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Heat Inactivation of Milk-Clotting Enzymes at Different pH

Department of Food Science, University of Wisconsin, Madison 53706

ABSTRACT

Thermal-death-time characteristics were determined for three milk-clotting enzymes and one mixture of enzymes for pH from 4.5 to 6.5 in Jenness-Koops buffer. Means for temperature dependence coefficients (z) in this pH range were 4.22 C for Mucor miehei protease, 4.62 C for Mucor pusillus protease, 5.40 C for rennet, and 5.23 C for a 50:50 blend of rennet and pepsin. Activation energies were calculated for each enzyme or the mixture of rennet and pepsin and means were 131.5 kcal/mole for Mucor miehei protease, 116.8 kcal/mole for Mucor pusillus protease, 97.0 kcal/mole for rennet, and 99.6 kcal/mole for 50:50 rennet-pepsin mixture. For each enzyme, heat resistance increased as pH decreased. To accomplish 90% inactivation of each enzyme and the mixture in 15 s at pH 6.0, these temperature treatments were required: Mucaor miehei, 77 C; Mucor pusillus protease, 71 C; and rennet and rennet-pepsin, 67 C.