Does the acetic acid in vinegar oxidize significantly?

Question

_{I apologize in advance for this question. I did very well in O Chem, but that was 50 years ago and I had to study very hard. I don't remember much of it now except what's common in basic biological pathways.}

About 4 years ago, I bought a gallon of distilled white vinegar for household purposes. After using about a third of it, I put it in a cabinet and forgot about it. That particular cabinet door is opened very infrequently, so it's dark, and the temperature is neither hot nor cold.

From my years of soapmaking, I know that the carbon-carbon double bonds of fats oxidize, and the fat will go rancid. When stored in a plastic bottle, the bottle will eventually begin to 'collapse on itself', presumably as the O2 is used up. The higher the number of unsaturated bonds, the faster it goes bad.

I took out my gallon jug of vinegar today and saw that the bottle looked like air had been sucked out of it, i.e. it looked like liquid fat does when it oxidizes. It smelled fine. I did a bit of research and saw that in certain light and at (very) high temperatures, acetic acid does oxidize. Atmospheric acetic acid oxidizes due to OH radicals in air.

The concentration of acetic acid in vinegar is pretty low (4-18%). At the lower range, it doesn't seem like there's enough acetic acid to use up that much 'air'.

Did my bottle 'collapse' because of oxidation, and if so, how did oxidation occur under the circumstances?

I did read this q/a but the circumstances are different.

Answer 1

• It's unlikely bottle shrinkage was due to a chemical change. Plastics are surprisingly permeable to water vapor, far more than to air! In data at Versaperm,

  • Polyethylene (PE) and polypropylene (PP), often used for food storage, are ~50 times more permeable to $\ce{H2O}$ than to $\ce{N2/O2}$ mix (air).
  • Polyethylene terephthalate (PET or PETE), a very common material for food containers, is >6,000 times more permeable to $\ce{H2O}$ than to $\ce{N2/O2}$ mix (air)!!!

  As water evaporates, right through that semipermeable plastic membrane, air cannot enter rapidly, and the bottle collapses.

  You can confirm that analysis with a simple titration: measure the concentration of acetic acid in a fresh bottle of distilled vinegar, and one that's been stored for some years in plastic (i.e., your sample). Plastics are likely far less permeable to acetic acid than to water (evidence: to me, a closed bottle of vinegar has no odor), so as water leaves the bottle, the acetic acid concentration should slightly increase.

• Also, what was the temperature when the bottle was closed, and when you later checked? A few degrees cooler can collapse a bottle noticeably.

Footnotes

• Thanks to Versaperm for that data, which rather surprised me. P. E Keller at PNNL has a more detailed analysis.
• There are many animalcules that can consume "mother of vinegar," such as the "vinegar eel". Distilled vinegar should last indefinitely, but freshly-fermented vinegar does decompose. However, the pressure inside the bottle would increase from their respiration products.