one’s chances of brett dramatically. Likewise, if one uses low or no SO2 in the elevage of the wines, this greatly increases the risk of brett’.  Preliminary studies by the AWRI show that there is a lot of genetic variability among Brettanomyces strains. This makes the correct use of sulphur even more important. If it is added in small, regular doses, winemakers might unintentionally be selecting for SO2-resistant strains of brettanomyces, or to put it another way super-brett strains that are then even harder to eliminate. So timing and magnitude of SO2 additions are important as well as the actual concentrations: the best way to get rid of brett seems to be large SO2 additions at strategic intervals.

Second, there is the move towards ‘international’ styles of red wine, made in an extracted style from super-ripe grapes. ‘These are higher in pH and are richer in polyphenols’, explains Grahm. pH is important, likely through its role in modulating the effectiveness of SO2 additions. The higher the pH, the less effective SO2 is and the more likely that brettanomyces will grow. Polyphenol content is important because these compounds are the precursors for the volatile phenols largely responsible for bretty odours.         

A vital risk factor is the presence of residual sugars and nitrogen sources left over at the end of fermentation. With the gradual rise in alcohol levels over the last 20 years, the last bit of sugar commonly isn’t being metabolised by the yeast. Godden suggests that one solution is to try to keep the wines warm while they are being pressed. As well as sugar, a nitrogen source is needed for brett to grow. In fermenting wine, S. cerevisiae uses amino acids as a nitrogen source. A recent winemaking trend has been to add diammonium phosphate (DAP) as a supplementary nitrogen source for yeasts, to reduce the risk of stuck fermentations. However, fewer than half of musts need actually use this additive, and DAP has been described as ‘junk food’ for yeasts – they’ll use this in preference to amino acids, leaving them in the wine as a nitrogen source that encourages the growth of brett.

Old barrels are frequently touted as the main culprits of brett, but Randall Grahm adds, ‘The received wisdom about old barrels, old foudres being the great repository of brett I think is somewhat mythical and simplistic: dirty barrels, dirty wines, q.e.d.’ Grahm adds that, ‘Since brett is largely ubiquitous, a rampant brett infection is often more of a function of a large inoculum coming in on the grapes.  

To gauge the extent of the current brett problem, Godden and his colleagues recently completed a survey of Cabernet Sauvignon wines in five major regions of Australia. He’s unable to give the actual results, because these are sensitive, and he thinks that the samples size, around 170 bottles, isn’t big enough to allow him to draw a firm conclusion. ‘But if a consumer were to go out and buy a mixed dozen,’ he told me, ‘several bottles would have more than 425 micrograms/litre 4-ethyl-phenol: if you drink wine regularly, you’ll have come across a lot of brett.’

Before the 1990s, brett was common in Bordeaux. The wines of several well known classed growths were well known for their distinctive ‘stink’. This was almost certainly because of brett infections, but without the data – and most properties would understandably be reluctant to own up to this – I can’t name any names. Since the early 1990s, however, brett has become much rarer, and this is mainly due to the groundbreaking work of Dr Pascal Chatonnet. In 1993 Chatonnet carried out a survey of 100 French wines, and showed that a staggering third of those tested had levels of volatile phenols above the perception threshold.

The conclusion seems to be that brettanomyces is widespread, and virtually every barrel of red wine has the potential to go bretty. Create the right environment for it, and you’ll have a brett infection. Thus the key objective for winemakers isn’t to create a sterile winery, which will never happen, but to make sure that their barrels aren’t a receptive environment for brett to grow in.

Brett, Mourvèdre or terroir? A case study
Brettanomyces is a favoured discussion topic among wine geeks, who’ll often enter into lengthy discussions about whether a certain wine is bretty or not. One wine that keeps cropping up in this context is Château de Beaucastel, the highly regarded Châteauneuf du Pape estate. To some, the distinctive earthy, slightly animal-like characteristics of many past vintages of Beaucastel have reflected an expression of terroir, or even the higher than average Mourvèdre content of this wine. Others think it’s because of brett infection. Who is right?

Back in early 1998, Charles Collins, an American wine collector, became so frustrated with the endless wine geek discussions about Beaucastel and brett that he decided to find out for himself. He got hold of some scientific papers on the subject and read up about the subject. ‘I realised that the presence of the compound 4-ethyl-phenol is a virtually certain indicator of the presence of a brett infection’, recalls Collins. He contacted a lab who does testing for 4-ethyl-phenol and sent them some Beaucastel from his cellar. ‘I opted to test two of the most famous vintages, the 1989 and 1990’, Collins told me. ‘These wines are supposed to represent what great Beaucastel is all about.’ He prepared the samples for shipment in sterilized glass 375 ml bottles and used fresh corks to seal them. The wines were labelled so that the lab had no clue as to their identity.

