What happens if you use buttermilk instead of milk?

Online Answer
Buttermilk has more acid than regular milk, which will reduce the carbon dioxide released and thwart the leavening process important to these recipes. To achieve the desired result when using buttermilk instead of milk, be sure to substitute baking soda for some or all for of the baking powder.
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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.