What happens if you use milk instead of buttermilk?

Online Answer
In recipes that call for buttermilk, it is not recommended to replace buttermilk with plain milk, because the absence of acid will not produce the same end result. But using an acidic ingredient combined with plain milk will create a substitute with properties closer to that of buttermilk.
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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.