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During the winter, I often leave milk on my porch to make room in my refrigerator.

During a recent warm spell, I had an unopened gallon of pasteurized milk on the porch for two days around 50 degrees Fahrenheit. When I opened the bottle to see if it had survived, it had no spoiled milk sour smell, but the consistency was similar to thin yogurt. I drank a little, and it tasted like normal unspoiled milk aside from the texture.

I was to nervous to consume the whole gallon, so I threw it away, but I'm curious about what process might have been at work here. Could it have been a yogurt like bacteria despite the pasteurization?

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  • Was the milk in an unopened container?
    – Mr. Mascaro
    Mar 19, 2015 at 14:44
  • Yes, unopened. I brought it home from the grocery store and put it on the porch.
    – 2cents
    Mar 19, 2015 at 14:45
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    It could have been some type of fermentation, but since you didn't initiate it, throwing out the milk was the right thing to do. Pasteurization is not sterilization. Some bacteria may remain.
    – Mr. Mascaro
    Mar 19, 2015 at 14:46
  • My mom would use it in baking if it had gone slightly off but not chunky. (even when it smelled off). If I were still consuming dairy, I'd have used it for pancakes, myself. (but I likely havea gut that's more used to it, and a higher tolerance for risk)
    – Joe
    Mar 19, 2015 at 16:11

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What you got is more properly characterized as buttermilk (in the current usage of the term) than yogurt. Pasteurization does not sterilize milk, and it can have been any kind of bacteria which can survive in milk. The process was the same as in any other cultured dairy: bacteria started multiplying, producing lactic acid which curdled and soured the milk.

The harsher the pasteurization, the less chance that you happened to have an abundance of a benign culture. You could have gotten something dangerous, or harmless-but-gross. Traditional buttermilk making by leaving raw milk unseeded with any culture out and hoping that it harbors no baddies is also unsafe by today's standards. So, if you want to produce buttermilk or yogurt, you are much better off starting with a culture. A few spoonfulls of prepared yogurt or buttermilk should work (unless their culture was killed), but for best results, you should match culture and fermentation temperature.

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as far as I know, Lactic acid (byproduct of fermenting yeast or bacteria) this is a natural preservative that inhibits the growth of harmful bacteria. In my case I know a person that actually make cheese in the same way you accidentally did. So to my knowledge once the milk doesn't smell bad you are good to go.

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