More Yeast Silliness

An announcement from the normally level-headed Australian Wine Research Institute regarding research into the selection of yeast strains which could less efficiently convert sugar to ethanol, thus allowing fuller grape maturity with less oppressive alcohol. A nice idea, but I'm afraid not practical.

As I mentioned in Some Like It Hot, yeast strains can’t change the conversion ratio of sugar to alcohol, at least not very much. The six carbon atoms in a sugar molecule have to go somewhere. Two atoms end up as carbon dioxide. The other four go to ethyl alcohol plus miniscule amounts of other flavors like glycerol and to the growth of the yeasts themselves. To change alcohol by 1 percent would mean 10 grams per liter of some other carbon material – an enormous amount which would render the wine undrinkable.






You strike me as an open-minded innovator, rather than a naysayer. You make money removing alcohol from wine. Has that affected your thinking? I would welcome lower conversion yeast. I believe S6U is one already available that has lower conversion, with glycerol being the byproduct- and that has a lot of carbon in it! If you have some biochemical insight please share it. Yes, that's a lot of carbon to use up, but it's a fraction of what already goes up as CO2. A moderate increase in CO2 production would suffice to lower alcohol yield. Never mind engineered yeast, we currently add residual sugar back to dry wine, and ferment grapes with high sugar to start. By the way, some of us think 15% alcohol is a yeast byproduct that "renders the wine undrinkable". High alcohol is a side-effect of the 100 point wine zeitgeist. Pick at 23% sugar, and quit worrying about alcohol. I know that's bad for wine scores, and the alcohol reduction business. No worries, though, there's always VA and vinyl phenols and TCA to remove. As for the scores, "build it, and they will come". The critics will anoint a certain number of the "right" wines with high scores no matter what their alcohol or the current trend may be. Writers are not objective arbiters of wine quality, they are "commentators" who are as deeply mired in this quagmire of their own making as we producers are. They have to award a certain number of 95 plus point scores, or people will quit reading them. Everybody wants to be told about a "great" wine. I'll bet that if all wines were made from grapes picked at 12% potential alcohol, the number of 95 point wines wouldn't change. Maybe I'm wrong. Maybe the critics are addicted to high alcohol. That's all they have ever known. Most of these writers weren't even around back when wine was 13% alcohol. I know you were, though!

Dear Mark:

Thoughtful comments as always. My point on low conversion yeasts is that there is no place for the carbon to go that wouldn't screw up the wine. Glycerol content in wine is pretty low; a myth made up by nontechnical tasters -- you always hear them cooing about "legs" as an indication of glycerol content. Glycerol is about the same sweetness as sugar (see, and at this level makes no contribution to viscosity. So you might as well just arrest fermentation if you want a low conversion ratio. My point is that glycerol is not a practical carbon sink for dry wines. Sure you could cheat with as much as 5 gm/L, but that's only a half a percent ethanol difference, and totally screws up your flavor profile choices. Sure I'm prejudiced by my bias towards alcohol adjustment -- the reason I commercialized it is it's the right way to do it. It doesn't require these kinds of trade-offs, and maintains the winemaker's freedom to make the wine she desires.


Dear Morton

Dear Morton
Having made wine since the eighties, I am one of the silly winemakers who thinks modern yeast produce more alcohol. I have never done a controlled trial, and maybe my use of yeast nutrients has had some effect. Nonetheless, I have seen alcohol conversion go up. Sorry. mark