Apples and singing

At rehearsal the other night for a Magic Flute next week, offstage discussion turned towards apples for dry mouth. Certainly anecdotally there is plenty of support for munching a small piece of apple right before you sing with the theory being that the apple is similar enough to your own mucus that it'll get things going. The thing is, I can't find any research to support apples over, say, chocolate.

Some swear by lemons, and others, like Lotte Lehmann had candy sewn into her costume. Anybody out there know of any science behind the apples thing? What do you use, if anything, to get saliva going when you need it, or to head the problem off at the pass?