There is a difference between soured and spoiled. Adding vinegar to milk is a reliable method of souring milk that many of have used at that "oh shit" moment when we realize we don't have a drop of buttermilk for a recipe which requires it. Letting it sit at room temp overnight will spoil it. You may need to evacuate the house and open all your windows and doors to clear the smell. Okay, that's the slightest of exaggerations. Still, I suspect you get the point: refrigerate it overnight. Microwave it the next morning for 30 seconds or so to chase the chill.
I wouldn't leave it out for more than a couple hours. If you are able to test acidity on food, anything below a 4.5, throughout, is generally safe enough to leave at room temperature for a limited period of time. That's a good rule of thumb for me. Buttermilk tends to hover around that level, so I would be careful.
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