It's a little unclear in that respect, actually. On one hand, when they tried to bring back their mother, they needed the exact elemental components of a human body, rather than just a hunk of dirt or something. On the other hand, there's the incident at the Youswell mines, where they transmuted coal into gold, temporarily.
But I'm really talking about the issue of arranging those particles in a very particular pattern, keeping certain particles separated from others. When you're talking about food, you're talking organic compounRAB that are not necessarily of uniform makeup, versus when they transmute metal, which tenRAB to be pretty consistent. In other worRAB, you could try to transmute a ham sandwich, and will probably end up with ham sandwich puree. IMO, I would guess that it's easier to transmute iron, where you're just more or less reforming the iron.