The results? According to Collins, ‘they showed indisputable evidence that significant brett infections occurred in both the 1989 and 1990 vintages of Beaucastel.’ Microscan and plating tests showed only small amounts of mostly dead brett cells, but the 4-ethyl-phenol levels were 897 micrograms/litre for the 1989 and a whopping 3330 micrograms/litre for the 1990. Collins concludes, ‘if you personally like the smell of brett, then none of this should you dissuade you from buying and cellaring Beaucastel. You should, however, give up the myth that the odd flavours are due to terroir—they aren’t.’ I would add that while I’ve detected what I’ve always assumed, in the absence of data, to be high levels of brett in some vintages of Beaucastel—the 1991 springs to mind as one of the brettiest wines I’ve ever encountered—in vintages since the mid-1990s I haven’t encountered any. But, of course, unlike Collins, I haven’t done the lab tests that would be needed to verify this.

‘We believe in natural winegrowing and winemaking, and I must admit that this has led us to have serious debates with scientists spanning three generations’, responds Beaucastel’s Marc Perrin. ‘In the mid-1950s, for instance, our grandfather, Jacques Perrin, decided to stop using chemical pesticides or herbicides on the vineyard. At that time, when scientists were recommending the use of such chemicals for productivity or lobby reasons, that seemed crazy and impossible. Now, it seems that people have changed their mind and more and more vineyards are turning organic. I could quote many more examples of opposition between a scientific vision of wine and our traditional/terroir oriented philosophy of wine, and the subject of Brettanomyces is just one more’, he explained. ‘There are certainly some Brettanomyces in every natural wine, because Brettanomyces is not a spoilage yeast (as many people think) but one of the yeasts that exist in winemaking. Some grapes, like Mourvèdre, are richer in 4-ethyl-phenol 'precursors' than others and we have a high percentage of these grapes in our vineyard. Of course, you can kill all natural yeasts, then use industrial yeast to start the fermentation, saturate the wine with SO2 and then strongly filtrate your wine. There will then be no remaining yeasts, but also no taste and no typicity. That is the difference between natural wine and industrial wine, between craftsmanship and mass-market product.’

Adding complexity?
Beaucastel has been widely acknowledged as one of the world’s great wines over recent decades. Yet from Collins’ limited sampling coupled with individual tasters’ experiences, it seems likely that some of the most successful past vintages of this wines have been marked by high levels of brett. This leads us to a critical—and fascinating—question: is brett ever a good thing? In small quantities, can it have a positive influence on certain styles of red wines?

If surveys such as those of Chatonnet and Godden are to be extrapolated across all wines, it is likely that many wines with above-threshold levels of brett have received critical acclaim and have been enjoyed by countless consumers. This leads to the conclusion that while most people won’t enjoy a really stinky wine, low levels of brett might not be a problem—indeed, a bit of brett might even add complexity to certain robust styles of wines.

Bob Cartwright, senior winemaker of Leeuwin Estate in Western Australia’s Margaret River region, acknowledges that ‘a lot of winemakers like to have some as a complexing character—the question is how much is too much?’. Randall Grahm is undecided. ‘I suppose this could theoretically add some complexity to a wine. The problem is that for now, this is not easily controllable’.

Pascal Chatonnet is opposed. He sees the problem of brett as a lack of fruit and loss of typicity. ‘If brett is able to grow in all the red wines of the planet—and this is the case—then all the wines will have the same odour, which is a pity’.

Godden is another who isn’t keen on the idea.My view is that if we could eliminate it altogether we would’, but he stressed that he wouldn't go so far as to say it is always negative. Godden cites some results from the PhD thesis of Phil Spillman, now winemaker with Villa Maria in New Zealand. In one study Spillman did some sensory analyses. The strongest relationship he found was an inverse correlation between levels of 4-ethyl-phenol and wine preferences. ‘I’ve not been able to find an Aussie winemaker who doesn’t find 100 micrograms/litre negative’, adds Godden. ‘In tests where brett character has been added, it has a severe adverse effect on the palate. 4-ethyl-guiacol can be interesting and complexing and doesn’t have the negative palate effect of 4-ethyl-phenol, but with brett infection you get 10 times as much 4-ethyl-phenol than 4-ethyl-guiacol.’

Randall Grahm has an novel suggestion, though: ‘It would be very interesting if we could isolate a strain of brett that worked in wine, depleting nutrients but producing very low levels of 4-ethyl phenol. In this way, one could inoculate one’s wine with brett, much the same way as one inoculates one’s wine with malolactic bacteria, thus depleting nutrients and rendering the wine safe from further microbial degradation.’ Now there’s a project for the microbiologists. Any takers?  

see also: Brettanomyces masterclass

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