Adding acid to milk causes the casein in the milk to coagulate. Casein precipitates when the normal 6.6 pH of fresh milk drops below 4.6. Whey proteins do not coagulate. Sources of acids include those from foods such as lemon and lime juices, tomato products, and certain fruits; or from bacteria-produced acids in cultured milk products.
Because of the coagulating effect of acids on milk products, some foods must be carefully prepared.
For example, extra caution is required when combining milk or cream with lemon-flavored tea, tomato soup, and coffee (which is acidic). The key to preventing the milk from coagulating is to add the acid to the milk base instead of the other way around. Avoiding high temperatures after milk has been mixed with acid also helps to prevent coagulation.
