Silly question, I know.
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pierino is a trusted source on General Cooking and Tough Love.
It will not have anything like the flavor.
To get a flavor closer to buttermilk, you can squeeze a lemon into the plain milk. Not exact but closer!
May I respectfully suggest you bake a different pie this time? But do try a buttermilk pie when you can. They are delicious.
It worked ok! A little different flavor, but the texture was fine. I added cinnamon for a fall flair, and it was a hit!
Deborah is a long-time speaker for the vegetable side of the plate and author of The New Vegetarian Cooking for Everyone.
Buttermilk has a wonderful tang, which is what makes it so good in a pie. Milk that hasn't been cultured just won't have the flavor. It will be a custard pie, not a buttermilk pie! It will be good, just not the same.