I simmered up about six quarts of chicken stock on Saturday, and poured it into three 2-quart containers, all pretty evenly filled. On Sunday, I noticed that one of them has bent inward, like the air had been sucked out of it. I opened it to have a look and smell, and nothing seemed out of order. However, I couldn't get the plastic to bounce back to shape, either. The other two containers are still perfectly round as usual.
Is this just some quirk of heating and cooling liquid and gas that I shouldn't fret about? Or could something have contaminated one of my containers? Could all the stock somehow be tainted, and only one container visibly indicates it?