@Zedrinbot probably 1-5 and 15-20 respectively. This would allow for food that not only has positive effects but negative effects as well, easily allowing for someone to play a kind of Alchemist for example.
@Zedrinbot 'cooking' stat, which in this situation might do more than one thing I dunno. Probably 'Culinary' or something. If time is of the essence, though, then players would roll for their Culinary stat. The effects would be lessened on a bad roll and increased on a good roll,