Milk + Lemon Juice or Vinegar (Best Option)
1 cup milk + 1 tablespoon lemon juice or white vinegar (let sit 5 minutes)This substitute works by acidifying the milk, mimicking the lactic acid in buttermilk that reacts with baking soda to produce carbon dioxide, which leavens the doughnuts. The acidity also tenderizes gluten, resulting in a softer crumb.
For best results, use whole milk for moisture and richness, and allow the acid to curdle the milk slightly before mixing. Avoid using skim milk as it reduces fat content, affecting texture.
Compared to buttermilk, this substitute yields very similar results in rise and tenderness, with minimal impact on flavor, making it the most reliable replacement in doughnut recipes.