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 trying to protect 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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