What sort of spoilage was this?

Tonight I was making hot dogs for supper, and realized that I had used the last of the bottle of mustard I had in the fridge the previous weekend. So I went to get another bottle out of the cupboard. The first thing I noticed about it was that the bottle was bulging inward, like something had decreased the pressure inside. That’s peculiar… Around here, you’ll often see things bulging outward, if they were packaged at sea level, but not much is packaged at significantly higher altitudes than us.

The second thing I noticed was that the color seemed off. Honey mustard is usually a fairly light color, but this was quite dark brown. That’s also peculiar… I don’t think it was that color when I bought it.

Alerted by these clues, of course, I didn’t just slather some on my hot dogs. I opened it up (the seal seemed to be intact, incidentally), and tasted a very tiny amount. Well, I’m not sure just how to describe the flavor, but I can certainly say that it wasn’t good at all, and with a very persistent aftertaste.

The bottle is closed back up and sitting in my trash, but I’m curious. What sort of spoilage would decrease the pressure in a bottle of mustard, and turn it dark?

It had to have been air exposure of some kind. Mustard isn’t all that perishable for a lack of organic proteins. That is weird though, that the container would be imploding like that.

Someone smarter will invariably come along.

Well, it wasn’t just mustard, but honey mustard, right? Honey mustard may have honey or, depending on the brand, may have corn syrup or sugar. Mustard being mustard, there’s also some water in there. Even if it’s actual honey, it’s too dilute to be antibacterial at that point. Water + sugar = nice bacterial breeding ground.

Which bacteria? I dunno. Probably something aerobic, though, to use up the air inside the bottle. There must have been contamination at the factory or a break in the seal during transport or storage, and that let bacteria in to colonize.

Regular mustard is pretty darn shelf stable, due to its very low sugar content and high vinegar content. Honey mustard is pretty perishable. Not as perishable as milk or leftovers, but it should be refrigerated and used within about 6 months of opening.

Yes, corn syrup is the second ingredient (honey and other sugars show up further down the line). I suppose it’s reasonable that honey mustard might spoil more easily than just vinegar-and-mustard-seed.