Curdle

Curdle is a verb that describes the process by which a liquid, most commonly milk, becomes thick and lumpy or separates into solid curds and liquid whey. Curdling results from the coagulation of proteins, especially casein in dairy, triggered by acid, heat, enzymes, or microbial activity. The phenomenon can occur in cooking, dairy processing, and food spoilage, and it may be intentional (as in cheese making) or unwanted (as in sauces or warmed milk that has gone sour).

Causes of curdling include acidity, heat, and enzymatic action. Acidic ingredients such as lemon juice or vinegar lower the pH and promote coagulation of milk proteins. Heating milk or dairy mixtures, particularly at high temperatures or with rapid temperature changes, can denature proteins and cause curds to form. Enzymes such as rennet are used deliberately in cheese production to coagulate milk. Bacterial activity during spoilage can also produce acids that trigger curdling. The exact outcome depends on temperature, pH, fat content, and processing time.

In cooking, curdling is usually undesirable because it yields a grainy texture in sauces and custards. Techniques to prevent curdling include warming ingredients gradually, tempering eggs with hot liquid, using fats or cream to stabilize emulsions, and adding starch to bind proteins. If curdling occurs, it may be mitigated by whisking, adding a small amount of cold liquid, or blending the mixture to restore smoothness.

Controlled curdling is essential in cheese making, where acid or rennet coagulates milk to form curds, which are then processed into various cheeses. In dairy processing and consumer foods, curdled milk may be either redirected to appropriate products or discarded, depending on safety and quality considerations.