Q: We have several of our winemaking club members dealing with mycoderma (a white surface film yeast) on their wines in carboys and barrels. We are wondering if using high-proof alcohol (190-proof) as a spray on top of the wine would kill the growth and then it could be filtered out?
— Dan Boykin • Camino, California

A: There is a great summary of what mycoderma is and ways to deal with it written by Daniel Pambianchi at www.techniquesinhomewinemaking.com/white-surface-film. Folks can check out that blog post for a deep dive, but to skim the surface (pun intended), mycoderma, or surface film yeast, is largely caused by oxygen ingress during wine aging and can be found in a thin skin-like growth in the necks of carboys and in slightly un-topped barrels. In Daniel’s post, he gives all sorts of very descriptive (and handy!) tips for cleaning your wine of this surface nuisance or racking your wine away from it. All of that effort will be for naught, however, if the wine isn’t stored in an air-tight way after the fact. 

Your suggestion to spray the surface with-high proof alcohol followed by a filtration is a decent one. Even 190-proof alcohol, however, is unlikely to “kill” a surface bloom like that because 1) the film yeast are perfectly happy living in an alcoholic (albeit lower-proof) environment and 2) once the high-proof ethanol hit any wine it would become diluted and/or evaporate, therefore lowering the toxicity of the high-proof spray. If you have some on hand, however, it won’t hurt to try some sprays; it will at least slow down and inhibit the film yeast colonies somewhat and slow its spread. 

Sterile filtration, on the other hand, will always exclude spoilage yeast, which are actually much bigger cells than bacteria. If you’re able to filter to a 0.45-micron nominal pore size, you’ll filter out both spoilage yeast and bacteria, leaving you with clean wine. The end result is the same, however. If you don’t have a topped-up and airtight way to store your wine afterwards, even sterile filtered wine can develop film yeast given enough time and the insidious ingress of oxygen.