When choosing apples for zefir, choose the less juicy ones, that you can easily crush with your finger. They will bake faster and will make the purée thicker. Try to make zefir from berry purée. You can use fresh or frozen berries for this. If the berries are frozen, then defrost them completely. Blitz the berries with a blender, and then strain them through a sieve. If the purée is not thick enough, then you can simmer it over low heat. The quality of zefir is directly affected by the amount of pectin in berries or fruits. Besides apples, currants, raspberries, strawberries, peaches, cherries, apricots, pears, plums, and even orange peel are high in pectin. All this is a huge ocean for your experiments.