Why did my vinegar not create the traditional volcano? At the weekend, my son and I tried the volcano experiment. You know the one: vinegar introduced to sodium bicarb (and a little red dye) within a conical &quot;volcano&quot;.

Not actually ours; photo borrowed from here.
So, after assiduously funnelling some (slightly clumpy) bicarb into the volcano and meticulously counting drops of red food colouring... nothing happened? You can imagine our disappointment.
As luck would have it, we have two types of vinegar in the house. So I went about swapping the &quot;non-brewed condiment&quot; (more on this below) that we had used previously for some fancy balsamic vinegar. Lo and behold, this time we were treated to an eruption to rival Vesuvius!
I'm left to conclude that the first vinegar we tried must have gone bad somehow. It still looked, smelled and tasted like malt vinegar, but why else would it have failed us when we needed it most?
So, my question: Why would our vinegar have failed to react with sodium bicarb?

Note 1: In the UK, &quot;non-brewed condiment&quot; is the glamorous name for the cheap type of vinegar that is very common in fish &amp; chip takeaway shops. It's basically identical to malt vinegar. (In fact, I only realised mine wasn't malt vinegar when preparing this question!) It's a brown, semi-transparent liquid with a sharp taste and smell. The principle ingredients are water, acetic acid, and caramel for colouring. Since acetic acid is the same acid found in real (i.e. brewed) vinegar, surely it shouldn't matter whether it's real vinegar or not?
Note 2: For completeness, it's worth mentioning that traditional (in the UK at least) vinegar bottles aren't really very air-tight. It has occurred to me that the acetic acid has been subject to some sort of chemical/physical reaction with the air. (Perhaps oxidation? Or even just evaporation?)
Acetic acid can be decomposed by methane-producing bacteria. Note that the citation is for soil bacteria, but these methanogens can be found almost anywhere.  Indeed, there is an entire microbial community in vinegar, including nematodes (worms). The more &quot;natural&quot; the product, the more organic material to nourish the microbes and to speed decomposition.
That said, check the labels for acetic acid concentration when purchased. The nominal concentration for distilled vinegar is between 5% and 10%, but some types of vinegar may be only 4% acetic acid.