Topic
Coagulation, temperature, pH, calcium and milk composition — explained approachably.
Raw milk generally coagulates readily and brings its own microflora and flavour complexity. Pasteurized milk is more consistent and is what most commercial operations use, but its heat treatment can affect curd formation — most severely with ultra-pasteurized milk.
Curd is the protein-and-fat gel formed when milk coagulates; whey is the liquid released as that gel contracts. Watching whey colour and clarity is one of the fastest ways to read how a make is going.
Curd firmness is determined by milk protein and calcium content, temperature, pH, coagulant dosage and how long the gel is left to develop before cutting. No single one of these controls it on its own.
A cleaner curd cut comes from cutting at the right firmness, with sharp tools, at an even pace, in a vat that set evenly. Cutting too early produces fines and loses yield; cutting too late gives an uneven, tight curd.
Milk composition changes how milk coagulates. Protein and calcium content have the largest effect on set time and curd firmness, and both vary with breed, season, feed, stage of lactation and animal health.
Calcium is essential for the aggregation phase of coagulation and contributes to curd firmness. Processing, cold storage and pH shifts change how much calcium is available, which is why some cheesemakers add calcium chloride to pasteurized milk.
pH strongly influences coagulation. A lower pH at the time of addition generally speeds the set, while pH through the make shapes curd firmness, whey release and the texture of the finished cheese.
Temperature affects both phases of coagulation: enzyme activity and the aggregation of destabilised micelles. Milk below the range your style calls for sets slowly and softly, and an unevenly heated vat sets unevenly.
Milk coagulation is the process of turning fluid milk into a gel. Clotting enzymes remove the protein layer that keeps casein micelles apart; the destabilised micelles then aggregate, in the presence of calcium, into the curd you cut.
Yes. Standard pasteurized milk is used with rennet every day in artisan and commercial cheesemaking. The problem case is ultra-pasteurized or UHT milk, whose heat treatment interferes with forming a cuttable curd.
Rennet is added within the temperature range specified by the cheese style you are making, and that range is part of your recipe rather than a universal figure. Coagulation slows noticeably when milk is below the intended range.
There is no single correct coagulation time. Set time is chosen to suit the cheese style and then controlled by dosage, milk composition, temperature, pH and calcium. What matters most is that your set time is repeatable.
