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Muri: A Mystery Solved

Posted in Beer on 2019-09-12 08:51

I wrote about the Muri mystery and how I didn't think we'd ever solve it, but yesterday I got an email out of the blue. Kristoffer Krogerus had noted a new paper on Saccharomyces eubayanus (one of the two parents of lager yeast) where the researchers had sequenced the genomes of a number of commercial brewing strains. He downloaded the data and took a closer look.

(If you're a scientist you may be better off just reading Kristoffer's own blog post on this.)

One strain caught his eye, because it was listed as a hybrid of Saccharomyces cerevisiae (ale yeast), Saccharomyces eubayanus (the other lager parent) and Saccharomyces uvarum. That's the exact same set of species that Muri seems to be a hybrid of. Which of course is intriguing.

The strain in question is White Labs WLP351 Bavarian Weizen. It's thought that WY3638 and Weihenstephan 175 are the same strain.

Kristoffer ran the genome sequence through his software and what do you know: it came out as a near-perfect match with Muri. Which makes a lot of sense. Bjarne Muri had been home brewing for a while when he tried to revive the family yeast, and it's not at all unlikely that he'd been using hefeweizen yeast.

For it to then hang around in his apartment for a while would not be surprising. Once they started trying to revive the family yeast they of course couldn't do it in completely sterile conditions, and so it would be enough for one cell of hefeweizen yeast to get into the starter. If the original yeast was dead, the hefeweizen yeast would take over the starter and outcompete everything else in it. Finally, they would have a culture of hefeweizen yeast, thinking they had kveik.

But how could they fail to notice that this was hefeweizen yeast? Well, they pitched it like it was kveik, in barley-based wort at 32C, and got flavours that were completely unlike hefeweizen yeast. Richard Preiss says he's experimented with WLP 351 and also found it to be sulphury at high temperatures. So that fits.

What should we take away from all this?

First of all, we can forget Muri in the context of farmhouse ale. All those tantalizing theories about it were wrong. It has nothing to do with Norwegian farmhouse ale, and was just an honest mistake by someone trying to rescue an old family yeast.

Secondly, it's reassuring to know that the microbiological work on kveik is solid enough that we can (eventually) weed out this sort of mistake.

Thirdly, and that's perhaps the main point, any attempt to revive a very old yeast is always an uncertain proposition. When it appears to succeed, it's very, very difficult to be certain which yeast it is that began to grow. Was it the original, or was it a contaminant?

Anyway, many thanks to Kristoffer for going the extra mile here to finally settle this mystery.







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Comments

James - 2019-09-12 14:42:05

This is why I'm extremely skeptical of those projects where they purport to recover ancient yeast from an archeological site in Egypt or wherever. It seems very likely to me that what they're "recovering" is much more recent than the artifacts it's found on.

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