So, what happens when you open a jar of Prego and let it sit for a bit? Well, it's not like the sauce is going to start growing a furry green mold overnight (although, that would be pretty funny). But seriously, the acidity in the tomatoes and the lack of oxygen in the jar do help preserve the sauce. However, once you open it, the sauce is exposed to air, moisture, and all sorts of bacteria just waiting to ruin the party.
The good news is that Prego sauce contains a lot of acid (think: tomatoes, vinegar), which helps to prevent the growth of those nasty bacteria. But, if you don't store it properly (i.e., in the fridge, with a tight-fitting lid), the sauce can still go bad. And trust me, you don't want to be responsible for serving up a science experiment to your dinner guests.
