A new review of familiar news on grape solids

I’ve just finished reading a (short, I’ll admit) review on the “characterization and role of grape solids during alcoholic fermentation under enological conditions,” and I’m delighted to report that there’s little to report. You’re not missing some manner of fascinating spanking new research on grape solids because you don’t have a subscription to the American Journal of Enology and Viticulture. If you got the picture that solids are important for yeast growth from your favorite old textbook for Winemaking 101, you still have the right picture. Chances are that you’re not doing solids wrong.

The major message about grape solids during fermentation continues to be that solids provide yeast with nutrients they need to survive high alcohol concentrations in low- or no-oxygen environments. With plenty of oxygen, yeast can make the lipids they need to maintain good, strong cell membranes. In the absence of oxygen, they can’t and need to absorb those lipids from their environment. Since a fermenting must doesn’t provide a whole lot of oxygen access, wine yeast need those external lipids. Per the classic “oil and water don’t mix” principle, lipids aren’t very soluble in water. (And per the very nouveau classique cocktail principle of fat washing, we’re familiar with the notion that lipids aren’t very soluble in alcohol, either.) The main source of lipids in a wine fermentation that’s essentially water, then, is the solid part.

Ergo, wine solids → lipids for yeast → strong, healthy cell membranes able to withstand the stress of finishing off the last sugars in that 15% alcohol Napa cab.

A few noteworthy points the authors make along the way to that conclusion:

  • Solids aren’t static through fermentation: small particles stick together into larger particles, and larger particles fragment into smaller particles when rising bubbles of carbon dioxide agitate the mix.
  • Solids may also help yeast by helping to remove carbon dioxide. Suspended particles act as nucleation sites for dissolved gases, allowing CO2 to be carried up and out of the must faster and in turn making the environment less yeast-toxic. This, however, seems to be a minor consideration.
  • The more yeast-assimilable nitrogen (YAN) the must contains, the more solid-associated lipids need to be available to support yeast growth to take advantage of all of that nitrogen.

All of which leaves plenty left for scientists to continue investigating. We still don’t have a good picture of how yeast take up lipids from grape solids, either in terms of the mechanism at the cellular level or in terms of how the dynamics of solids settling and mixing matters. Our understanding of how solids move during fermentation could be more detailed (which sounds like a job for the Champagne physicists who study the science of celebratory bubble movement, though I imagine that the CIVC may have them busy enough on other projects). How solids play with wine aroma is a whole category of interesting questions, and research in that arena might even have something useful – and new – to say to winemakers. In the meantime, the news from the research front is that grape solids are still a matter of the good care and feeding of your yeast friends.