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What is the film forming inside my bottles?

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I am making wine from my own grapes in Alpine, California and have noticed that occasionally a light film forms on the inside of some bottles. I am very meticulous about keeping my winemaking area clean and sanitary. The bottles I use, if new, get a light sulfite rinse prior to bottling. If they are used I scrub them with a bottle brush and one-step cleaner, then I rinse with water and give them a light sulfite rinse. Most of the wines do not have this problem, only a small percentage. Any idea what might be the culprit? David Didier Alpine, California You might be encouraged to know that finding a light film inside some bottles after wines have been aged for a few months is entirely normal, especially if your wine is un-fined or unfiltered. Minimally-handled wine (many homemade wines fall into this category) carries more tiny solids and precipitate precursors like tannins, phenolic compounds and tartrates, which can fall out of solution over time. They can collect in a light dust-like layer at the bottom of bottles or