What gives buttermilk its unique flavor is it’s butterfat content and soured taste. Liquid buttermilk may give you some problems with sporadic softening of the dough if you don’t scald it before using it, or I’d recommend just getting some bakery grade dried buttermilk solids. This will keep a lot better, and doesn’t require refrigeration. The amount to use will depend upon the flavor you’re looking for, but I’d probably want to start at about 5% of the flour weight and bench mark from there.
You can get dried buttermilk solids from any Bakery Ingredients supplier, or, check with a local retail bakery to see if they have some that you might be able to buy or barter. Keep in mind that you wil need to increase the absorption of your dough by the same weight as the buttermilk solids, ie: for every pound of buttermilk solids that you add, you will need to add one additional pound of water to your dough. Also, remember that it is a browning agent, like sugar, so you might want to delete any other sugar from your dough formula when using buttermilk in any form.
Tom Lehmann/The Dough Doctor
