Heat Inactivation of Milk-Clotting Enzymes at Different pH

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Abstract
Thermal-death-time characteristics were determined for 3 milk-clotting enzymes and 1 mixture of enzymes for pH from 4.5-6.5 in Jenness-Koops buffer. Means for temperature dependence coefficients (z) in this pH range were 4.22.degree. C for Mucor miehei protease, 4.62.degree. C for M. pusillus protease, 5.40.degree. C for rennet and 5.23.degree. 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 M. miehei protease, 116.8 kcal/mole for M. 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: M. miehei, 77.degree. C; M. pusillus protease, 71.degree. C; and rennet and rennet-pepsin, 67.degree. C.