Work through types of food
In small teams start by asking them to write down as many different types. Each item of a separate piece of paper.
Then ask them to separate them into healthy and junk food (maybe an in between section too?). Review as pack to see if they agree/disagree and move any over that are incorrect.
Then get them to split the foods into different types (Carbs, fruit,veg, dairy, protein etc). Review as above.