Friday, August 26, 2011

The missing lager yeast strain has been found!

You probably know that beer is alcoholic because of yeast. Yeast grow and divide in sugary starting solutions, and the byproducts of their metabolism (or fermentation) include ethanol.

If you make beer yourself, you also know that the two major categories, ale and lager, are fermented at very different temperatures. Ales are fermented at room temperature (68-72 degrees Fahrenheit), while lagers are fermented at much cooler temperatures (45-55 degrees). This difference is because the yeast "like" to grow and ferment at different temperatures, but why? A research group thinks they figured it out, and they're publishing their findings next week in the journal PNAS (Proceedings of the National Academy of Sciences).

Ale yeast is the species known as Saccharomyces cerevisiae (the same yeast that people use to make bread). Lager yeast is slightly different. It's a hybrid species that came about because S. cerevisiae mated and hybridized with another species. This hybridization allowed the lager yeast (known as Saccharomyces pastorianus) to ferment at much cooler temperatures than would be possible with S. cerevisiae alone.

The problem was, the other contributing yeast species was unknown. People tried looking at the genome of S. pastorianus to figure out the identity of the other species, but because many yeast species are closely related and have similar gene sequences, these efforts were not conclusive. It was thought that the other strain was Saccharomyces bayanus because this yeast grows much better than S. cerevisiae at lower temperatures. However, S. bayanus is also a hybrid species, which complicates the analysis. Based on gene sequence similarities, it was also proposed that the other strain is Saccharomyces monacensis, but this identification is also controversial because S. monacensis might also be a hybrid species.

Researchers from Argentina, the US and Portugal seem to have finally identified the mystery lager yeast. This species, named Saccharomyces eubayanus, was found on large nodules (galls) that grow on beech trees in Patagonia (southern South America). Tree galls are full of sugary compounds that the yeast like to feast on, so the researchers collected galls from different tree species in different locations. When they isolated the yeast from these Patagonian beech trees, they discovered that this yeast's genome was 99.5% identical to the non-cerevisiae portion of the S. pastorianus genome. The most interesting piece of evidence supporting the true identification of S. eubayanus as the "lager" portion of the hybrid yeast species is that these beech forests exist in an Alpine environment with consistently low temperatures, and they seem to be the yeasts' preferred habitat.

A yeast species that is 99.5% identical to a large portion of the S. pastorianus genome and naturally grows at lager temperatures seems like pretty convincing evidence. There's one giant question remaining, though: how did a yeast species from Patagonia make it to Germany, where lagers were born, in the 1400s? The prevailing thought is that lagers have been made in Germany since the early 1400s, which is long before Patagonia was explored by Europeans. There are several possible explanations...

First, maybe the Germans got their dates wrong. It's possible that lagers were not produced in Germany until after South America was explored by Europeans.

Second, Patagonia might have been explored by other groups before the Columbian era. There is some evidence that Vikings and other seafaring groups found parts of North and South America a long time before the Western Europeans. That alone doesn't explain why lagers were first made in certain regions of Germany. You'd expect Viking lagers if this were true, right?

Third, and the most likely possibility in my mind, S. eubayanus might not be specific to Patagonian forests. There are plenty of Alpine environments in Europe, particularly regions of Europe that are easily accessible to Germans. Now that people know to look at tree galls in cold climates, we may discover that S. eubayanus is common to all forests in cool climates.

Cited paper:

Microbe domestication and the identification of the wild genetic stock of lager-brewing yeast

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