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.
A beautiful paper recently published in Cell (high-octane journal, but this article is open-access, thanks to European equivalents of policies like these) stands up in favor of domestication, at least for beer yeast. It’s a beer paper: the scientists* investigated genomes of 102 industrial beer yeasts and only 19 wine yeasts, 7 from sake, 7 from “spontaneous fermentations” (of…?), 5 from bioethanol production, 4 from bread, and 2 used in scientific research. So, while they make a few claims about how yeast strains have developed outside breweries, those claims are standing on sparse foundations.
The beer yeast genomes show well-known signs of domestication: they’ve become less fit, less able to compete for survival against wild yeast in uncontrolled situations outside their brewery homes. They’ve lost some genes and gained extra copies of others, some quite specifically in ways that suit their profession. Compared with other Saccharomyces cerevisiae, beer strains tend to have more genes for using maltose – the predominant sugar responsible for the alcohol in beer, which comes from wort, which comes from malt, which comes from keeping moist grain warm until its enzymes split long starch molecules down into maltose molecules. They’ve lost the function of genes for 4-vinyl guiacol, a clove-flavored molecule that constitutes a sensory fault in most styles of beer.
Wine yeast – at least based on the few they studied – don’t look very domestic at all. Those conclusions make for a nice, logical story. Beer is often brewed throughout the year with one batch directly inoculated from its predecessor; wine is a once-a-year event with no real chance of directly transferring bits of one freshly-fermented vintage into the next. Beer gives everyone more time to get used to each other and to choose their colleagues deliberately; wine sends yeast out into the surrounding world to survive and only ends up collecting some of them back the following fall, so populations mix rather than form a tight little microcosm. Wort → beer is also a much less stressful environment than must → wine. The wort offers plenty of sugar but not too much, not too acidic, lots of nutrients, modest alcohol concentrations at the end of the day; wine yeast, on the contrary, have to cope with more sugar than is really good for anyone, and lots of acid, and lots of alcohol, and questionable nutrient conditions. The logic goes that wine yeast can’t afford to become too plump and pampered in the master’s house because they’re still out fighting in pretty darn wild environs.
This is great basic science work, and full of fun findings, but it’s fair to ask: why does this domestication question matter?
- First, the basic science part. Studies like these help us learn more about yeast genetics and how populations have changed over time, which in turn helps us understand population dynamics in general. Observing how beer yeast genomes have changed also adds to what we know about gene functions. Beer applications notwithstanding, that’s where the heart of this paper lies.
- Second, the industry development part. Knowing more about the genomes of current brewing strains can point to genetic markers for developing new, perhaps better brewing strains.**
- Third, the sustainability part. Understanding more about what effects human cultivation has heretofore had on yeast populations helps suggest how our actions might continue to affect yeast populations. Those trains of thought are becoming ever more important as we use yeast as “cell factories” to make more and more products and as we change yeast genomes in ever-more aggressive ways.
Finally, papers like these are good for thinking about how we live with and depend on and mutually change other species in our environments, including the ones too small to see. Have we domesticated yeast? Has yeast domesticated us? How can we keep taking good care of each other?
And just because this paper is so full of them, a few more fun findings:
- The United States has its own uniquely American group of beer yeasts which seem to have split off from British yeasts roundabouts the Revolutionary era.
- Yeast strains used in industrial processes are sometimes related to yeast used for beverages in the same country. In Brazil, yeast used for bioethanol resemble yeast used for cachaça .
- The beer yeasts formed two groups, one of which was much more similar to the wine yeasts, which the authors take to suggest that humans domesticated yeast at two separate times, one involving both wine and beer, the other just beer.
*Who are mostly academics from Belgium, but also include a few folk working at Californian companies.
**This paper cites a review article in Nature on improving crop productivity as a model of what might be done with yeast. The paper is from 2008 and many of the techniques it describes are outdated, but it remains good as a picture of differences between classic genetics and genetic modification (GMO) techniques at a technical level.