That half-used can in your fridge isn't dangerous and the reason why will change how you cook foreverYou scoop out a spoonful of tomato paste, shove the rest of the can in the fridge, and forget about it. The USDA confirms that leftover canned food can be refrigerated right in the can, there's no microbiological danger in doing so. Acidic and salty foods, tomato paste, chipotles in adobo, pickles, brined beans, coconut milk are hit hardest, since acid and salt dissolve metal into the food faster. Tomato paste, chipotles, coconut milk, and sauces freeze well in small portions, so you can thaw just a tablespoon at a time. Move leftovers to an airtight container or the freezer, the moment you open the can.