Why stuck fermentations are like Mad Cow Disease

Stuck fermentations — when sugar levels stop dropping and the winemaking process stands still — are one of the more persistently frustrating mysteries in winemaking. Like most winemaking mysteries, we understand part but not all of the situation. Bacterial contamination is one of numerous known causes of sticking: lactic acid bacteria can compete with wine for access to sugar, but it’s also long seemed that something else is going on. Researchers now have a better idea of what that something else is, and it involves prions.** Yes, prions, best known by nearly everyone as the infective agent in bovine spongiform encephalopathy, more fondly known as Mad Cow Disease.

Briefly, bacteria are producing some kind of small signaling molecule that provokes Saccharomyces cerivisiae to shift from preferentially fermenting glucose into alcohol to consuming other energy sources indiscriminately. Bacteria release the molecule, yeast take the molecule up and begin expressing a prion, and in some as-yet-unknown way, the prion jams the mechanism that normally tells yeast to consume only glucose when it has both glucose and other energy sources available. Bacteria don’t tolerate alcohol as well as S. cerivisiae, so it’s in the bacteria’s interest to get the yeast to make less of it. S. cerivisiae can use all manner of different molecules for energy, but a specific control mechanism ensures that it (usually) eats glucose first when glucose is around.

These findings tie into an overwhelming lot of very interesting, very intricate biology, the fullness of which is a bit much to discuss here. But (understanding that there are others), a few reasons why this research matters to scientists and to winemakers stands out.

To scientists:

  • Bacteria and yeast are talking to each other. Or, rather, bacteria are controlling yeast for the bacteria’s benefit. Bacteria produce lots of small messenger molecules — a bit like hormones in the human body — to communicate amongst themselves. But the idea that they use a similar molecule to control the behavior of a different species is exciting. Bacteria probably do this all the time, too, but microbiologists are behind on learning about it because we traditionally study one type of microbe at a time, by itself, in a test tube or beaker. Imagine studying 12 year-old boy behavior by putting lots of 12 year-old boys in a room by themselves and watching them for a week. That’s what we’ve been doing with bacteria. Microbiology as a field is increasingly realizing that there are better ways (which are, of course, more complicated, and therefore harder…)
  • The mechanism involves prions, which are cool because they’re a relatively recent discovery and we’re finding them in places we didn’t see coming. It’s still not clear how they’re working in this setting, but finding out will almost certainly involve learning some new and interesting biology

To winemakers:

  • Winemakers who are adamant about avoiding stuck fermentations are probably also vigilant about trying to keep bacterial contamination out of their wines, so I imagine this news doesn’t change much. Nonetheless, some folk might end up using more sulfur dioxide in an effort to knock down bacteria in ferments that tend toward stickiness.
  • More interestingly, researchers may be able to develop yeast that don’t respond to the bacteria-induced switch, maybe with a mutated form of the prion protein. Non-stick yeast?

**The research is published in two complementary papers (here and here) in the journal Cell and, as happens with particularly interesting stuff like this, the editors have put together a short summary. It’s still pretty dense stuff unless you have a background in molecular microbiology, but you can find it here if you’re interested in the details (and if you have institutional access to the journal).