Curdles

Curdles (the curdling of liquids) refers to the coagulation of proteins in a liquid, resulting in the formation of solid curds with liquid whey separating. In dairy systems, milk proteins called caseins are normally stabilized in micelles. When this stabilization is disrupted—most commonly by acidification, heat, or enzymatic action—the proteins aggregate and separate, producing curds and whey.

Common causes include acidification from bacterial fermentation or added acids (lemon juice, vinegar), heating or boiling temperatures, and enzymes such as rennet used in cheese making. Spoilage bacteria can also sour milk, leading to curdling as proteins denature.

Contexts and outcomes vary. In cheese making, intentional curdling through rennet or controlled acidity separates milk into curds (rice-like solids) and whey, forming the basis for cheeses like paneer, ricotta, and many fresh cheeses. In cooking, curdling of sauces or custards occurs when dairy-based liquids are overheated, overly acidic, or beaten too aggressively, producing a grainy or lumped texture. In spoiled milk, curdling is a sign of deterioration and the product is generally unsafe to consume.

Prevention and treatment depend on context. To avoid unwanted curdling in sauces, keep temperatures moderate, add dairy slowly, and consider stabilizers or emulsifiers; to rescue a mildly curdled sauce, remove from heat and whisk in cold liquid gradually. Curds used in cheese production are typically separated, collected, and processed further.