This question usually pops up in my head every time I step into Home Depot and see all the orange buckets around.
What makes a bucket food grade?
It's the type of plastic, and how it's manufactured (it has to do with the chemical properties of the plastic). A food grade container will not allow most foods to leach toxic chemicals from it under normal operating conditions.
Look for the recyclable marks on the buckets, usually on the bottom. There should be three arrows in the shape of a triangle with a number in the center. If the number is #1 or #2 then it is food grade. Usually #2 which is HDPE or high density polyethylene.