Flor yeast are something like that one strange guy with the office down the hall, the one who’s always pleasant, who has a kind word when the rest of the world seems out to either hate or bother you, who sometimes holds the door open when you’re carrying coffee, and who does goodness-only-knows-what when he closes the door to that office of his. You’re really glad he’s there, but he’s a mystery.
I’m stretching, but you get the point: flor yeast are useful, we’re glad they’re there for us, and we don’t understand them. Flor are the film-forming, surface-dwelling yeast best known for their role in making sherry, though they’re also key actors in Hungarian dry Szamorodni**, Xeres from Jurez, and Jura’s Vin Jaune. “Biological aging” — the yeast metabolizing and spitting out acetaldehyde, glycerol, a bit of ethanol, volatile acids, and some other stuff — is responsible for much of the unique and complex nutty-freshness these wines share. Though it’s entirely possible to purchase and inoculate flor yeast, the film often comes and goes on its own in traditional production schemes. All of this happens after ordinary alcoholic fermentation finishes.
Flor are Saccharomyces cerevisiae, but they’re evidently not the same yeast responsible for alcoholic fermentation in these wines. Previous flor research also tells us that being flor is more about a lifestyle choice than about the yeast’s innate characteristics. Being flor is about surviving under stress: the yeast develop as a “velum” or skin on a high-alcohol (fortified) wine with little remaining nutrients. Yeast strains that survive here are generally those that have been able to make a few rapid, key genetic changes that researchers see as a common pattern across flor. By definition, one of those changes is the ability to form a film or skin on the wine’s surface, which signifies that the yeast have become more hydrophobic than usual: they don’t mix well with water, so they stick to each other and find the least-liquidy environment they can…given that they’re living off of a liquid.
Flor aren’t exactly like that guy down the hall, unless he’s been the subject of 34 studies published in the American Journal of Enology and Viticulture. Even still, how and why flor come and go isn’t something we understand well, and neither is what exactly makes flor yeast flor in the first place.
Wine microbiology research is often very local: researchers study the vineyards and wines near them (they’re convenient, but they’re also, often, the primary interest of whichever regional body is helping to fund the research). A study of flor yeast recently published in PLoS One (which means that it’s open-access; no paywall!) is notable for including samples from Hungary (Tokaj), Spain (Jerez), France (Jura), and Italy (Sardinia). This group of French enologists set out to build a better genetic tree for these yeasts, both to see how they’re related and to establish more data about what makes them flor-ish. My genetics knowledge is too poor to comment on how their data look, but it’s satisfying to note that they used more than one method.
How they’re related — The strains from different locations formed genetically similar groups: all of the strains from Jura were more similar to each other than to the strains from Jerez, and Jura strains were similar to other strains from the same cellar, too. My genetics knowledge is too poor to comment on how their data look, but it’s satisfying to note that they used more than one method to judge relatedness and kept coming up with the same relationships.
What makes a flor yeast a flor yeast — The best that can be said here is that this study makes one more incremental step forward in solving a big, complex problems. The researchers found two genetic changes shared across all of their flor yeasts, involving the duplication of two known genes. We know they’re genes; we don’t know much about what they do. So goes the story of modern genetics, caught between the past of not understanding how DNA mapped to functions at all and the future in which (we hope and presume) we’ll know what all (heck; I think we’d settle for most) genes actually do. This study also reinforces some previous findings about genetic characteristics shared by flor which, again, all point toward them having some kind of shared ability to deal with their high-alcohol, low-nutrient, clingy lifestyle. Gosh, now I feel as though I’ve really insulted that guy down the hall…
** An earlier version of this piece wrongly named the Hungarian wine as Tokaj Hegyalja, which is how the authors of the paper name that wine. A reader working in the Tokaj Hegyalja wine industry helpfully pointed out that Tokaj Hegyalja refers to the region and that the name of the wine I’m calling out is actually dry Szamorodni. She suggests that good descriptions of this wine can be found on the producer Samuel Tinon’s website. Thanks, Katherine.