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As the lovely [livejournal.com profile] desireearmfeldt and [livejournal.com profile] kirisutogomen point out, it's been ages since last I posted.

I've got a really poorly-planned trip to Athens coming up in a week or so (we'll be talking programming languages, though, which I enjoy). Meanwhile [livejournal.com profile] desireearmfeldt is working on her dissertation, and it's tool, tool, tool hereabouts.

But that's not what I came here to talk about. I came here to talk about yeast morphology.

Those of you who know me know I'm a huge fan of sour beers like lambic and berliner weiße. There's now (to my great joy) quite a burgeoning set of microbreweries making American Sour Beers, and events like the Extreme Beer Fest that showcase the emerging new styles.

What distinguishes these beers are the organisms used during fermentation. Most beer is fermented by one or another strain of Sacchromyces Cerevisiae (it's right there in the name – think cerveza). This is the same yeast you use to make bread and wine, and the one used in most genetic research on yeast.

Sour beers add one or more other organisms to the mix. The first is Lactobacillus. Strains of this bacterium occur naturally in grain – when we make beer, we normally boil it (killing the lactobacillus) and add hops (an antibacterial, preventing Lactobacillus from taking hold later on). You're already familiar with various strains of Lactobacillus – they make sourdough bread sour (another mix of Sacchromyces and Lactobacillus), and they are responsible for turning milk into yogurt, sour cream, or buttermilk. They've also become the all-important basis for fermented hipsterpickles of the sort made by bearded, pierced, and tattooed entrepreneurs in Brooklyn studio kitchens. Wherever they occur, they're converting sugars like lactose into a mix of lactic acid and alcohol. Generally we select for the acid.

The other, more intriguing addition are yeasts from the genus Brettanomyces. These "wild" yeast are much more aggressive than Sacchromyces – they'll chow down on long-chain dextrines and other complex sugars and generally ferment bone-dry. By contrast, Sacchromyces will give up while there's still some residual sugar (and we can deliberately sweeten a beer or add body by increasing the long-chain dextrines that don't get fermented). Brettanomyces tend to reproduce relatively slowly, so they'll get out-competed by Sacchromyces for smaller sugars. And when there's oxygen around they love to produce acetic acid – so they can turn your beer vinegary if you don't keep it under a fermentation lock. Also, these guys love to burrow into wood – they can actually digest cellulose to a very small degree. They were first discovered in barrels of English ale about a century ago; they were considered a big contributor to beer spoilage if you kept the beer in the barrel for too long. A quick glance at the Wikipedia page explains why (beyond the acetic acid, of course): when they munch on the by-products of Sacchromyces fermentation they produce flavors described as "barnyard", "mousy", or "horse blanket". Or often just "funky". When Brettanomyces was discovered most beer was getting drunk in a matter of a month or so after it went into the barrel, so this wasn't a big deal as it takes months for the flavors to really develop. What sour beer brewers tend to do is deliberately infect a barrel with Brettanomyces and then age beers in that barrel. You can keep reusing a barrel if you like the beer that's coming out of it.

A side note here is that warehousing that quantity of beer in barrels for a year or so turns out to be expensive – barrels take up a lot of space and require a lot of maintenance – so sour beers tend to be on the pricier side.

We're now also starting to explore fermentation using only Brettanomyces, with no Sacchromyces or Lactobacillus at all. Interestingly, it tends to yield fruity flavors rather than funky flavors when used on its own, especially if you don't drop the pH with the help of our friend Lactobacillus.

Quite apart from its funky tendencies, Brettanomyces has a distinctive morphology. If you've ever seen a picture of yeast – nice round cells with little buds becoming daughter cells – you've probably seen a picture of Sacchromyces Cerevisiae. Brettanomyces by contrast forms oval or long sausage-shaped cells.

When they're fermenting they also look pretty different. Sacchromyces forms a foamy krauzen on top of the beer (this is the foam you see if you've ever proofed baker's yeast). By contrast, Brettanomyces forms a pellicle – a thin, solid-looking mat on the surface of the beer. The pellicle is actually formed in response to oxygen, and it's theorized that it protects the beer from the oxygen – thus keeping out aerobic organisms like the vinegar-forming acetobacter that might muscle in on the territory.

The interesting thing about the pellicle is that it starts to make Brettanomyces look very fungus-like indeed. It's not the only yeast in the family that has interesting large-scale morphology. Candida albicans is a cousin of Sacchromyces and Brettanomyces that lives in your mouth and gut and elsewhere on your body. Most of the time it's round and yeasty. It can also turn oval like Brettanomyces. But every now and then it decides to morph into a fungal filamentous form and invade the body. This is responsible for thrush and yeast infections. It's also one of the big killers of immunocompromised individuals like AIDS patients – without an immune response the invasion can continue unchecked. That's also the reason cancer patients get thrush during chemotherapy.

Why does this all come to mind right now? Genetics and morphology! One of the most popular "Brettanomyces" strains used in home brewing today is White Labs WLP644, Trois. This yeast is derived from lambics brewed by Drie Fonteinen in Belgium (Drie = 3 = Trois, geddit?) and was sold as a strain of Brettanomyces Bruxellensis, one of the characteristic lambic species. But there's been rumbling for a while that this yeast might not be Brettanomyces at all. But what is it? Its morphology often matches the oval shape of Brettanomyces, but sometimes it's the round shape of Sacchromyces Cerevisiae instead. It also produces flavors that are strongly reminiscent of Brettanomyces, and behaves in a Brettanomyces-like way during fermentation – though it's known as being rather more vigorous of a fermenter than Brettanomyces typically is.

Genetic sequencing has revealed that Trois is not Brettanomyces at all – but a subspecies of Sacchromyces, probably distinct from the ones that we've already classified. This resulted in a reclassification this week by White Labs. This has one potentially very interesting consequence. A number of competitions (such as the Great American Beer Festival) have defined categories for beers fermented with Brettanomyces, and a number of beers fermented with Trois have done quite well in those competitions! Are these relatively exotic fruity/funky beers worthy of living together in the same category now that we know how different the yeasts really are? This is interesting in part because these tend to be the most diverse categories in the competitions, and comparing the beers can be a bit like deciding whether you prefer apples or oranges (whereas most categories are just searching for the poodliest poodle). Ongoing genetic sequencing of the various strains revealed the miscategorization. Lots of great discussion of that can be found in the Sui Generis Brewing post on the subject.

So, another victory for modern genetics in teasing apart differences between species with highly flexible morphologies and apparently similar behaviors!
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