Practice question

Cheese manufacture fundamentally depends on enzymatic coagulation of milk casein micelles to form curd network separating from whey. Rennet extracted traditionally from fourth stomach abomasum of suckling calves contains 80 to 90 percent chymosin EC 3.4.23.4, aspartic protease with two aspartate residues Asp34 and Asp215 in active site, plus 10 to 20 percent pepsin. Chymosin shows exquisite specificity for Phe105-Met106 peptide bond of kappa-casein which forms hairy layer stabilizing casein micelles of 100 to 300 nanometer diameter via steric hindrance and negative charge of glycomacropeptide portion. Cleavage releases soluble caseinomacropeptide into whey, removes steric stabilization and permits calcium-mediated aggregation of para-kappa-casein with alpha-s1, alpha-s2, beta caseins into three-dimensional gel entrapping fat globules and moisture determining rennet cheese texture. Optimum activity pH 5.5 to 6.0 and temperature 30 to 32 Celsius matches cheesemaking conditions. Microbial rennet from Mucor miehei and recombinant chymosin expressed in Aspergillus niger or Kluyveromyces lactis now supply over 80 percent global demand due to ethical, cost and vegetarian considerations, while lipase and protease adjuncts modulate flavor during ripening.