Hi Alex, most often when a starter develops a skin or crust on top, this is a sign that it needs to be covered more carefully. When we recommend covering a starter "loosely," this is not because the starter requires oxygen in order to ferment, but because fermentation gases can build up in a closed container and pop the lid off. It's fine to cover the container securely with plastic wrap (which will stretch rather than pop off) to protect your starter from drying out. A towel or cheesecloth allows too much air flow, and can contribute to the surface of the starter drying out. If you don't think dryness is the problem, I'm wondering if the juices you use may be contributing to the developing film? I've only ever used juice in a starter during the first few days of the creation phase, so I'm not sure how the added sugar that juice provides will affect the biochemistry of the starter over time, or if this could cause a film to form. If you'd like to attach a photo you can email us at customercare[at]kingarthurflour[dot]com. Barb

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