The renneting of milk : a kinetic study of the enzymic and aggregation reactions

The rennet-induced clotting of milk was studied under various conditions. The kinetics of the enzymic and aggregation reactions was analysed separately and, where possible, related to the physico-chemical properties of the casein micelle and its environment.The effects of important variables, such as temperature, pH, divalent cations, ionic strength, casein concentration and pre-heat treatment of milk, are given and discussed in relation to the reaction kinetics and to the implications for the control of the renneting process.The kinetics of the enzymic reaction in milk differs substantially from that observed in pure χ-casein solutions. This discrepancy was ascribed to the immobilised state of the substrate in the relatively large casein micelles. The calculated upper limit of the enzymic reaction was close to the experimentally obtained reaction rate, suggesting that the reaction may be diffusion-controlled. Interactions between the enzyme molecule and the micelle surface are discussed.The relation between the rate of aggregation and the degree of χ-casein conversion was deduced from the change in viscosity during renneting. The correct interdependence between clotting time and enzyme concentration could be predicted with a kinetic model combining the enzymic and aggregation reactions