This foam is called an "acetic uterus". Remove it is not necessary, on the contrary it is necessary to mix foam with juice. And in order not to damage this very "vinegar uterus", you do not have to rearrange the dishes with the prepared vinegar from one place to another. But for you the recipe for cooking apple cider vinegar: We collect mature and even overripe apples. Ideally, it should be apples, fallen ...