TRIVIAL PURSUITS: Why is buttermilk so named?

So why is it called buttermilk?

Because originally, buttermilk referred to the liquid left over from churning butter from cultured or fermented cream. Today, this is called traditional buttermilk. Traditional buttermilk is still common in many Indian, Nepalese, and Pakistani households, but rarely found in Western countries.

Commercially available cultured buttermilk is milk that has been pasteurized and homogenized (with 1% or 2% fat), and then inoculated with a culture of Lactococcus lactis plus Leuconostoc citrovorum to simulate the naturally occurring bacteria in the old-fashioned product.