Rennet & Coagulation

Coagulation is the physical-chemical process by which milk transitions from a pourable sol into a semi-solid gel called the curd, releasing the water phase (whey) in the process. It is triggered either enzymatically — by proteases in rennet that cleave the kappa-casein micelle coating — or acidically, by lowering pH until the negative charge on casein micelles collapses and they aggregate. Animal rennet, traditionally extracted from the fourth stomach (abomasum) of suckling ruminants (calves, lambs, kids), supplies chymosin and pepsin as its primary proteases; chymosin is considered the gold standard because it cleaves kappa-casein at a single Phe105-Met106 bond with high specificity and low proteolytic side activity that would otherwise cause bitter peptides in aged cheeses. Microbial rennets (e.g., Rhizomucor miehei) and fermentation-derived chymosin (FPC, produced in Aspergillus, Kluyveromyces, or Trichoderma by genetic engineering) now supply the vast majority of global cheesemaking and are considered vegetarian-suitable. Plant rennets — cardoon thistle, fig sap, nettles — predate animal rennet in Mediterranean and Iberian traditions and produce cheeses with characteristic herbaceous or bitter notes. The quality of coagulation — the firmness and speed of gel set — is assessed by the vat operator through the 'clean break' test: inserting a flat spatula and lifting; a properly set curd snaps cleanly with a sharp angle. Gel strength determines cut size, syneresis rate, and ultimately moisture content and texture in the final cheese.

“After adding the rennet to the milk held at 32 °C, the cheesemaker waits 45 minutes for coagulation to achieve a firm clean break before making the first cuts.”

• TechniqueAcidification
• ConceptCheddaring
• TechniquePasta Filata

1. 1.Cheese and Fermented Milk Foods — Frank Kosikowski & Vikram Mistry (3rd ed., 1997)
2. 2.Technology of Cheesemaking — Barry Law & Adnan Tamime (2nd ed., 2010, Wiley-Blackwell)
3. 3.Dairy Science and Technology — P. Walstra, J. Wouters & T. Geurts (2nd ed., 2005, CRC Press)
4. 4.The Oxford Companion to Cheese — Catherine Donnelly, ed. (2016, Oxford University Press)

Confidence: high