Taming The Yeast
The Age
Tuesday December 16, 2008
Scientists have unlocked the secrets of wine yeasts, says Ben Canaider.
WORKING very late one night, possibly in a bar, a wine industry professional described what his perfect wine might be. "Imagine owning an ancient vine, and that vine - being so old - only producing one single grape. Imagine that in a perfect vintage, in perfect ripening conditions, that single grape fermenting within its own skin to the point where you, as the owner of the vine, could gently pluck the berry from the stem and squeeze the juice - the wine - into your mouth. It would be the most pure and the most perfect wine in the world ..."Nonsense, of course, but a wonderfully romantic idea.Even if it could happen, wild or natural fermentation wouldn't keep us in much wine. Indeed, what separates man from the beasts - alongside language and tiled bathrooms - is his mastery of yeasts and the process of fermentation. Without it, there would be, among many other things, no bread, no beer, no wine, no salami and, perhaps most importantly, no soy sauce. These are all fermented products.Australian wine researchers have recently conquered a final frontier, enhancing our knowledge of fermentation. They have sequenced the wine yeast genome. In less than a year, the Australian Wine Research Institute in Adelaide has - on a budget of less than $10,000 - mapped the genome of Saccharomyces cerevisiae. This is the species of yeast that most wine yeasts come from. It's a breakthrough that should have big implications for the way winemakers use different grape varieties and wine styles, and could also potentially help identify wine yeast strains that produce less alcohol in finished wine."We were lucky. We adopted the right approach," says Wine Research Institute managing director Professor Sakkie Pretorius (pictured).Together with fellow researchers Anthony Borneman, Angus Forgan, and Paul Chambers, Professor Pretorius took the two sets of a wine yeast cell's 16 chromosomes and separated them. "Instead of trying to read the two sets of chromosomes at the same time, all mixed together, we took them apart and read one, then the other. It was much quicker."Professor Pretorius did this with the Australian Genome Research Facility's sequencing machine. "It took a half a day, and we were able to use it for free, because I told them that this wine yeast gene sequencing would make their machine famous!"Professor Pretorius' motivation for mapping the genome is straightforward: "We want to find out what makes a wine yeast a wine yeast."Up until now, the 200 or so different wine yeast strains that are available have been used by winemakers in a trial-and-error manner, slowly working out which yeasts suit their varieties, their vintage and their style of winemaking. Professor Pretorius thinks that with the wine yeast genome breakthrough, winemakers will be able to use new "cocktails" of yeasts to make a certain style of wine."Take sauvignon blanc, for instance. The chemical compound thiol gives it its unique aroma and flavour. Some yeasts help release this compound, but might not convert it into a volatile so that it may be identified by a wine taster. We've already developed two yeasts called Alchemy One - which releases thiol - and Alchemy Two, which converts it."In another area we have also identified a wine yeast strain that acts as a hydrogen sulphide negator. For winemakers this means they do not have to blow off any hydrogen sulphide (that rotten egg gas smell) they might produce in their ferments - because this action blows off some of the good things in the ferment too. The incredible thing about this yeast is that it lacked one specific gene; without that one gene it can negate hydrogen sulphide."Perhaps most importantly, this new genome map can help identify new wine yeast strains that ferment grape juice to dryness without creating as much alcohol in the process. Professor Pretorius says that with warmer vintage conditions and a trend in winemaking towards full physiological ripeness in grapes, Australian wines have become more alcoholic in recent times. In laboratory conditions Professor Pretorius has already genetically modified a wine yeast to prove it is possible for such a wine yeast to biologically exist.This laboratory prototype can ferment grape juice to dryness with alcohol levels 2 percentage points lower. Finding a non-GM strain that can do the same thing is proving a little harder. "We are committed to non-GM production of wine, and so far we've found a yeast that can ferment wine to 0.3 per cent less alcohol; but I am confident we will soon find one that can take away a full 1 per cent."The interesting part about this lower alcohol yeast is the way it diverts the grape's sugar. Instead of turning it into ethanol, it turns it into glycerol, and yet the glycerol produced is so low it can't be detected by the human palate. A neat trick from Mother Nature: less alcohol achieved using natural wine yeasts that, although hidden, are among us.
© 2008 The Age
