Why does an apple go brown? And if you squeeze lemon juice on it how does it keep from going brown?

Question

This is for a science assignment :S

Answer 1

becaz acid converts blu litmus into brown litmus& becaz of oxallic content apple colour is changed...........

Answer 2

Cutting the apple releases enzymes from the cells which react with the oxygen in the air, oxidising and in doing so turn the apple brown. Enzymes are proteins and tertiary molecular structure of  proteins becomes denatured in the presence of acid. The lemon juice thus destroys the enzymes and prevents the apple turning brown.