Coagulation

Coagulation is the moment Milk stops being a stable dispersion and becomes a gel. Cheese keeps the gel’s solids; yogurt is a different acid gel you usually do not cut for whey in the same way.

Milk proteins come in two groups. FAO’s cheese notes put casein at about 75–80% of total protein and whey proteins at about 20–25%; it is casein that coagulates in ordinary cheesemaking ¹. Casein micelles stay apart because of charge and bound water. Acid or enzyme removes that stability.

Acid coagulation #

Adding acid, or growing Cheese cultures until they make lactic acid, lowers pH. Near the isoelectric region the micelles no longer repel; they aggregate. FAO notes that this reaction is temperature-sensitive: warmer milk can set at a higher pH ¹. The gel is typically soft and breaks easily. Cottage and some fresh cheeses live here. Acidity is the shared chemistry; this page is the dairy application.

Rennet coagulation #

Rennet (and other clotting enzymes of animal, plant, or microbial origin) cuts a stabilizing piece of κ-casein. Micelles can then hydrophobic-bond. Calcium ions help the second, aggregation phase; cheesemakers sometimes add calcium chloride as a coagulation aid, which U.S. cheddar identity explicitly allows in a small dose ². FAO describes an enzymatic phase then a coagulation phase, with a typical combined window on the order of 30–45 minutes depending on temperature, pH, and enzyme dose ¹. Optimum enzyme temperature is higher than the vat many cheesemakers actually use, because culture activity and moisture targets share the same kettle.

Most aged cheeses use both: culture acidifies, rennet sets a firmer, more elastic gel ¹.

What to observe #

Look. A skin of whey on the surface; the mass pulls slightly from the vat wall.

Feel. The clean-break test: a finger or knife lifted from the gel should split it with whey filling the cut, not smear like yogurt. Too early and you shatter protein into fines that cloud the whey and steal yield. Too late and the gel is tough to cut evenly.

Listen. Almost nothing. This is a quiet step. Rushing it with a clock instead of a break is how vats lose control.

Variables #

Temperature, pH at renneting, enzyme dose, milk heat-history, and calcium all move firming rate and final firmness ¹. Pasteurization and cold storage can weaken a set; that is a milk-handling fact, not a moral failing of the culture packet.

When the gel is cut, you have left coagulation and entered Curd.