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I have found online that unopened and unrefrigerated yogurt will last for a few days without refrigeration. However, is this also true for opened and unrefrigerated yogurt? By "opened" I mean that I have already removed the seal and taken a few spoonfuls out of the tub before leaving it unrefrigerated.

EDIT: I see this question has been marked as duplicate. I think that the link provided does not answer my question. The case for yogurt is a very special one since it is a fermented food and cannot just be casually thrown into the general category of "dairy product" (as does the top answer in linked question). Moreover, my rough understanding is that yogurt has a low pH (from the high amounts of lactic acid) which prevents the growth of external pathogens. Hence, unopened yogurt will be safe at room temperature for at least a couple of days. However, if you wish me to be pedantic, my question would be: is it likely that by opening the yogurt tub I introduced pathogens that can replicate rapidly at the low pH of yogurt given that it was left unrefrigerated?

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    I think this question is worthy of it's own answer, so I am going to answer it below taking relevant tidbits of the discussion @Cindy shared.
    – Caleb
    Dec 20, 2016 at 0:26
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    As answers are currently not possible: I think yoghurt fits into the dairy category for spoiling. It is sour, but not exceptionally so (pH around 4 - 5, like tomatoes and bananas. Apple juice is more sour), and dairy-typical protein content makes it more prone to spoil fast. Open as in truly open vs. taken something out with a fresh spoon and then put a tightly closing lid does make a whole lot of difference in my experience (also inside the fridge). Outside the fridge are not very well specified conditions in terms of what to expect. Right now, my larder (?) has 11°C, the kitchen ca. 16°C...
    – cbeleites unhappy with SX
    Dec 20, 2016 at 17:07
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    ...(didn't cook today). I wouldn't expect trouble leaving an open yoghurt with closed lid over night at these conditions. But: it's winter over here now and there are no flies. Flies are attracted by yoghurt (and other protein-rich stuff) and leave their microbial footprint (if not their eggs) on the yoghurt can just where the lid sits. (Experience from Italy - with room temperature ≈ outside temp around 15 °C). Otherwise spoiled yoghurts that I've seen so far lead me to guess that usually a single something falling in is the cause (usually single little island of mold, not multiple spots).
    – cbeleites unhappy with SX
    Dec 20, 2016 at 17:20

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When you open a food product, you are exposing it to microbes in the air. The food is sealed in a sanitary environment and bacteria is kept out. Once you open the container of yogurt, those microbes begin working on the yogurt. Further, when you leave the yogurt at room temperature the bacteria can rapidly multiply as the yogurt is in the danger zone between 40 F and 140 F. At temperatures in this range, any yogurt exposed to bacteria in the air will become rancid rather quickly.

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  • Your danger zone IMHO starts very low: over here (Germany) the recommended temperature in the middle of the fridge (which is also the recommended place for dairy products) is 7 °C (45 °F).
    – cbeleites unhappy with SX
    Dec 20, 2016 at 16:14
  • Yogurt actually spends quite a long time right around 100F making itself hospitable to yogurt cultures and much less hospitable to other microbes. It's not a chunk of raw chicken; it's a fully populated bacterial colony that's not nearly as hospitable to random growths.
    – Ecnerwal
    Dec 21, 2016 at 2:11
  • @Ecnerwal yes the yogurt is full of cultures already, but I don't think that means other microbes can't grow and spoil the food.
    – Caleb
    Dec 21, 2016 at 3:12

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