Have we domesticated yeast? Yes.

Common sense says that winemakers – and beer brewers, and bread bakers – were developing specialized Saccharomyces cerevisiae yeasts a good long while before Red Star marketed its first dried and packaged commercial product to the industry in 1965. Winemakers weren’t inoculating ferments with an aluminum foil packet they bought at the store, but that doesn’t mean they weren’t inoculating, maybe with a little bit of an already-active ferment, maybe just by having a conducive winery environment where the right kinds of yeast were happy to make a home. Either way, the yeast you’d find in any given winery or brewery weren’t the same as the yeast you’d pick up off the street, or the same as what you’d find in the next alcoholic beverage factory down the road.

Plenty of evidence, old and new, supports that story. But did those yeast become different simply because they were isolated from each other, like Darwin’s famous Galapagos finches? Or did they change because they became domesticated, because brewers and winemakers cultivated and selected them? In other words, what kind of difference did the humans make to the yeasts’ evolution?

The theory basically goes like this. If yeast populations developed in different ways just because they were physically separated, then their genomes should look like what you expect from “wild” yeast. If humans domesticated them, they should be less genetically fit, because they’ve grown accustomed to being specially cared for and protected by humans and have lost some of their capacity to live on their own.

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Human intervention increases yeast biodiversity? Sort of.

A new article in PLOS ONE (which, being open-access, you can read for yourself) headlines with the promising title “Yeast Biodiversity in Vineyard Environments is Increased by Human Intervention.” Unfortunately, the paper probably doesn’t mean what you’re thinking it means.

The authors collected yeast from vineyards across the Azores, an archipelago off the shore of Portugal where, following the usual story, viticulture arrived with European settlers in the 15th century. The article says that more than 85% of vineyard acreage has recently been abandoned in the course of “social and economic change,” creating an array of cultivated and abandoned vineyards geographically isolated from each other and the rest of the world. This sounds like a fantastic research setting, even if you don’t take into account “doing fieldwork” meaning “walking around vineyards in the Azores.”*

As advertised, the authors took samples across these vineyards and found that the cultivated vineyards harbor higher numbers of individually distinct yeast strains than vineyards that have been abandoned for at least five years. What that means, however, is a little tricky.

The scientists picked bunches of grapes directly from vines into sterile plastic bags, crushed the grapes inside those bags, and then spread the juice onto what microbiologists call rich media** in Petri dishes, and then used DNA sequencing to identify (some of the) yeast colonies that grew on the surface of the media (jello, essentially) in the dishes. So:

  • We’re only looking at yeast on grapes, not in the soil or “in the environment” more generally. A different, maybe more interesting picture of “vineyard diversity” might have come from microbes in the soil.
  • We’re only looking at yeast willing to grow into visible colonies in two days under standard lab conditions (and the scientists also only sequenced some of those colonies). Most yeast will be happy to oblige, but not all yeast cells present in the environment will become visible colonies in dishes (and some will grow slowly. A lot of microbiology research these days side-steps that problem by sequencing all of the DNA present in a sample, but that option is, as one might expect, more expensive and more difficult. These techniques don’t mean that the older grow-in-a-dish options are completely not-useful or wholly obsolete, but they’re a good reminder that growth-in-a-dish always gives us a limited picture of a microbial world.
  • We’re not comparing cultivated vineyards with the untrammelled wilderness. We’re comparing cultivated vineyards with previously cultivated vineyards that are no longer being maintained as such. We don’t actually know anything about environments on this island where human cultivation hasn’t happened.

The authors are right: this study works against the idea that human activity always decreases ecosystem diversity. And it does say something interesting: that (at least in this setting), human maintenance increases the number of yeast species on grapes. This study continues to support the hypothesis that humans and/or their equipment are a source of vineyard yeast. It’s a good reminder, too, that “human intervention” (you could also say “humans living and working as part of the environment,” if you felt like being contrary) isn’t necessarily detrimental. Though it’s also worth remembering that increased biodiversity isn’t necessarily either “natural” or universally beneficial, either. If humans intervened to increase species diversity in the Arctic tundra, would that be a good thing? We might work on finding better ways of listening to environments telling us how they’re feeling. In the meantime, I suppose that this is a start. 


*Part of it is also a UNESCO World Heritage site.

**Rich media = lots of nutrients = easy for most yeast to grow.

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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