Leaving buttermilk out overnight, recipe for food poisoning? In the recipe for corn bread in The Bread Baker's Apprentice, the first instruction reads:


  "The night before baking the corn bread, soak the cornmeal (1 cup) in the buttermilk (2 cups). Cover and leave at room temperature overnight."


Similarly (just found this randomly), a recipe I found online for "Overnight waffles" instructs: 


  "Before going to bed, combined the dry ingredients, then stir in the
  milk, butter and vanilla. Cover with plastic wrap or a tight-fitting
  lid and let stand overnight at room temperature."


Wouldn't the buttermilk spoil overnight and give you food poisoning, even with baking the bread in the oven at 350F?
Let's assume that we're talking about cultured buttermilk (the modern, sour kind that smells like yogurt), rather than the old-fashioned by-product of making butter, sweet cream buttermilk.

Cultured buttermilk is pretty acidic, with a pH of about 4.5. Acidity is one of the conditions that control the growth of foodborne pathogens and 4.5 is outside of the optimal growing range for most of the bad-guys. 

If the buttermilk is fresh, it should have a living colony of good-guy, lactic acid bacteria. This should mean that as it warms up, your existing good-guy bacteria will multiply and lower the pH a bit more. Lactic acid bacteria (the guys that make cheese, yogurt, buttermilk, and sour kraut) help preserve things by making the food a hospitable environment for themselves and an inhospitable environment for bad-guy, pathogenic bacteria.

Since there are existing good bacteria, the pH is low, and neither the dry corn nor buttermilk is likely to bring in contamination from bad-guy bacteria, this instruction seems reasonable. 

In fact, maybe the author wants the buttermilk to get a little more acidic (sour) overnight by allowing the lactic acid bacteria to develop a bit more. 

That said, the buttermilk package says Keep Refrigerated, and it would be possible for contamination with a bad bacteria to happen and for pathogens to multiply overnight. So there is some risk.

You could soak it in the refrigerator if you wanted to play it safe. 



Sources: reading about making cheese, making cheese, watching buttermilk get thicker and more sour in the fridge over time.