How Long Can Mustard Sit Out Of The Refrigerator After Being Open?

I personally never refrigerate my mustard. It just sits out all the time.

I have looked in the container - sometimes it is in a glass jar - many times now it comes in a plastic container. I have removed the lid and looked inside of my current mustard, which happens to be a squeeze container, and it looks - smells - and tastes as fresh as when I bought it 6 weeks ago.

I usually purchase mustard once every 2 months. In the summer a bit more often - we use it summer cooking more than winter.

I do not believe you need refrigerate mustard at all. However as mentioned it would be prudent to follow instructions on the container and/or call the manufactor for further information.

By the way I eat plain yellow mustard - some of the gourmet ones may have different ingredients that require refrigeration.

It will certainly be fine after one night. You could see if there are any instructions on the jar - eg I've just looked at mine, which is Tesco's french mustard, and it says to consume within one month of opening. Yours may be somewhat different, but no mustard goes off quickly. You should be fine for a few weeks unless you got a dirty spoon in the jar.