Pour 1 cup (240 ml) fresh milk into a glass measuring cup or small mixing bowl. Make sure the milk looks and smells fresh; do not use already spoiled milk. Room-temperature milk sours slightly faster, but cold milk also works.

Stir in 1 tablespoon (15 ml) distilled white vinegar or freshly squeezed lemon juice. Use white vinegar for a neutral flavor or lemon juice for a subtle citrus note. Add a small pinch of salt if desired and stir gently to combine.

Leave the mixture at room temperature for 5–10 minutes. After 3–4 minutes, give it a quick stir and check the texture. The milk should thicken slightly and may look lightly curdled around the edges—that’s expected.

If after about 10 minutes the milk has not thickened or changed much, stir in an additional 1/2 teaspoon of acid and let it rest a few more minutes. For a milder flavor, you can use only 2 to 2 1/2 teaspoons of acid per cup of milk; for recipes with lots of baking soda, you can increase up to 1 1/2 tablespoons.

The sour milk is ready when it has a mild tangy aroma, coats the back of a spoon slightly more than plain milk, and shows small flakes or gentle curdling. It may look a bit separated but will blend smoothly into batters and doughs.

Stir the sour milk gently, then measure and use it immediately in place of buttermilk in your recipe. If making ahead, transfer to a covered container and refrigerate for up to 24 hours. Stir before using and discard if it develops an off or rancid smell.