So you open the lid to your bucket or peer through the glass of your carboy and what do you find, some gnarly looking whitish film, crust, or even little island floating on the surface. What is it? It has a variety of names such as film yeast, yeast rafts, flor, and pellicle. Is it harmful? Does it mean your cider is spoiled? How did it happen?
First, there is a difference between yeast and mold. I covered this in details in my post on mold. Mold is generally colorful while yeast is white or tannish. Film yeast, yeast rafts, flor, and pellicle are formed by yeast and not mold. In fact you might have noted in a previous Mālus Trvium that there is a gene in yeast that can promote the creation of flor, or flower in French or Portuguese. Besides the impact from the yeast that is present, there are also environmental aspects that help form a flor. These include temperature, humidity, pH, alcohol, and sulfur dioxide (SO2). However, the one thing that always has to be present is oxygen. Without oxygen, film yeast, flor, pellicle, or whatever you want to call it won’t form.
Is it harmful? No. Does it mean your hard cider is ruined? No. Is it desirable? Maybe. This is actually your cider protecting itself. This film or flor creates a barrier that limits the additional exposure of your cider to oxygen. It’s also a method used in the production of certain wines. For example, sherry is made by aging wine in oak barrels that are partially filled and have a loose bung in the barrel. The goal is to create a flor and let that evolve the flavor profile of the wine. So does a film on your cider mean it’s spoiled? Definitely not. It does mean it will taste differently. If you are trying to make a sherry-styled cider, it may be perfect. Ultimately, if you don’t want film yeast as part of your hard cider process, avoid oxygen.
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PricklyCider.com 
Thank you so much for the explanation. I am so tickled that my 5 gallons of cider are ok. Many Blessings!
`Pearl
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I have the white film on my pear cider in a gallon jug. How do I get it out from under the yeast?
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Siphoning is the best method either with an auto siphon, which will allow you an easier way to start the process or just a tube where you need to suck the cider to get it started. I would plan to bottle it immediately with some sugar to avoid getting the film again.
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Thank you for your reply. How does sugar deter the growth? How much should I add per gallon? I ended up siphoning and running it through coffee filters. More for my peace of mind than anything else.
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Film yeast requires oxygen. That can be from dissolved oxygen or oxygen exposed through the headspace. Dissolved oxygen is added when you do things like siphoning, pouring, or doing things to your cider. They best way to remove this oxygen is to restart fermentation. Adding a little sugar (2.6 grams per liter) will create about 0.6 liters of CO2 per liter of cider. The yeast will process the available oxygen, release some CO2, which will fill your headspace. This means the film yeast can’t work. Bottling and carbonating with sugar does the same thing. Coffee filters would add a lot of dissolved oxygen and generally won’t filter out anything in the cider as they are too coarse.
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