Does freezing buttermilk kill the cultures?

Online Answer
Does Freezing Buttermilk Kill the Active Cultures? Nope, the cultures become inactive when they're frozen, but as soon as the buttermilk thaws they become active again. ... Just allow the buttermilk to thaw in the fridge (not the microwave), and you're good to go.
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Buttermilk is a good source of calcium and phosphorus, as well as vitamin D if it has been fortified. Full-fat varieties are also rich in vitamin K2 ( 5 , 6 ).
Pour buttermilk (1/4 cup for a quart jar or 1/2 cup for a half gallon jar) into your clean jar. Top off the jar with your plain milk. Tightly screw lid to the jar and shake vigorously for 1 minute. Place in a warm (but not hot) area out of direct sunlight.
So if you are baking cakes, cupcakes, biscuits, and such and looking for it to be even more similar to real buttermilk, use heavy cream or half heavy cream and half milk so it's just as thick as buttermilk, then add in the vinegar. ... All will work well in many recipes no matter what type of milk you choose to use.
Some common traits of bad buttermilk are a lumpy texture, a slight discoloration and a very distinctive unclean sour smell. If you notice any of these changes, your buttermilk has gone bad and should not be used.