Troubleshooting Your Wine
The faults and flaws and their analysis and resolution presented below are from my latest book, Modern Home Winemaking, where you can find additional information should you wish to explore these further.
WHITE SURFACE FILM
DESCRIPTION
A whitish film on the surface of wine is the result of surface spoilage yeast that thrive because of the presence of oxygen. The condition is often referred to as mycoderma, named after the most common of these aerobic surface yeasts, Candida mycoderma. It is most commonly seen in seemingly forgotten wines aging in poorly topped up barrels.
This condition always results in some oxidation and increase in volatile acidity (VA) although there may not be any perceptible organoleptic flaws if caught early.
CAUSES
Surface spoilage yeast needs oxygen to grow and develop. It starts off by forming a small islet that grows and is able to do so even in the presence of SO2, and therefore, this clearly points to an oxygen ingress problem, typically because of poor topping in barrels or a defective bung or airlock in barrels or carboys.
ASSESSMENT
Any surface film points to the presence of excessive oxygen and surface spoilage yeast activity. The cause is readily obvious in barreled wines that have not been topped up properly or stored above 15 °C (60 °F). Remember that a barrel acts an oxygen-transfer system and, although the headspace is largely a vacuum, there is sufficient oxygen for aerobic spoilage yeasts (and bacteria) to start their destructive effects.
As you assess the problem, do not disturb the film; the film will otherwise break up and disperse on the entire surface and possibly drop into the wine.
Perform a visual examination of both the bung or seal and airlock to see if there are any obvious defects that would allow air to enter into the vessel.
Certain kinds of bungs tend to lose their form or elasticity and not form a proper seal when seated in a carboy.
S-style airlocks too can be a source of air infiltration. These airlocks have a seam that can split open ever so slightly, invisible to the eye, and let air in from below the liquid level in the airlock. It happens. You can test the integrity of the (empty) airlock by blowing into one end and blocking the other with a finger. If you are able to keep blowing or if you feel some air through the seam, then you have a defective airlock. Another problem is the extra plastic material protruding from the seam, which, when inserted into a bung, creates a small air gap into the carboy.
If you are using dry airlocks, verify that they work properly. If you blow from the bottom end of a dry airlock, it should vent properly at the other end, but once you blow from the other end at the top, the airlock should provide an absolutely tight seal. Marble-type airlocks on variable-capacity tanks (VCTs) are notorious for letting air in; it is best to change those with better airlocks.
Also assess for potential oxidation and volatile acidity (VA) problems.
REMEDIAL ACTIONS
As a short-term solution, if the surface film is high in the neck of a full carboy, which definitely points to a defective bung or airlock, try and remove the film by gently inserting a paper towel and giving it a gentle twist to scoop up as much film as possible. Be gentle as it is very easy to break up the film and cause it to spread further into the wine. You can also try sucking the film up with a kitchen baster although this may prove more difficult.
The best technique though is the “overflow” or “flooding” method. If you are able to insert a small tube into the wine without disturbing the film too much, add wine via the tube into the carboy until the film overflows and floats out. Wrap the top of the carboy with paper towels to avoid making a mess.
Use the same overflow technique for film in barrels as you will not be able to scoop it out given the large surface area and headspace. With a baster or wine thief, remove wine to bring it down to a level where you can properly insert a bung. If still using an airlock, make sure that it is not defective and sand off the excess plastic material on the seam to a smooth finish to avoid creating that air gap into vessels.
Since this technique never completely removes all the surface film, no matter how careful you are, the recommended longer-term solution is to gently insert a racking cane to just above the sediment, if any, and transfer the wine to another vessel being careful not to transfer film particles as the surface of the wine approaches the bottom. As it has surface tension characteristics, most film will cling to the wall of the vessel as the wine surface drops. Stop the transfer when you see film particles approaching the racking tube inlet; discard the leftover wine.
A coarse filtration in conjunction with the above techniques helps separate excess film if you were not able to remove it effectively and particularly if some dropped back into the wine. Remove as much film as possible to avoid carrying it through.
Measure FSO2 to see if there was any significant depletion, and make any necessary adjustments according to pH and perhaps an additional 10–15 mg/L to deal with DO.
If you detect oxidation or volatile acidity (VA) problems, consult those pages for further remedial actions.
PREVENTIVE ACTIONS
Always keep vessels, and especially barrels, ideally at around 13 °C (55 °F), never exceeding 20 °C (68 °F), completely full and topped up, and wines at the proper FSO2 level according to pH. Make sure bungs are inserted as tight as possible. Aerobic spoilage yeast cannot thrive in the absence of oxygen.
Sand off to a smooth finish the extra plastic material at the seam on new airlocks. Visually inspect bungs, seals and airlocks before use.