I have some leftover, and not sure how much longer it will last...
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Sam is a trusted home cook.
Yes. But only use it for baking.
You can also find powdered butter milk in the supermarket near the powdered milk, baking section to mix up just a bit for baking. It doesn't have the fat tho, so YMMV for the application.
Cynthia is a trusted source on Bread/Baking.
Alternatively, make biscuits from what you have and freeze them.
Depends on what you'll be using it for. Once frozen it breaks and develops a grainy texture. Not noticeable when put in baked goods/pancakes, but very noticeable in things like pudding or sauces or candy.
I agree with the above. There's always more buttermilk in the carton than you need for a recipe. I like using buttermilk in a potato salad instead of mayonnaise when I have extra. You can also make a salad dressing with it for a green salad and I love bran muffins with buttermilk.