Gram stain of buttermilk, showing Streptococcus lactis (purple) with a pink background of milk protein (casein)

This is a recipe I have been using for years to make buttermilk in large quantities. I like to use it for baking as well as drinking. It makes pancakes, waffles, cakes, and the like rise very well.  It works because Streptococcus lactis (purple round spots in picture above) ferments the lactose in milk to lactic acid.*

The acidic pH causes the protein in milk (most prominently casein, pink in the picture above) to precipitate, thickening the liquid.  Because the lactose has been broken down to lactic acid, it would cause no problem for those who are lactose intolerant.

INGREDIENTS AND EQUIPMENT:

1 cup active cultured buttermilk
1 quart whole milk
very clean 2 quart container
whisk

BUTTERMILK RECIPE:

1. Pour the milk into the 2 quart container.
2. Add the buttermilk.
3. Whisk until thoroughly mixed.
4. Pour inoculated milk back into the original containers.
5. Let sit at room temperature over night.
6. Refrigerate until needed.

Because buttermilk is acid, it will keep well in the refrigerator for weeks (or longer if you are using it for baking).  However, if it gets too old, the bacteria will die, and it cannot be used for inoculating fresh milk.

You can make any volume of buttermilk you like, so long as you hold to the proportions of 1 part buttermilk to 4 parts whole milk. (I have used this for years to prepare buttermilk (an ingredient in my cornmeal waffles) for an annual waffle breakfast I serve at Clermont College, serving about 120 people, with 1 quart buttermilk plus 1 gallon of fresh milk, mix, and pour back into the original containers.

It may be that less starter is needed, but it has worked so well that I have not wanted to mess with the proportions.