There are always court cases about silly things; you don't need taxes for that. Five years ago, a Massachusetts judge ruled in a commercial leasing lawsuit that a burrito is not a sandwich: http://www.democraticunderground.com/discuss/duboard.php?az=...
No doubt it gets to be a little weird, but unprepared is a pretty well defined thing if you don't do what the federal food stamp program in the US does and type of food shouldn't really matter. If it requires cooking or is primarily an ingrediant then no tax.
US weird stuff: If you buy a frozen burrito it is ok. Unless the store has a microwave and you use it, then it is not ok. If you go home and microwave it then it is ok.
I don't have to define it, it is in the law books in several states - even the fed food stamp program has its own version which I mentioned is problematic.
milk and fruits are often considered ingredients in these laws - chocolate is too, chips are often taxed. stuff gets complicated depending on who wrote the law.
what's with the down votes - I didn't write the damn laws like this
And then there are court cases about whether this is a biscuit or a cake. http://en.wikipedia.org/wiki/Jaffa_Cakes#Cake_or_biscuit.3F