Innovation in sparkling wine production: Trust the yeast

Numerous recent studies have been playing with how yeast can work above and beyond the usual call of duty in sparkling wine production. The Australian Wine Research Institute’s (AWRI) superb yeast biologist Jenny Bellon continues to convince yeast to reshape itself to our needs by breeding across the usual species lines.* Hybrid yeast (open-access article) with a Saccharomyces cerevisiae strain as one parent and a Saccharomyces mikatae or other close cousin Saccharomyces species as the other, generate different secondary metabolites compared with conventional straight-up S. cerevisiae strains, and we somehow end up interpreting that difference as “complexity,” and liking it.

The goal in those cases is to produce new and different (and better) flavors by using these more metabolically complex yeasts for tirage or in-bottle fermentation.

The interesting thing about tirage yeast, though, is that they do a good portion of their winemaking work after they die. While alive, yeast are useful for their insides: the enzymes they house convert sugar to alcohol and numerous other valuable metabolites. In dying, yeast are useful for their outsides: they release mannoproteins from their cell walls that improve wine quality in numerous ways, by enriching mouthfeel, by stabilizing mousse, and by adding lovely bready or toasted aromas. (Find more detail on those effects in this embarrassingly badly written article from 2012).

When yeast cells die, they don’t just turn off; enzymes split open the cell from the inside (autolysis), releasing good-for-winemaking compounds. However, autolysis happens inefficiently under standard winemaking conditions: yeast are most inclined to self-sacrifice around pH 5 and at warmer temperatures; sparkling wine generally sits below pH 3 and is fermented cool. Here’s where two recent scientific studies about innovating in sparkling wine production meet.

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Indigenous yeast in Sauternes, multi-species family wineries, and Casale’s Trebbiano orange wine

A group of French scientists, mostly based in Bordeaux, have recently published evidence* that the same Saccharomyces cerevisiae populations have lived at their winemaking homes in Sauternes for at least 23 years.** Two decades is a brief moment, whether you’re measuring in human-history-of-Sauternes time or in yeast-evolution time, but their work still supports the idea that yeast populations become associated with wineries and stick to them.

A main point the scientists aim to make is that these longstanding, tradition-following Sauternes estates haven’t been badly polluted by modern, commercial winemaking yeasts. Only seven percent of the S. cerevisiae strains they collected could be clearly connected to specific commercially produced strains. This news is excellent for those in the natural wine camp who want to call spontaneous ferments “natural” or “wild.” In this little corner of Sauternes, at least, it seems that wines allowed to ferment spontaneously aren’t just being fermented by commercial yeasts persistent in the environment. Most of the work of fermentation is indeed being done by yeasts which very likely existed in the area prior to Lallemand and the rest introducing their tidy little foil packets of active dried specimens.

A good case can be made, I’d argue, for calling those non-commercial yeast something like “domesticated indigenous” rather than “wild:” human winemakers have selected and bred them up for desirable winemaking characteristics over many generations, just as human dog-keepers have bred up select canine features or orchid aficionados have carefully developed new plants. Domestication can happen even if living things are never bought and sold.

Regardless of what we choose to call these yeast species, though, this sort of research says that those species are part of the traditional winemaking environment, part of the terroir. Terroir is apt because that complex term incorporates human traditions and activities, the way winemakers and the rest of the community have shaped the land and its various characteristics: soil, plant life, microbial life, architecture, machinery, maybe some canines. Terroir doesn’t force us to make a choice between those winery-associated yeasts being wild or cultivated. They’re both, and winemakers and winemaking have evolved with them.

The Casale orange trebbiano*** I opened last night is made by a winery with whose winemaking history in their neck of Chianti can be traced back to 1770. The wine itself is a 2012 vintage and the current release, kept on the skins for 30 days and in stainless steel for two years. The property is biodynamic, the wine spontaneously fermented.

Even if the mid-palate is fairly empty, the nose and the finish are more than enough to rescue that deficiency: savory, nutty, sweetly orange blossomy and honeyed up front all at once, and acidity with great energy and tension (minerality, if you wish) on the tail, with a pleasant light astrigency across the breadth of the middle. This isn’t just a wine for thinking with, and I’d readily pour it for anyone who thinks that all biodynamic orange wines must smell like hair salons and taste like dirt, because it doesn’t. But it’s a beautiful example of a wine good both for drinking and for thinking, that tastes pleasant and nourishing and that provokes pleasant and nourishing trains of thought.

When we care about family wineries – for the purely aesthetic value of tradition as well as the economic value of maintaining local businesses and the winemaking value of passing down knowledge and physical infrastructure – we should care about the extended family, not just the humans but the yeast (and maybe even other species) who have also lived on a winemaking property for generations, who make the wine with their human coworkers. Taking care of the family can’t be just about loving your brothers, or even about supporting the other families who might work with you, but about caring for your non-human brothers and coworkers. Now, that’s a pleasant and nourishing train of thought.

*Full text of the article behind an academic journal’s paywall, unfortunately.

**A bit more detail on that point: the team collected samples from three Sauternes estates over 2012, 2013, and 2014, isolating 653 individual yeast strains, and compared them to commercial strains and to 102 “library” isolates collected from one of the three estates between 4 and 23 years ago. The comparisons to determine which strains are related to which other strains rely on 15 “microsatellites,” or specific, small sections of DNA the sequence of which is highly and characteristically variable amongst strains. Much could be said about how we define what constitutes an individual “strain” on the basis of the tools we have available to do so – appearance (phenotype, in biology jargon) in the past, genetic today – but I’ll forgo that conversation for today.

***Purchased for £15.50 from Wood Winters in Edinburgh.

Why do yeast make alcohol?

Ever wonder why yeast make alcohol? Probably not, I realize, but you should. Yeast throw off ethanol in the process of metabolizing sugar, so alcohol is a byproduct of survival; fair enough. But alcoholic fermentation is, in fact, a surprisingly inefficient way to get energy. The standard oxygen-requiring way of breaking down sugar used by most cells, our own included, wrings somewhere between 30 and 38 ATP (38 is the ideal number; it’s probably never quite that high in practice) out of a single glucose molecule. (ATP is the cellular currency in which energy is transferred and spent.) Nevertheless, alcoholic fermentation has the distinct advantage of not needing oxygen and so it makes perfectly good, intuitive sense for Saccharomyces cerevisiae to use it when oxygen isn’t available.

Here’s the quirk: S. cerevisiae uses inefficient alcoholic fermentation even when it does have access to oxygen, even though it has the machinery for the much, much more energetically worthwhile aerobic metabolic process. Yeast will only switch to aerobic metabolism when the amount of sugar available for them to eat is very low. Why? A good question, and one microbiologists haven’t had much success answering.

Our best hypothesis according to a brand-new review on the subject comes in two parts:

  1. Alcoholic fermentation lets yeast act fast to use up the “public goods” while squirreling away private resources for later. Every microorganism you’ll encounter in grape juice can consume sugar. Very few can also consume (and get energy out of) ethanol, but yeast can. So, by converting sugar to ethanol, S. cerevisiae can starve out other microbes and leave itself with a food source for later.
  2. As an additional and maybe even bigger benefit, ethanol is toxic to most yeast and bacteria at concentrations that Saccharomyces can tolerate with relative ease

Possibly the most bizarre thing? We don’t know much about what determines the circumstances under which S. cerevisiae, our long-time compatriot and coworker, produces alcohol versus making energy in some other way. We’ve looked at when and where different yeast genes are expressed and when and where it makes different byproducts but, like so much else in the wonderful and frustrating world of modern-day genetics, putting together the whole story is still a work-in-progress.