Rennet

Rennet is the enzyme tool. FAO: acid or starter makes a soft curd that breaks up easily; rennet makes a firm, elastic curd ¹. Most aged cheeses use both: culture acidifies, enzyme sets. The full physics lives on Coagulation.

U.S. cheddar identity allows “rennet and/or other clotting enzymes of animal, plant, or microbial origin” ². Traditional calf rennet is chymosin from the fourth stomach of unweaned calves; FAO also notes you can buy enzyme preparations ¹. Plant and microbial coagulants are the same job with different flavor and yield side-effects — a make choice, not a moral category.

Two phases, then a window #

FAO: a primary enzymatic phase cuts a strongly negative piece of casein so hydrophobic bonds can form; a secondary coagulation phase aggregates para-casein, helped by calcium and magnesium ions. Combined, they give a typical 30- to 45-minute window depending on temperature, pH, and dose. Optimum enzyme temperature in their notes is 40–42 °C; vats often run around 30–32 °C because culture and moisture share the kettle ¹. Rennet “does not work” above 55–60 °C in their teaching ¹. Too much enzyme, or too high a temperature, they warn, makes a soft curd — the opposite of the intuition to “add more so it sets harder.”

Calcium chloride is a coagulation aid. Cheddar identity caps it at 0.02 percent of the dairy ingredients ². FAO’s small-scale range is 5–20 g per 100 l of milk ¹. Those are different documents; do not average them.

What to notice #

Clean break: whey fills a cut, the gel does not smear like yogurt. Too early: fines, cloudy whey, lost yield. Too late: tough, uneven cubes. Store rennet cold and dark; FAO says do not keep homemade or small lots more than three months ¹.

Acid-set cheese skips this enzyme. Curd begins when you cut. CDR still wants moisture, fat, pH, and salt verified after the make ³. Return to Cheese.