Film yeast in apple cider brewing

Today at practice a fellow Dominionite and brewer had a question about her hard apple cider, and one that unfortunately, due to a similar experience, I could answer... Her 4 gallons of cider had developed a thin film of white on top, and she wondered if it had gone bad. Unfortunately, that means the yeast has a competitor, but fortunately, it does not perse mean it needs to be thrown, as it will be fine to cook with. Add a Campden tablet when you think it's sour enough when vinegrating, or immediately with a film yeast, decant without the film into sterilized containers, keep cool, and enjoy your cooking!

A sample of film yeast (Photo: Alison Steele, 2019)

At first, when it happened to me, I was puzzled as to what it was. Not until I got lucky at our local book sale and found small publication on hard cider making, did I learn what exactly had happened. As this little booklet is not in standard publication, I will transcribe the appropriate section below.

"Chapter 6 Possible Faults in Cider Making

There are a few faults or problems the amateur cider maker may encounter when making cider for the first time, one of the commonest being acetification. The reason for this is that too much air has been allowed to reach the cider, causing the acetic bacteria to turn the alcohol into vinegar. Bad storage is one of the causes, therefore it is essential that containers are kept topped up at all times. Once the first rapid ferment has settled down, keep fermentation jars full and insert airlocks containing a little metabisulphite. In the initial states the yeasty scum covering the surface will prevent the air coming into contact with the juice.
   Darkening of the juice can occur, due in most cases to oxidation, and one way to avoid this is to add one Campden tablet to each gallon of juice as it is pressed. Over-ripe or rotten fruit could have the same effect, therefore discard any fruit that is severely blackened and mouldy as both there could cause the cider to have unpleasant flavours in addition to the enzymic darkening.
   Film yeasts can develop on a weak, low-acid, still cider if it is exposed to the air, and the first indication of this is a grey powdery film that develops on the surface. If left unchecked it will eventually turn the cider to carbon dioxide and water. There is a remedy if it is caught in its early stage. Remove as much of the film as possible then filter it into another jar using a very fine piece of linen or filter papers. Add a Campden tablet (crushed) to the gallon of cider and make sure the jar is filled to the top to exclude air."

From Making Cider by Jo Deal, Amateur Winemaker Publications, Ltd. 1985