The coagulation of differently sized casein micelles by rennet

Fractions of bovine casein micelles of different sizes were prepared by successive centrifugation steps, and dilute suspensions of the different fractions were reacted by rennet. The molecular weight increase with time after addition of rennet was measured by light scattering. After an initial lag stage, where the increase was zero, and a short intermediate stage, the molecular weight became linearly proportional to time, indicating complete conversion of the kappa-casein substrate by the enzyme. The slope of the final portion of the growth profile can be used to define values of the Smoluchowski rate constant governing the aggregation. No significant differences in this constant were found with micelles of different sizes, and the value of the rate constant suggested that the aggregation reaction of completely renneted micelles is only moderately retarded relative to the diffusion-controlled process, even at room temperature.