I probably shouldn't really answer as a) I've never made compost with apples, and b) I'm a beginner myself, but - won't they reduce to a mouldy slime? Would they make a better compost with some other 'brown' stuff added, e.g. shredded paper, dry leaves/grass, etc? Also, they're quite acidic, but I don't know what pH they come out as once they've decomposed, perhaps the acid gets converted into something else.