Monday, May 23, 2011

The fittest Yeast

Wine Spectator

How Evolution Made Nature's Perfect Winemaker

Scientists discover how one species of yeast came to dominate wine, beer and bread
Jacob Gaffney
Posted: May 20, 2011
Yeasts are diverse—biologists estimate that there are at least 1,500 different species of the fungal microorganisms. But when it comes to fermenting wine, one yeast is much more common than any other. The same is true in brewing and baking—food and drink chemists overwhelmingly prefer Saccharomyces cerevisiae.
How did this one species earn most favored yeast status? Cellular biologists at Lund University in Sweden used genetic sequencing to study several kinds of yeasts and have found evidence that S. cerevisiae underwent a series of rDNA changes 100 to 200 million years ago. This evolution helped S. cerevisiae outperform its fellow yeasts and dominate the fermentation of wine and beer.
When S. cerevisiae, present either on grape skins or added by winemakers, comes in contact with grape must, it instantly begins to consume the sugars, primarily glucose and fructose. In doing so, it produces alcohol, carbon dioxide, heat, and other compounds to make wine. It also avoids breaking down the antioxidants and other organic compounds associated with wine's health benefits. But it's the alcohol production that is the key to this yeast's success.
S. cerevisiae is not the only yeast that does this. So do species such as Dekkera bruxellensis, which separated from the S. cerevisiae lineage nearly 200 million years ago. "But it wasn't until the emergence of the modern fruit humans consume today, around 150 million years ago, that the two became direct competitors," said Jure Piskur, a cell and organism biology professor at Lund University and lead author of the report, which was published this month in the journal Nature Communications.
"Every autumn, when fruits ripen, a fierce competition for the sugars starts within microbial communities," write the authors. The scientists performed an examination of mitochondrial DNA in several yeast species, and found that around 150 million years ago S. cerevisiae's genes dramatically changed. Fifty million years later, it had split off from another yeast species, Saccharomyces kluyveri, and began to dominate fermenting fruit.
The changes, the authors believe, made S. cerevisiae more efficient at producing ethanol (the alcohol in wine). As alcohol levels increased in fermenting fruits such as grapes, other yeast species where knocked out, leaving all the sugars for S. cerevisiae. "Saccharomyces produces ethanol, accumulates it, and kills the competing microbes, and then, when alone, uses it as a substrate to react with any oxygen present," said Piskur.
S. cerevisiae also has the ability to ferment both in an oxygen environment or without it. Eventually, S. cerevisiae's ability to flourish in dark, contained environments made it the preferred yeast among brewers and winemakers. Other yeasts simply can't compete with its ability to withstand high levels of acid, alcohol and heat.
The Lund team thinks that their discoveries may pave the way to genetically modify strains of S. cerevisiae to produce lower alcohol wines. It could be a way to keep alcohol levels down, even as better farming and climate change are producing riper fruit. "Now that we understand better how ethanol production is regulated at the gene level, this knowledge can help when we genetically breed different strains which would produce less or more ethanol, if so desired," said Piskur.
That would require more research, but for now, wine lovers can be grateful that S. cerevisiae evolved into the perfect little winemaker. "Yeast did not develop the ability to produce ethanol to please mankind, but as a tool to out-compete other microbes," said Piskur. Pleasing mankind, it seems, is only a delightful byproduct of S. Cerevisie's desire to dominate.

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