I Say To-may-to, You Say To-mah-to
Yeast is controversial. Maybe not "Sarah Palin doesn't believe in Darwin" controversial, but certainly slippery and befuddling enough to those passionate about this sort of thing. The genesis of this contentious finger pointing emanates from applying increasingly complex DNA sequencing tools to the genome of the humble wine yeast. As minute alterations in tiny DNA fragments are elucidated, these DNA differences are magnified into conflicts of nomenclature of colossal proportions. As recently as 2 months ago, my UCD professor wrote, "I need to point out that there is a lot of controversy amongst wine yeast microbiologists as to the uses of S. cerevisiae andS. bayanus." But what does that mean? I turned to Ribereau-Gayon's latest Handbook of Enology for details. He has been classifying and organizing wine yeast since the 1960's. When life was simpler and JFK was President, the two principal wine yeasts (S. cerevisiae andS. oviformis) were distinguished by their ability to digest certain sugars. Then in the 1980's, in a clandestine move somewhere between leg warmers and acid jeans, S. oviformis became S. bayanus. Soon thereafter, fancy DNA assays supplanted simple sugar tests, rendering the sugar-based classification nearly obsolete. While the microbiologist- taxonomists (the DNA geeks) were busy in their labs, the enologists (wine makers) themselves decided to add the varietal name to S. cerevisiae, leaving us with S. cerevisiae var. cerevisiae or S. cerevisiae var. bayanus. OK, this is clearly worse.
Some semantic discord is resolved by recognizing that S. cerevisiaeis by nature one slippery eel of a species. Amazingly, it alters its physiologic properties faster than Jessica Simpson discards last week's Prada bag. Over time, yeast samples acquire and lose the ability to ferment certain sugars. This alters their characteristic profile and affords them opportunity to leap across the recognized boundaries of scientific taxonomy. Unstable like rehab Britney, their DNA is so squirrelly that the Saccharomyces clan now is lumped together under the umbrella rubric Saccharomyces groupsensu stricto. Indeed DNA tests now verify the existence of S. cerevisiae and S. bayanus hybrids, with unique, intermediate DNA sequences that defy traditional classification.
Now let's focus our microscope at higher magnification yet. Like pinot clones, yeast has clones, too. (Let's say it together- "aaargh!"). Clones, called "strains," not only change year to year in the same winery but also may share identical DNA across wineries miles and miles apart! Luckily for us, scratch DNA researchers enlist a panoply of DNA sequencing tricks for ever more detailed yeast strain analysis. The techniques are identical to the "DNA fingerprinting" used in paternity tests and CSI: Miami. So why is this important? Differences between yeast strains dictate fermentation speed, alcohol tolerance, temperature tolerance, and even fermentation to dryness. Sometimes a number of different strains coexist, and when none can assert itself to the exclusion of others, fermentation gets "stuck." Conversely, one or two dominant strains characterize rapid and complete fermentation. In fact, even different proportions of the same few strains are recognized at the start, middle, and end of fermentation. Now extrapolate these yeast strain differences across the world's vineyards and wineries. The possibilities are infinite!
So what do we really know? S. cerevisiae is an organic, fluid, shape shifter, polyclonal by nature, and always reinventing its essential self by mixing up its DNA. And these changes, whether minute like a few single nucleotides or greater like whole chromosomes, reverberate in the winery and ultimately in your wine glass. As yeast strains vary from year to year and vat to vat, each aliquot of wine is rendered indelibly unique.