Buttermilk is soured, fermented cow's milk. Traditionally, buttermilk was the low fat, sour liquid residue from the cream which has been used for butter-making after churning, which is why some people think of it as ‘whey’. Its sourness is due to lactic acid. Nowadays this is known as ‘traditional buttermilk’ as modern methods create ‘cultured buttermilk’ by adding Streptococcus lactis to skimmed milk. ‘Bulgarian buttermilk’ is made with Lactobacillus bulgaricus and is more sour. Buttermilk does differ from place to place, so it is worth trying to find out how it is produced locally if you have an interest in these things. If you were hoping to use buttermilk in a recipe for bread but there is none in the fridge, you can add a teaspoon of lemon juice or white vinegar to each 250 ml of fresh milk as a substitute.