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Signs of strains

To stress the increasing importance of yeast in modern winemaking, Lallemand, the Canada-based yeast giant, held a two-day conference in Bordeaux last week entitled 'Advances in Oenology: The Impact of Selected Yeast and Bacteria on Winemaking'.

Although there was a certain amount of 'spin' involved there were no presentations, for example, extolling the virtues of spontaneous fermentation Lallemand's central message, that yeasts are an important 'tool' for winemakers and have a noticeable and distinct effect on wine style, certainly seemed to ring true. The same can also be said (although, arguably, to a lesser extent) about the strains of bacteria used to perform malolactic fermentation.

According to Dr Andrew Markides, Lallemand's man in Australia and the former head of wine science at Adelaide's Roseworthy Agricultural College, the science of wine yeasts has come a long way since the first commercially produced dried selected yeasts were made available in the 1970s. 'We are beginning to separate the good from the bad yeasts,' he said. 'We're isolating those with bad attributes from those with good attributes. Some have a greater tolerance to alcohol, some might have more tolerance to sugar. Others can ferment more efficiently. Some protect fruit-derived grape compounds better, and some can provide the winemaker with the ability to obtain yeast-derived positive attributes. Yeasts are now seen as wine quality-enhancers.'

According to Markides, it is also becoming apparent that different yeasts are better suited to different regions and grape varieties. 'You might have the same variety in different regions and the performance of the yeast strain and the resulting wine will be different depending on the conditions prevalent and the nutrient profile of the juice,' he explained. 'A yeast that does wonderfully well in Barossa might perform awfully in Yarra.'

But what of the current vogue for 'wild' ferments that are supposed to give more complexity to the finished wine than just using one strain of commercially produced Saccharomyces cerevisiae (the most common genus of wine yeast by far)? Markides answers that, leaving aside the risks of sluggish or stuck fermentations, modern developments can allow the winemaker to choose exactly what 'interesting' traits he wants. 'It's about control,' he said. 'And if the winemaker wants certain attributes, we can provide them. You don't have to use just one yeast. You could use five or six different strains, ferment them separately and then blend them back together to get the wine style you want.'

The only problem with this argument, as Markides admitted, is that non-Saccharomyces yeasts (which generally start the fermentation in 'spontaneous' ferments before a Saccharomyces cerevisiae strain takes over at around 5% to 8% alcohol by volume) are not produced on a commercial scale.

Markides did point out, however, that Lallemand is trialling a number of Saccharomyces bayanus strains said to produce a very slow ferment that results in nutty, secondary aromas in order to give winemakers the ability to add more 'complex' aromas to their wines. Penfolds, for example, has experimented with using a Saccharomyces bayanus yeast for its Adelaide Hills Chardonnay to emphasise its nutty, toasty notes.

To illustrate his argument, Markides led a number of tastings of finished wines made with different yeasts. Arguably the most striking were three Entre-Deux-Mers Cabernet Sauvignons made from the same fruit in the same way. The entire tasting group, which included two MWs and representatives from across Europe, agreed that the results were striking. One remarked the wine could even have come from 'different grape varieties'.

The Cabernet made with the most common yeast used in Bordeaux, Davis 522, was more 'traditional' in profile, whereas the one fermented with BM45, a yeast isolated in Brunello, was clearly fruitier and softer with a broader middle palate. The final wine, made with a Burgundian yeast, was, in contrast, distinctly meaner and greener.

According to Christophe Coupez, director of the Centre d'Etudes et d'Informations Oenologiques de Pauillac, who advises 300 Bordeaux chteaux in eight appellations, the Old World regions are finally waking up to the potential of yeast choice. 'When I started in 2002 everybody I worked with used the traditional Davis 522 yeast, but that is changing,' he said. 'Davis 522 is not particularly suited to the styles modern consumers like. I think about 40% have looked at changing their yeast, many to the BM45, for example.'

But what does this mean for traditional notions of terroir? According to Markides, 'The terroir effect comes ultimately from the grapes and the climate. I believe yeasts can enhance this and enable fuller expression of the fruit. Remember that you still can't add anything artificial to the must. All selected yeasts were wild once.' The effect on typicity, however, is more debatable.

