Why are nouns like meat, salt, or smoke not proper nouns? They name a specific food, don't they?
A proper noun refers to a single specific thing. The Eiffel Tower is a specific building in Paris. "Meat" doesn't refer to a single specific thing, nor do "salt" or "smoke".
A proper noun names a single specific thing. For example, the Eiffel Tower names a specific building in Paris.
There are nouns that name specific things that in one sense can be a proper noun and in another sense can be a common noun. For instance, if I use the word "colosseum", I can use it as a common noun to talk about the local concert hall, or I can use it as a proper noun "Colosseum" and refer to a specific building in Rome (vs. LA Coliseum, which refers to a specific building in Los Angeles).
So let's go to the words in question: meat, salt, smoke. These words refer to things (for instance, "meat" refers to the flesh of animals) but they don't refer to a single specific thing. For instance, when I say "meat", I could be referring to chicken meat, beef meat, pork meat. If it's beef, is it steak? hamburger? ribs? And even if we have narrowed the choice to, say, hamburger, when we go to the store, can we point to a specific package of meat and say that that one and only that one in the entire world is Meat and everything else is something else? No.