How would you heat food in an airport? Most restrooms don't have really hot water, although I guess lounges have water for tea but would certainly frown on someone cooking outside food there. I can't think of any airports where I've seen microwaves (or ovens, stovetops, toasters, grills, etc.) for public use. Maybe the old college method of using aluminum foil and an iron, so that you just need an outlet? Most other cooking equipment would be too large to bring through security.