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 pathogen can survive in wine.” What she means is that no bacteria or virus that can harm you can live in the high-acid and high-alcohol environment of table wine. There’s a reason the ancient Romans and Greeks used wine as an antiseptic in their ancient medical practices.
It’s a shame, though, to put all your hard work at risk. I wonder why you don’t just bottle all the wine at once? You’ll protect the wine from a high-oxygen environment and will develop secondary (and in most cases, desirable) bottle bouquet aromas and textures by doing so. I always say that “wine ages better in the bottle” meaning after it’s done with its bulk aging for about 9 months for a still white wine like a Chardonnay to 12-18 months for a bigger red like a Cabernet, the wine will most safely and gently develop in the bottle.
If you don’t want to bottle it all up at once for whatever reason, there are some steps to take if you still choose to store your wine long-term in jugs. Here’s a list:
• Don’t over-feed your fermentations. An often-overlooked source of spoilage organisms is “leftover” yeast nutrient from the primary fermentation. If there are excess nitrogen or micronutrient molecules in the wine after the primary fermentation is complete, they can unintentionally nourish undesirable yeast and bacteria.
• Make sure that your wines are stored with enough Free SO2 to help keep the microbial population at bay. I tend to shoot for about 28–35 ppm FSO2. You don’t want to add so much that it affects the taste and aroma of your wine but you want some in there to be effective at least against some of the organisms.
• Make sure your wines are fermented dry. Residual sugar and residual malic acid are just invitations for spoilage yeast to take hold.
• Don’t constantly open up your carboys and storage containers. Opening your storage containers (carboys, barrels, etc.) after the wine has finished fermenting not only lets in oxygen but also potential spoilage organisms. If you must open a carboy, spray the surface of the wine and underneath the cap with a strong SO2 solution before you close it up.
• Use an air filter (UV) in your cellar. To cut down on ambient spoilage organisms, I’ve heard some wineries, caves and cellars are having success installing air filters that rely on UV light to kill microbes.
• Keep cellar temperature low. Higher temperatures encourage spoilage growth whereas cold temps suppress it. Even a 10 degree Fahrenheit change, like 52 °F (11 °C) rather than 62 °F (17 °C), can make a big difference.
• Try for lower pH wines. The higher the pH (and the lower the acid) the higher the chance you’ll experience populations of spoilage yeast and bacteria. Even big rich red wines can be delicious at a pH of 3.45-3.55 so if you want to thwart oxidative and microbial spoilage in your wines, this is one thing to consider.
I hope that this gives you some direction for how to curb those unsightly white film yeast colonies. Eventually they can oxidize and spoil your wine so the next jug you open you might not be so lucky. I very much recommend that you bottle your wine all at once instead of leaving it so vulnerable in gallon jugs.
