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Ron Wilson — Hamilton, Ontario asks,

My father-in-law makes wine from grapes, de-stemmed and put in pails. He does the usual vatting and filtering and transferring demi-johns, etc. Once the wine is ready, we put it in gallon (3.8 L) bottles and store it in our basement cold cellar until we need some, at which time we transfer it into 750 ml bottles. When we open the gallon (3.8 L) jugs, there is sometimes (not always) a topping of what appears to be mold or something. It doesn’t smell or anything. The father-in-law says to carefully remove it, but often some bits remain, which we filter with a strainer when bottling. The wine always tastes fine, but I wonder if it is okay to drink?

Thanks so much for sending over the pictures, they are very helpful. Even though it’s impossible for me to diagnose down to the organism just based on images, I’d wager you’ve got a very thick colony of spoilage yeast growing on top of your gallon jugs. Sometimes called “film yeast” and similar to the classic “flor yeast” bloom intentionally encouraged to live on top of wine during Sherry production in Spain, these are aerophilic organisms that can metabolize ethanol, glycerol and organic acids as food. If conditions are right, they can grow to such an extent that they form the white fluffy colonies on the surface of the wine. Though they can be Saccharomyces species, film-forming yeast colonies can also be comprised of Pichia, Candida and Hansenula strains. I’m glad that, even with these visible spoilage colonies, you’re finding that the wine tastes fine. The wine should also be safe to drink and not poisonous for any reason (unless something funny was added to it!). To quote one of my favorite UC-Davis professors, the newly-retired Dr. Linda Bisson, “No human
Response by Alison Crowe.