Curdling — flocculation — happens when plant proteins clump out of suspension.
The cause
Plant proteins are stable in a narrow pH range. Coffee is acidic — pH 4.8 to 5.2 — and when the milk’s pH drops below its stability point, the proteins aggregate.
Heat accelerates it substantially.
What makes it worse
Light roasts and bright coffees, which are more acidic. This is the single biggest factor.
Very hot coffee.
Cold milk poured into hot coffee — the thermal shock destabilises it further.
Soy and almond, which are more susceptible than oat.
Old or near-date plant milk.
The fixes
Use a barista formulation. They contain buffers — commonly dipotassium phosphate — that hold pH stable. This solves it most of the time.
Warm the milk first. Pouring warm milk into coffee is far more stable than cold into hot.
Pour the coffee into the milk, not the other way round. The milk is then diluting gradually rather than being shocked.
Use a less acidic coffee — a medium or dark roast rather than a bright Kenyan.
Let the coffee cool a little before adding milk.
Is it safe
Yes. Curdled plant milk is unappetising, not spoiled — the proteins have clumped, nothing has gone off.
Switching to oat
If it happens constantly, barista oat is the reliable answer.
Last reviewed 8 August 2026