I have learned in my research that oxygen reacts with the enzymes in an apple to turn it brown. If I can coat my apple slices with a commonly found food substance they will not turn brown."
This is even better! However, I still think that you can refine this a bit. You already know that lemon juice works. Lemon juice is acidic and has antioxidant properties. I can name several other food items that fit one or both of those descriptions: tea (acidic and antioxidant), tomato juice (acidic and antioxidant), olive oil (antioxidant), coffee (acidic and antioxidant), Orange juice (acidic and antioxidant), vinegar (primarily acidic). On the other hand, things like milk, water, soda water, are not. You could hypothesize that one or more of these substances will also prevent browning based on their known properties while other will not. You could also switch it up a bit and hypothesize that lemon juice will also prevent browning of other food items, like potatoes.
Here are some other resources that may help: http://www.education.com/science-fair/a ... ing-brown/http://www.scientificamerican.com/artic ... urn-brown/