Curdling occurs when milk proteins break down and clump together, turning the liquid into a semi-solid or solid mass.

Curdling happens when milk proteins break down and clump together, transforming the liquid into a semi-solid or solid mass. While curdling is necessary for making delicious things like buttermilk, cheese, and yoghurt, most of us don’t want lumpy milk in our coffee! 

Milk contains proteins that are sensitive to temperature and pH changes. When milk gets too hot or an acidic substance like lemon juice is added, the proteins denature (or ‘unfold’) and start to lump together, causing curdling. 

If you notice your milk curdling when heated, here’s what to do: try warming it gently at a slow, steady temperature. Stirring with a whisk can also help keep it smooth and prevent clumps from forming. Bottoms up!