In the majority of instances when looking at expenses for food/meals we are talking travel and you can get your best guidance on travel policies and procedures here.
For those instances not involving travel, the simplest way to determine what is allowed when it comes to this type of expense is to refer to the Disbursement Grid. Whether a purchase is allowable or not depends on the specific type of expense and the type of funds to be used in paying for it. The Disbursement Grid summarizes most any type of situation.
If an expense is allowable based on the Disbursement Grid, you then have to determine HOW to pay for it. Some basic guidance: