Yeast is the undeniable heart of winemaking, the true winemaker that works the magic of fermentation while the rest of us watch and tinker around the edges. So it’s not surprising that a huge descriptive vocabulary has grown up around yeast, much like the proliferation of descriptors for wine itself. And like winespeak, some yeast talk is quite precise, and some fairly fuzzy.
Two commonly used descriptions for yeast strains--"neutral" or "enhances varietal character"--seem pretty fuzzy.
"Neutral" turns out to be a relative descriptor; yeast strains called neutral don't produce nearly as many aromatic compounds as other strains, and most of what does get produced fades rapidly.
Varietal character yeasts have been shown to extract or transform certain classes of compounds--like the thiols in Sauvignon Blanc or terpenes in Riesling.
At the Unified Wine and Grape Symposium in Sacramento this past January, I made the rounds of the mind-boggling tradeshow floor, visiting one yeast-enabled booth after another on a personal mission to pin down two terms that have always made me scratch my head: yeast that is “neutral,” and yeast that “brings out varietal character.”
When a yeast strain purports to bring out apricot aromas, we all know what that means and can judge the strain accordingly. But can anything provoking such a profound transformation of juice into wine truly be “neutral?” And the “varietal character” of what? Gewürztraminer? Tannat? Gamay? All of the above?
I followed up my tradeshow floor tour with phone calls, hoping to pin down these two powerful, problematic attributes.
Neutral…compared to what?
As I suspected, in technical terms, no yeast is neutral in the ordinary sense of the word. As Duncan Hamm, a commercial enologist for Chr. Hansen, put it, “We use ‘neutral’ a lot in the industry—especially winemakers—but strictly speaking, that’s not correct. If there was such a strain, you’d get alcohol and grape juice. All yeast do something to precursors to make flavors juice doesn’t have.”
Every yeast strain encourages (or discourages) biochemical change, every one spins off a long list of sulfur-related compounds, every one emits some of the familiar fermentation esters that make wineries smell like wineries during the crush.
Which means that “neutral,” far from being an absolute descriptor, is a relative term—a bit like “somewhat unique.” It gets applied to strains that have proven, during the years, not exactly to produce nothing in the way of aromas and flavors, but rather to produce not much. I love the analogy provided in an e-mail from Valérie Nouirissat, U.S. sales representative for Burgundy-based Enovine: “We can distinguish aromatic yeasts from neutral ones like aromatic grape varieties (Muscat for example) against Chardonnay (neutral).”
The classic examples are the so-called Champagne yeasts—EC1118/Prise de Mousse, Premier Cuvée and the like. Champagne producers have traditionally preferred yeasts that do not add much (at least nothing strong and distinct) to their fermentations, and these strains have proven over time to fit the bill. Similarly, notes Marco Bertaccini, general manager of the AEB Group (Fermol yeasts) for the U.S., distillers like those in Cognac have traditionally favored both neutral grapes (Ugni Blanc) and neutral yeasts, letting barrel aging determine character.
What few esters these (fairly) neutral yeasts generate, says Toni Stockhausen, an Australia-trained winemaker and now technical services manager for American Tartaric Products, “don’t impart much to the finished bottle, often don’t even persist through dryness. Nothing of significance is added to the varietal.” Stockhausen and Charlotte Gourraud, a former researcher for Laffort at the University of Bordeaux and now in the U.S., both note that the neutral strains are also known for what they do not contribute—hydrogen sulfide, elevated levels of volatile acidity or encouragement for Brettanomyces or bacteria. Primarily, they are valued for their performance characteristics—alcohol tolerance, fermentation straight through to dryness and so on.
And, as several people point out, they’re cheap—not a trivial consideration. Russ Robbins of Laffort speculates that you’re likely to find traces of PDM in most fermentations in the U.S.; it has become a kind of resident national yeast strain.
Several of the prominent neutral strains are Saccharomyces bayanus yeasts, not cerevisiae—or, rather, these used to be called bayanus before the new yeast taxonomy of recent years. Now they’re classified as cerevisiae bayanus. Gordon Specht of Lallemand tells me that in the new taxonomy, those properly classified as bayanus “are anything but neutral.”
Varietal…for what variety?
The notion of yeast strains that “enhance varietal character” opens up another whole kettle of fish—speaking of strong aromatics. Besides the question of varietal character for what variety, there’s the question of who decides what is the character of a grape variety.
Are herbaceous, green overtones part of Cabernet’s varietal character? Depends on whom you ask. It also may be that the yeasts that do most to express varietal character are really those that add the least—our friends the neutral strains.
