Kosher for people who are Modal Logic Geeks

(Expressed using standard Kripke-semantics.)

Some foods are Kosher in all possible worlds. Foods like fruits and vegetables (which rigidly designate), are always kosher.

Some foods are Kosher is some possible worlds. Beef, for example is Kosher if it inhabits a possible world in which it was killed according to some particular kosher guidelines. Worlds in which the beef was not prepared this way are not kosher. So a particular piece of beef is kosher in some worlds and not others.

Some foods are Kosher in no possible worlds. There are no possible worlds in which shrimp and pork are kosher. It would have to be a contradictory world where pork was not pork, and thus that world could not exist.