Why dairy curdles
Milk's main protein, casein, floats as tiny clusters that repel each other. Acid or high heat strips away that protection, so the clusters clump. That's curdling. It's wanted when you make paneer and a problem when a cream sauce turns grainy.
Heat and acid together curdle dairy
A little acid or a little heat is usually fine. Both together, like wine or lemon juice in a boiling cream sauce, can make the proteins clump. Add acid last, off the heat or at a bare simmer, and don't boil the sauce afterward.
Temper dairy before adding it
Tempering means warming a cold ingredient slowly. Stir a spoonful of the hot liquid into the dairy, then another, then pour the warmed mixture into the pot. The proteins adjust to the heat gradually and stay smooth.
Test yourself
Questions from the interactive lesson, with the answer and the why.
You stir lemon juice into hot milk and white clumps form. What are the clumps?
Milk proteins that clumped together. Acid and heat make casein stick together into curds.
Fill the blank: Cream, with about 36% fat, curdles less easily in hot sauces than milk because the ___ gets between the proteins.
Cream, with about 36% fat, curdles less easily in hot sauces than milk because the fat gets between the proteins. Fat keeps the proteins apart, so they're harder to clump.
What went wrong? You pour cold yogurt straight into a simmering curry. It breaks into grainy bits.
The yogurt hit the heat all at once. Stirring a spoonful of the hot liquid into the yogurt first warms it gradually.
When should you add lemon juice to a cream sauce?
At the end, off the heat. Contact is brief and the heat is low, so the cream doesn't curdle.
True or false: Milk can only curdle when it spoils; acid and heat never curdle it.
False. Acid plus heat makes milk proteins clump, which is how paneer is made.
