Since buttermilk always smells sour, how does one know when it has gone bad?
When we asked this question of the folks at the dairy farm that produces the buttermilk we use in the test kitchen, they told us to consume their product within five to seven days after opening. However, guidelines from agricultural programs at various universities extend that period to two weeks. Then there’s our experience, which has shown that refrigerated buttermilk won’t turn truly bad (signified by the growth of blue-green mold) until at least three weeks after opening. That it can last this long is not surprising, since buttermilk is high in lactic acid, which is hostile to the growth of harmful bacteria. That said, we wondered if the flavor of buttermilk changes the longer it’s stored. To find out, we held a series of tastings, comparing pancakes made with freshly opened buttermilk with those made with buttermilk that had been refrigerated for one week, two weeks, and three weeks. We found that as time went on, the pancakes tasted increasingly bland.
Here’s why: The bacteria in buttermilk produce lactic acid and diacetyl, a flavor compound that gives buttermilk its characteristic buttery aroma and taste (diacetyl is also the dominant flavor compound in butter). As time passes, the buttermilk continues to ferment and becomes more acidic. The abundance of acid kills off virtually all of the bacteria that produce the buttery-tasting diacetyl. So three-week-old buttermilk will retain its tartness (from lactic acid) but lose much of its signature buttery taste, giving it less dimension. The good news is that there is a way to prolong the shelf life and preserve the flavor of buttermilk: Freeze it.