You pour oat, soy, or almond milk into your coffee, and after a few seconds you see little flakes. This is called curdling. It isn’t dangerous, but it doesn’t look appetizing and the coffee tastes less smooth. Fortunately, you can often prevent it.

Why does plant-based milk curdle?

Coffee is acidic and hot. Plant-based milk contains proteins that are sensitive to this. Espresso is much more acidic than soy milk (about pH 4.5 compared with pH 7 to 8). When acidic, hot coffee and milk come together, the proteins clump. You see this as flakes or grains in your cup.

Four tips to prevent curdling

1. Don’t heat it too much, and pour right away

Don’t heat plant-based milk too much when frothing it. Around 55 to 60 °C is often enough. Pour the milk right after frothing, because plant-based milk separates quickly if left to sit.

2. Add a splash first, then the rest

Let the hot coffee cool slightly. First, pour in a splash of milk and stir. This helps the milk adjust to the coffee. Then add the rest.

3. Choose a barista version

Barista versions are made for coffee and often stay more stable. You’ll usually see the word “barista” on the carton.

4. Choose a milder coffee

Acidic coffee makes milk curdle more quickly. Many people find a medium or dark roast less acidic. This is a guideline, not a guarantee: flavor and acidity vary from bean to bean.

Here’s what to do

Tip	What to do
Don’t heat it too much	Heat milk to ± 55 to 60 °C, then pour right away
Add a splash first	Let the coffee cool, add a splash of milk, stir, then add the rest
Barista version	Choose a carton labeled “barista”
Milder coffee	Try a medium or dark roast