So I buy a tin can of Furmanos pizza sauce (it’s the cheapest), open it, use a little to make mini pizzas, and stick the rest in the fridge. When I was a kid my mom would never let me keep tin cans in the fridge, she said it was unhealthy, though I can’t for the life of me remember why that was supposed to be.
So: Was my mom right? Is there any problem with keeping a half empty tin can of pizza sauce in the fridge?
If covered properly, you shouldn’t have any trouble. However, it’s difficult to put a good seal on an open tin can, which can lead to rapid spoiling. You can buy plastic, tupperware-like lids for open cans at your local kitchen supply shop. (Plastic wrap won’t maintain a good enough seal.)
A good way to deal with leftover sauce is to pour it into a freezer bag, seal it, flatten it out and then pop it in the freezer.
I forget the exact reason, but it is something to do with acidity in food and exposure to oxygen and tin leading to food spoilage (not food poisoning). Too long since I did my food hygiene course and too lazy to look up notes.
My mom said it would eventually cause the food to take on a metallic taste. But if you’re going to use it in a couple of days, I don’t think it’ll matter.
Once open, a can with acidic foods like tomato sauce or citrus fruit can develop a metallic taste in reaction to the air and metal, or at worst cause some of it to leach into the food resulting in an upset stomach. (This usually doesn’t happen with tin, though, more likely with zinc or copper) Most canned food doesn’t need preservatives, so you should treat it as fresh and use within a day or two.
I stick by my original post. As long as you seal the can well, it should last as long as any other fresh product.
Also, most “tin” cans are actually steel with a very fine layer of tin. Here’s an excellent site with everything you ever wanted to know about the canned food industry: http://www.cannedfood.org/
Your mum must have gone to the same school as mine. Growing up in the fifties and sixties, in Australia, my Mum told me the same thing. But only for certain items- for instance salmon. It was quite normal to keep cans of jam there for months.
If you’ve ever tasted apricot nectar that was left in it’s can after opening for a day or two, you’d know what your mother was talking about. It just tastes so… Bleh.
If you open the can and pour it in a pitcher instead of leaving it in the can, it still tastes great even after several days.
there’s definitely some sort of reaction happening with the metal and whatnot, but what it is sure beats the hell out of me. 
There are some things that don’t have any noticable difference after leaving them open in the can. Soups, for example. So, it probably has to do with the pH, or something.
<mumbles on for several more minutes>