Relation between syneresis and rheological properties of particle gels

The relation between the tendency to exhibit syneresis of rennet (pH 6.65) and acid (pH 4.6) skim-milk gels and the rheological properties of these gels is discussed. Based on the syneresis model for milk gels of Van Dijk and Walstra 1, 4, 14, 19] it is reasoned that the average lifetime of the protein-protein bonds in the casein strands forming the gel network, the fracture force of these strands, and their flexibility are the main mechanical properties of importance. It is argued that the ratio of the loss modulus to the storage modulus (tan δ) as a function of the time scale of the measurement in dynamic experiments is a good measure of the first property. Results are given for rennet skim-milk gels at 25, 30, and 40°C and for an acid skim-milk gel at 30°C. Moreover, the stress strain relation and fracture behavior of both gels were assessed in creep measurements as a function of the duration of the applied stress. Rennet skim-milk gels have a lower fracture stress σ _f and therefore probably a lower fracture force per strand and a higher tan δ over long time periods than acid gels, while tan δ increases with temperature. As predicted, low fracture stresses and high tan δ correlated with an increased tendency to exhibit syneresis.