Dear Wine Wizard,
My partners and I, for the first time, bought Merlot grapes last year. We followed our normal procedure, which we have been using for the past five years for the red grapes we used, including Zinfandel, Barbara and Carignan. We plucked about 75 percent of the grapes off the stems prior to crushing. We used pasteur Champagne yeast and the grapes were in the primary fermentation container for five days. We now have the wine in carboys. It has cleared up in good shape but there is a yellow ring around the top of the wine. The wine tastes good and has a good aroma, but what’s with the yellow ring?
Wine Wizard replies: Sounds like ring around the bathtub to me! Or maybe we should call it ring around the fermenter! Going on the information you’ve given me and if my wino intuitions are correct, your “yellow ring” is probably a deposit of post-fermentation residue, especially if you put the five-day fermented wine into the carboy before the wine had finished fermenting to dryness.
As I’m sure you’re well aware, winemaking is not a sterile process, and well, wine tends to leave goop, glop, and globs of gook wherever it goes throughout the entire production process. I’ve seen my share of thick layers of lees, skins, and other winemaking detritus in fermenters, barrels, and bottles as well as on the periphery of carboys or stainless steel tanks. In your particular situation, what seems to have happened is that your yeast wasn’t quite done with its job yet when you put it into your carboys. As the yeast cells sputtered through their last degrees or two Brix, they decided to, in cohorts with evolving carbon dioxide, minute particles of suspended pigments (that aren’t always necessarily red), and stirred-up yeast lees, effect a deposit of post-fermentation gook up the sides of their container. Add more time (the time it might take for newly fermented wine to clear up) and the small amount of evaporation that can occur in a carboy with a fermentation lock on it, and the wine level will sink ever so slightly, revealing the debris as a ring-around-the-carboy that even though it looks ugly, is entirely harmless.
As a more microbiologically involved answer along the same lines, it’s possible that along with your Pasteur yeast you somehow got some surface-dwelling microaerophillic organisms (there are a few bacteria and yeast species that come to mind). These guys can live in wine and especially like to hang out in that just-right zone between the non-oxygenated wine in the carboy and the barely-oxygenated layer of mixed carbon dioxide gas and air that exists in the headspace of a post-fermentation carboy. These organisms that have paid you a visit, thankfully, do not seem to have spoiled your wine.
If what you’ve described in your question seems to be at the wine-carboy interface and can be categorized into the general “wine crud” category, do not despair. I’d just rack when you’re ready into a clean container and continue to enjoy your wine. If not, try to examine every step of your procedure and find out what it was you did differently this year. Ring around the carboy is common; rings of vibrant yellow slime are not.