Apple : apple is among the most difficult fruits to juice. There are two reasons why you can't juice apples; one is too thick, and the other problem is hardness. An apple's peel contains cellulose and pectin which makes it very hard to break them into small pieces. You should use a sharp knife, grater or blender instead of your fruit juicer or centrifuge juicer. You can also take advantage of natural pectin that helps make it soft in order for you to get through those hard apples. However if it still continues like this, then consider using your food mill as a way to get rid of the apple peels off from your juice extractor and help in making an easy process easier on your machine with less troubles caused by all types of peels inside your apple extracts.