Does Buttermilk, lowfat density change after the container has been opened for several days?

Marginally. Butterfat can separate or bloom, but measured weight per cup/mL stays within 1 % as long as the product is within its use-by date.

Can I swap whole Buttermilk, lowfat for skim at the same gram weight?

For mass yes, but fat content changes flavor, browning and texture. In baking, match the variant the recipe expects rather than the raw gram count.

By Fidamen-Chefsolver Team | Reviewed by Chefsolver Staff

Lowfat buttermilk is tangy cultured milk used for tender baking and dressings.

Ingredient

Use in pancakes, biscuits, fried chicken brines, ranch dressing, and marinades.

Does Buttermilk, lowfat density change after the container has been opened for several days?: Marginally. Butterfat can separate or bloom, but measured weight per cup/mL stays within 1 % as long as the product is within its use-by date.
Can I swap whole Buttermilk, lowfat for skim at the same gram weight?: For mass yes, but fat content changes flavor, browning and texture. In baking, match the variant the recipe expects rather than the raw gram count.