Scaling relations between structure and rheology of ageing casein particle gels

Abstract

Mellema, M. (Michel), Scaling relations between structure and rheology of ageing casein particle gels , PhD Thesis, Wageningen University, 150 + 10 pages, references by chapter, English and Dutch summaries (2000).The relation between (colloidal) interactions, structure and rheology of particle gels is discussed, especially the properties and the spontaneous ageing behavior of rennet-induced casein(ate) or skim milk gels.Methods involved were Brownian dynamics simulations, confocal microscopy, permeametry and rheometry (large- and small deformations). A categorization of relevant (fractal) scaling models and types of structural rearrangements (particularly those affecting the rheological properties) for particle gels has been made, and applied to experimental data of rennet-induced casein gels.Using Brownian dynamics simulations, the relation between colloidal particle interactions and gel structure was obtained. The simulation model used, included a repulsive barrier between the particles; the bonds formed were irreversible. The aggregation was delayed by a long-range repulsive barrier, and a high fractal dimensionality of the gels (2.4-2.5) resulted. This value was independent of the details of the interactions and volume fraction of particles in the range of 3-10 vol% particles. The simulation results agreed well with experimental results on rennet-induced aggregation and gelation in skim milk.The (fractal) structure of rennet-induced casein gels, as studied by confocal scanning laser microscopy and permeametry was monitored during aging, at various pH values (5.3-6.65) and temperatures (20-30 °C). At low pH, a gradual coarsening of the structure was observed; the size of the pores and compact structures increased. This was reflected in a decrease in apparent fractal dimensionality (from 2.4 to 1.7), and in an increase in pore size and lower cut-off length of the fractal regime (from 0.5 to 1.5μm).A scaling model was developed for the rheological behaviour of particle gels as a function of structure and particle volume fraction. The main structural parameters are the fractal dimensionality, the size of the compact building blocks and two parameters describing the number of deformable links in the strands and the dominating type of deformation of these links. Application to rheological data (storage modulus, maximum linear strain, yield stress) as a function of volume fraction (5-9 vol%) showed that rennet-induced casein gels contain straight, elastic strands.Application of the model to results on measurements of the storage modulus during ageing, showed that at low pH rearrangements induce a dramatic increase in compact building block size and early disappearence of the fractal structure. It is concluded that the main types of rearrangements in rennet-induced casein gels after gelation, are particle fusion and stretching and breaking of strands.Keywords : colloid, structure, rheology, casein, rennet, gel, fractal scaling, computer simulation, confocal microscopy.</p