Rennet coagulation of milk

Caseins (~80% of milk protein) occur in milk in the form of large, multi-molecular aggregates called micelles. Casein micelles are approximately spherical aggregates of the 4 types of casein, alpha s1-, alpha s2-, beta- and kappa-casein, together with inorganic ions collectively referred to as colloidal calcium phosphate. There is an uneven distribution of the different caseins throughout the micelle. In particular, kappa-casein is located principally on the surface of the micelle. kappa-Casein stabilizes the micelles and prevents them from aggregating together in the presence of Ca2+. Were it not for kappa-casein, the other caseins would aggregate together as they are highly phosphorylated.

kappa-Casein is divided into 2 parts. Residues 1-105 (approx. two-thirds of the molecule) are hydrophobic and associate with the other caseins. The C-terminal region of kappa-casein (residues 106-169, approximately one-third of the molecule) are hydrophilic (usually containing complex sugar groups esterified to Thr residues) and protrude into the environment, stabilizing the micelle due to steric reasons and the reduction of its zeta-potential.

Enzymatic coagulation of milk involves modification of the casein micelles via limited proteolysis of kappa-casein by proteinase preparations ("rennets") followed by Ca2+-induced aggregation of the rennet-altered micelles.

kappa-Casein is the only casein hydrolyzed during rennet coagulation. kappa-Casein is hydrolyzed at its Phe105-Met106 bond to produce para-kappa-casein (kappa-casein fragment 1-105, kappa-CN f1-105) and macropeptides (also called glycomacropeptides or caseinomacropeptides; kappa-CN f106-169). Macropeptides diffuse into the aqueous phase; para-kappa-casein remains attached to the micelle core. Macropeptides (~30% kappa-casein or 4-5% total casein) are lost. This is an unavoidable loss and a consequence of rennet coagulations but it does have consequences for cheese yield. Proteolysis of kappa-casein by the proteinase(s) in rennet preparations is referred to as the FIRST STAGE OF RENNET ACTION.

Removal of the macropeptides from micelles reduces zeta (surface) potential of the micelles from -20 to about -10 mV and also removes the steric stabilizing layer. When about 85% of total kappa-casein is hydrolyzed, colloidal stability of the micelles is reduced so much that they coagulate at temperatures above about 20oC in the presence of Ca2+. This event is called the SECOND STAGE OF RENNET ACTION.

The Phe105-Met106 bond of kappa-casein is many times more sensitive to rennet action than any other bond in the caseins. Actually, Phe-Met residues aren't necessary; you can replace either amino acid residue in kappa-casein without changing the sensitivity of the bond to rennet action very much. In fact, human, porice and rodent kappa-caseins have Ile or Leu at position 106. Interestingly, the proteinase of C. parasitica does not cleave at Phe105-Met106 but rather Ser104-Phe105 (i.e., the bond next to the Phe-Met bond). Much of the work done in this area has involved synthesizing short peptides with the same amino acid sequence as this region of kappa-casein. The smallest peptide hydrolyzed by rennet is kappa-casein fragment 104-108 (Ser-Phe-Met-Ala-Ile); extending this peptide out towards the N or C-termini of kappa-casein increases its susceptibility to rennet action. Peptide kappa-casin fragment 98-111 is hydrolyzed as well as intact kappa-casein. Certain amino acid residues appear important (e.g., Ser104, Leu103, Ala107, Ile108). The conformation (shape) of kappa-casein renders the Phe-Met bond very susceptible to rennet action.

See also

Fox, P.F. and P.L.H. McSweeney (1998). Dairy Chemistry and Biochemistry. Blackie Academic and Professional Publishers, London, 478 pp. (Reprinted with corrections, 2003.)