curdling

Curdling is the process in which milk proteins, especially casein, coagulate and separate from the surrounding liquid to form solid curds and liquid whey. It can occur in dairy products such as milk, cream, and sauces, as well as in non-dairy milks when proteins become destabilized. In cooking, curdling is often an unwanted change, but it is also a controlled step in cheese making.

The most common triggers are acid and heat. Acidic ingredients such as lemon juice, vinegar, yogurt, or fermentation-produced lactic acid lower the pH toward the casein isoelectric point (about pH 4.6), causing curds to form. Heat can also promote curdling by denaturing whey proteins and disrupting protein micelles, especially in high-protein liquids or when milk is overheated. Enzymes such as rennet can coagulate milk as part of cheese production by cleaving κ-casein.

In fresh milk, curdling appears as soft clumps separated from a watery serum. In sauces, curdling produces grainy texture and liquid separation, which is typically undesirable. Spoilage can also cause curdling as bacteria produce lactic acid and other byproducts that destabilize proteins and flavors.

Prevention and management include heating gradually, avoiding sudden addition of acid to hot milk, tempering eggs, and using stabilizers or higher-fat dairy products to improve stability. In cheese making, curd formation is controlled through precise acid or enzyme addition and temperature management. Understanding the cause helps decide whether to repair, use, or discard the product.