During the aging of wine in tank or barrel, it is not uncommon that a film may start to appear on the surface of the wine. What is causing this film and what are the consequences for your wine? The most likely cause of the film is growth of oxidative film yeast such as species of Candida, Pichia, and Hansenula as well as the aerobic bacteria Acetobacter. All these microorganisms are commonly found on grapes and winery equipment but due to their oxidative nature they typically do not grow during alcoholic or malolactic fermentation where the production of CO2 creates an anaerobic environment. However, if tanks and barrels are not well topped there can be sufficient air to allow these microbes to grow and produce various metabolic products that can be detrimental to wine quality.
Film yeast form a film on top of the wine due to continuous budding of mother and daughter cells that remain attached to each other forming chains and branches rather than separating into individual cells. The film can initially look like small “floating flowers” and may grow into a film that covers the entire wine surface. Films formed predominately by Candida vini (formerly C. mycoderma) have a dry, chalk like appearance rather than the slimy, scummy film that Acetobacter can form. Acetobacter films also tend to begin around the edge of the wine barrel/tank rather than in the middle. While the formation of the film is visually unappealing, it is the production of certain flavor/aroma compounds that can result in spoiled wine. Acetobacter produces acetic acid via metabolism of ethanol and under less oxidative conditions will also produce acetaldehyde. Film yeast can produce high concentrations of acetaldehyde, ethyl acetate, and/or acetic acid utilizing various substrates (ethanol, organic acids, glycerol) under oxidative conditions. For example, Candida can produce over 300 mg/L of ethyl acetate (nail polish remover odor) which is well above the sensory threshold of this compound in wine (60-100 mg/L). Acetaldehyde produced by film yeast and/or Acetobacter may also result in a bruised apple/sherry aroma in the nose or palate. Acetaldehyde also rapidly binds free SO2 rendering it ineffective as an antimicrobial. In fact, the rapid consumption of free SO2 can be an early sign that oxidative microorganisms are growing in your wine. pH and color changes in red wines can also become apparent due to the production of acetic acid and acetaldehyde.
Once these microbes are established in a wine it can be difficult to control their growth with SO2 given the binding of free SO2 by acetaldehyde. Preventing the initial growth of film yeast and Acetobacter during wine aging therefore becomes essential. Minimizing exposure of wine to air is the primary way to prevent film yeast and Acetobacter growth given their oxidative nature. This may include systematic topping, use of inert gases to fill headspaces and flush lines, and managing tank volumes to minimize partially filled vessels. Low pH, low storage temperature, and high ethanol will reduce the spoilage risk of certain wines but under the right set of conditions, film yeast and Acetobacter will grow in most wines. Complete removal of film yeast or Acetobacter can only be achieved by appropriate filtration, but populations can be reduced by carefully removing/racking the wine from underneath the film and leaving the top portion of wine and the film behind. Care is needed to minimize disturbing the film as it may break apart and mix in with the rest of the wine. Barrels that contained spoiled wine need to be well cleaned (high pressure warm water) and sanitized before being used again. Film yeast and Acetobacter are less challenging to remove from a barrel than the spoilage yeast Brettanomyces as they don’t tend to penetrate into the wood in contrast to Brettanomyces. As with most microbial wine spoilage issues, prevention is better than remediation so maintain well topped barrels and tanks, keep your free SO2 at a pH appropriate concentration (and keep an eye out for rapid consumption of free SO2), and sparge headspaces with inert gases if you can’t maintain full vessels. — By Dr. James Osborne, Professor and Enology Extension Specialist, Dept. Food Science & Technology, OSU