THE STRUCTURE OF CASEIN AGGREGATES DURING RENNETING STUDIED BY INDIRECT FOURIER TRANSFORMATION AND INVERSE LAPLACE TRANSFORMATION OF STATICAND DYNAMIC LIGHT-SCATTERING DATA, RESPECTIVELY

Aggregation of casein micelles after addition of the proteolytic enzym e chymosin has been studied by static and dynamic light scattering at three different concentrations of casein corresponding to dilutions 1: 100, 1:500, and 1:1000 of native milk. The static light scattering dat a have been analyzed by an indirect Fourier transformation method whic h gives the distance distributions as a function of timer From these c urves radius of gyration and an average number of casein micelles in t he aggregates have been derived as a function of time. The dynamic lig ht scattering experiments give the hydrodynamic radius as a function o f time after the addition of rennet. The initial radius of gyration fo r the intact casein micelles is 140 nm. The corresponding hydrodynamic radius is also 140 nm. This shows that the casein micelles are not so lid spheres. Inspection of a plot of relative mass versus radius of gy ration for the aggregates appearing after the addition of chymosin sho ws that two processes take place. First extended linear aggregates are built up to a relative mass of the aggregates of about 10 and then re structuring of aggregates occurs such that increasingly compact object s are formed. Whereas the first process exhibits a relatively fast gro wth in size, the aggregates grow slowly in size during the second proc ess. Further evidence of the formation of linear aggregates followed b y more dense aggregates was obtained by forming the ratio between the radius of gyration and the hydrodynamic radius. This ratio increases t o values of about 2.5 (indicating that linearly extended molecules are present followed) by a decrease to about 1. The log-log plot of mass versus radius of gyration ils linear up to relative masses of about 10 with a slope of about 2. This extends up to sizes of 1 mu m in diamet er. The slope then increases to values indicating branching and thereb y the formation of more compact aggregates. For relative masses below 10 and sizes below 1 mu m sedimentation is unlikely to occur and infor mation about the mechanism of aggregation can be obtained. The aggrega tion number as a function of time has been analyzed in terms of Smoluc howski's equations with a rate constant including both functionality a nd a changing barrier height as a function of the extent of proteolysi s. The functionality obtained from Smoluchowski's equations is about 2 .1. (C) 1995 American Institute of Physics.