I know, for example, Riesling winemakers who swear by EC1118 precisely because it lets Riesling be Riesling. Similarly, John Katchmer, an enologist at Vinquiry (Enartis yeast), says that yeasts like Prise de Mousse are “popular with terroir-seeking folks for not producing polysaccharides for mouthfeel, not producing aromas, just fermenting the stuff.”
Nearly everyone I spoke to brought up two groups of compounds that certain yeasts can maximize as the way to understand what it means to enhance varietal character: thiol production for Sauvignon Blanc, and terpenes for Riesling and other aromatic grapes. These are the two best-studied areas of yeast-precursor interaction; red wine characteristics, by comparison, are still largely uncharted territory in hard scientific terms.
The mechanisms for optimizing thiols and terpenes are quite different. For thiols, Robbins explains, yeast can have a gene that turns sulfur aroma precursors into desirable thiols—the basis for the passion fruit, boxwood, grapefruit, cat pee and similar things favored in Sauvignon Blanc. The thiols (and the cat pee) aren’t in the grapes; they have to be created by the yeast.
Terpenes, on the other hand, are in the fruit, but bound with glucose, and need a yeast that can cleave those compounds, freeing the volatile terpenes. In Laffort’s case, VL3 is your ticket to thiols, VL1 the liberator of terpenes.
Stockhausen emphasizes that yeast can find, transform, express and stabilize aromatic and flavor compounds, but they cannot create them out of nothing. “You can use an aromatic strain on Chardonnay,” she says, “and it will maintain Chard character; there’s not so much there to act on. Those wines are more likely to be enhanced by lees, barrels, malolactic strains and so on. But for the varieties that have aromatic components, yeast can bring something to the table.”
Specht and others note the extensive work on thiol production done by the Australian Wine Research Institute. In those studies, some strains were found to be better at releasing thiols, some better at converting precursors to more stable compounds, some better at producing compounds perceptible at lower sensory thresholds.
Marco Bertaccini notes that some strains can encourage the formation of anti-oxidants, thus helping preserve the aromatics that are created. Vinquiry, among other suppliers, can fix you up with enzymes that help free the precursors, so the yeast can do more of their thing.
Varietal…according to whom?
Robbins notes that many strains have been isolated in places famous for certain wine styles and certain grapes, and sometimes the yeast strains get named overtly for those grapes or growing areas. Robbins thinks this convention has a lot to do with marketing—in some cases, very successful marketing.
Which takes us to the question of how it’s decided that the varietal character of grape X is…whatever it is. Gordon Specht notes that the style of Sauvignon Blanc created in New Zealand (thanks in part to all that Australian thiol research) became not only accepted but a world benchmark, a market-driven re-definition of varietal character. There was a time, Katchmer recalls, when many people thought Sauvignon Blanc should taste like Chardonnay.
How varietal character is expressed and how it is constituted varies widely by wine region. Specht says you can take the same equipment and the same yeast strains that produce the New Zealand effect, and work with different fruit from somewhere else, and come up with a different wine.
Katchmer takes the case of Pinot Noir: “One Pinot yeast might work great in Carneros, another in the Anderson Valley—varietal one place, varietal another place.”
All of which helps explain why, after decades of research, salesmanship and fermentation, there are no generally accepted answers for precisely what yeast goes with what grape. Not only do the yeast companies compete with each other—Lallemand, for example, with a very large yeast portfolio, suggests at least eight different strains that might do the job. Not surprisingly, the more popular the grape, the more numerous the yeast strains that promise to snuggle up to it.
The drive to associate yeast strains and grape varieties isn’t universal. Robbins and Gourraud both think this is a U.S. thing, maybe a New World thing. Winemakers in France, they say, are more likely to want to know a yeast’s general characteristics—does it produce forward fruit, does it build mouthfeel, and so on.
Chr. Hansen’s Duncan Hamm isn’t so sure of this Old World/New World comparison, but does see much more concern about varietal typing among boutique, artisan wineries and more concern for general performance characteristics among larger, mass-market producers.
So, back to “neutral” and “varietal character.” If there’s a way to summarize all these angles, it comes down to something most winemakers already know: The only way to find out how a particular yeast strain works on your particular fruit is to try it.
Tim Patterson writes about wine and makes his own in Berkeley, Calif. Years of experience as a journalist, combined with a contrarian streak, make him interested in getting to the bottom of wine stories, casting a critical eye on conventional wisdom in the process. Contact him through email@example.com.