Buttermilk is the liquid left over after producing butter from full-cream milk by the churning process. It has a slightly sour taste. It is quite popular as a refreshment in India and a variant called lassi is sold commercially. Many breads are made with buttermilk, and it is also used in creamy soups and sauces. For breads made solely with grains, buttermilk may contain amounts of calcium from its dairy products, such as nonfat dry-milk powder.
Most of the modern, commercially-available, "buttermilk" in supermarkets is not genuine buttermilk but rather cultured buttermilk, that is milk to which souring agents (Streptococci bacteria) have been added to simulate the original product. The sour taste, or tartness of "cultured buttermilk" is owing to the fermentation process, which, if making buttermilk, begins with a commercial product of buttermilk which has become chemically active, or rather fermented. In this reaction the starter bacteria, streptococcus bacteria, turns lactose into lactic acid. As the pH drops in this reaction the milk becomes tart. At this point, casein, a milk protein, precipitates as it is no longer soluble under acidic conditions, causing what is called clabbering. The acidity of buttermilk accounts for its long refrigeration life. This process can be repeated when making sour cream with slight alterations.
Note that it may be difficult to find buttermilk that is not low-fat in some areas, but it is possible to augment it with cream that has risen to the top of some varieties of store-bought whipping cream.
Buttermilk used in baking recipes can be replaced by adding 2-3 tsp. of lemon juice or vinegar for every one cup. This method can be used with soy milk or rice milk for vegan/vegetarian consumption.
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