Storage of partially used jar of mustard? Because I use mustard for my own consumption only occasionally, I am buying smallest available packages (350 g) but after opening I can see unused mustard slowly changing into "water" and solid component. I understand that every food deteriorates but I would like to understand something more about this.


Why it does not deteriorate this way in unopened package?
Is there some advice to slow down the process? Or even a way to simply renew its original consistence?
Mustard often separates, it doesn't mean it's going bad. It's likely perfectly fine, mustard stays good for years in the refrigerator. Just stir it up before using it to recombine. 

As for why it doesn't separate unopened, sometimes it does. Often you are buying a product that's been manufactured recently, so it hasn't had time to separate yet.