Sometimes I bring yogurt to work as a snack. This is the common single-serving container, sealed at the factory with a sheet of foil on the top (the brand varies). Every now and then I skip eating it, and it sits on my desk for a day or two.
Or three.
OK, sometimes four, or even more.
And then I eat it. And it tastes perfectly fine, and I don’t get sick.
I can’t remember the longest time that I’ve left a cup of yogurt on my desk like this, but I’m pretty sure it’s more than a week. And I’ve never seen the lid balloon up, which is the sign of trouble I would expect if there was dangerous bacterial activity taking place within.
I’m a bit puzzled by this, since yogurt is supposed to contain bacteria that are beneficial to human health. So why does yogurt have such an amazing shelf life, even when kept at room temperature? Shouldn’t those little bastards be having a field day in there, fermenting to their everlasting delight, and creating a super-sour witch’s brew for me?
Balllooning would take place if there was something releasing gases, but one, that wouldn’t necessarily equal “dangerous”, two, “dangerous” stuff doesn’t necessarily release gases, three, isn’t it a bit incoherent to expect beneficial bacteria to be doing dangerous stuff?, and four, whether there is or isn’t live bacteria depends on stuff ranging from how long that yoghurt has been around to your local regulations (some governments allow pasteurized products to be sold as yoghurt, some do not).
Frankly I’m skeptical about any yogurt from the dairy aisle even if it says Contains Live Cultures on it. One of the things I read over and over on yogurt making websites is that if you use commercial yogurt as a starter for your homemade yogurt, you’ll only get one or two good batches and it will pretty quickly stop working. This is because the commercial cultures degrade and only last for a few iterations. I wonder if your issue is degradation of the culture along with a lack of material for the bacteria to yogurtize.
AFAIK, the bacteria that turn milk into yogurt generally don’t produce a lot of gas, but they do produce lactic acid. So it’s entirely possible that your yogurt has become slightly more sour than it already was.
This is assuming that the bacteria has sugars to eat, and isn’t being inhibited by the concentration of lactic acid. It’s probably a lot like alcoholic fermentation- each yeast has a specific tolerance for alcohol, and will only ferment to that level, regardless of how much sugar there may be.
I’ve found yoghurt stays tasty for at least a week at cool office temperatures. I would typically bring in five on Monday, keep in my desk drawer, and eat one per day. Never had a problem. I wonder if the micro-organisms run out of food. They may not be able to digest what’s left.
I’ve since switched to kefir, and it is a lot more active. The bottles will bulge and if not carefully opened, contents will spray out. The kefir will get a lot more sour, too.
Bacteria can live in a dormant state, and start reproducing again when food is vailable and poisons are gone. The yogurt in my fridge would probably go off it it wasn’t pasturatised: it’s nothing like the yogurt we made ourselves in the 70’s.
Sauerkraut, which is colonized by some of the same bacteria responsible for fermenting yogurt, can sit out at room temperature for a year or more, according to some people. I let mine sit a minimum of three weeks before transferring it to a refrigerator. Longer than three weeks tends to affect the texture, and I like it a little crisp. It becomes sauerkraut in a week or so, depending on temperature, but it develops more flavor if allowed to ferment longer.
The beneficial bacteria, namely Leuconostoc mesenteroides, Lactobacillus brevis, Pediococcus pentosaceus and Lactobacillus plantaru develop an environment inhospitable to dangerous bacteria as long as you start off with sufficient concentration of sodium chloride to give the beneficial ones a head start and keep air away from the cabbage.
Yogurt doesn’t go bad because it’s already “gone bad”. If you let those bacteria work, yogurt is what you get.
Note, by the way, that even uncontrolled spoiling of milk won’t make it dangerous. It’ll just (probably) taste bad. Or maybe not even that, if it spoils in just the right way, in which case you end up with yogurt, or cheese, or any of a wide variety of other dairy products.