Research

The latest research into yeast technology apparently involves isolating yeasts that can let the winemaker better target desired levels of glycerol and volatile acidity. Another 'hot topic' is that regarding Polysaccharides, a mysterious group of molecule levels which different yeast strains produce in different quantities and which apparently (although no one is exactly sure why) have an effect on mouthfeel, fruit quality and tannin structure. 'A lot of work is going on in this area as it is becoming clear that the right yeasts can boost the concentration of both the aromas and the middle palate,' said Markides. 'If you are forced to pick the grapes a little bit less ripe because of vintage variation, you need to ask yourself if changing yeast, or using a mixture of different yeasts, will make a difference to the finished wine.'

So what of the 'Holy Grail' of yeast scientists, a strain that can ferment a Barossa Shiraz with great extraction and colour but at the alcohol level of a Bordeaux? Still some way off, it seems. 'When we monitor the sugar conversion rates of our yeasts we find a difference of around 0.2%,' said Markides. 'For a yeast to noticeably effect the alcohol level of the finished wine it would have to have a difference of about 4%.'

There is one exception, however: 'Using genetic modification, research institutes have already proved that it is possible to have a yeast that is a weak converter, producing around 4% less alcohol with the same amount of sugar,' said Markides.

Genetically modified yeasts are currently illegal in winemaking although this is not the only problem, as Markides outlined. 'This doesn't mean genetic modification is the way forward, as there are no indications yet as to what the implications are for wine quality,' he explained. 'The result of less alcohol being converted from sugar could mean more undesirable compounds being produced instead.'

Markides does feel, however, that genetic modification will eventually provide the solution even if the resulting 'Frankenstein' yeasts are never actually used commercially. 'If we can produce a genetically modified yeast that is a poor sugar converter and then understand how it works, then we might be able to produce the same results in a natural way,' he said. 'There are lots of possibilities still open for us. It's all about further understanding how yeasts actually work.'

Bacteria

The arguments regarding using selected bacteria, or 'starter cultures', to get wine to undergo a malolactic fermentation (MLF) are similar (although far fewer winemakers use them at present, as most 'natural' MLFs pass off safely and easily). As well as enabling the winemaker to choose the time when the wine undergoes MLF, rather than waiting for nature to take its course, Dr Sibylle Krieger, who researches the role of bacteria in winemaking for Lallemand, said the choice of bacteria can have an effect on wine quality. 'Recently it has been found that the metabolic activity of malolactic bacteria influences aroma compounds,' said Krieger. 'It has also been found that individual strains of malolactic bacteria make distinct flavour contributions. Some strains produce characteristic buttery, yeasty or nutty aromas, while others support fruity flavours and reduced vegetative aromas.'

A winemaker can now decide how much diacetyl (largely responsible for 'buttery aromas') he or she wants as a result of the malolactic fermentation. 'If you want to reduce the level of harsh malic acid in your Chardonnay, that doesn't mean you have to have buttery characteristic as a result,' she said.

Krieger added that she is currently doing a lot of work on simultaneous fermentation and malolactic fermentation, something that previously was thought to produce high levels of volatile acidity. 'Our work proves that this is not necessarily so,' she said. 'Running the two processes simultaneously can in fact boost some aspects of a wine's character, particularly fruity aromas in white wines.' This new process also means that wines can be finished almost a month earlier than previously, which is good news for high-volume producers of lower-end wines (the technique is less successful for reds and high-quality whites).

So how widespread is the use of these modern yeast and bacteria strains? Despite the majority of the wine world using selected yeasts to start fermentation, Lallemand says many, many winemakers don't understand the full capabilities of modern strains. Indeed, one world-famous winemaker, asked his opinion on using different yeast strains at a tasting held the day after the conference, said he doubted yeasts had much effect on the style and flavour of red wines.

According to Coupez, however, it doesn't take much to convince winemakers of their potential: 'When you taste different samples of wine with winemakers and then they start to experiment themselves, they are quite quickly converted.' (And he doesn't work for Lallemand).

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