Before singing you should avoid eating for at least 2 hours if possible. In particular, there are certain kinds of foods that can irritate even the toughest songbird. Citrus, dairy, chocolate, fatty foods and anything that is difficult to digest should be at the top of your “DO NOT EAT BEFORE SINGING” list. If you (like the majority of everyone else in the world) happen to be even slightly sensitive to one or more of these types of food, you will have an raging river of thick mucous flowing that no amount of throat clearing will cure.
Rather, opt for a light snack and drink plenty of water until after your performance. Your loyal vocal cords will thank you.