STRUCTURE OF ACID CASEIN GELS .2. DYNAMIC MEASUREMENTS AND TYPE OF INTERACTION FORCES

The time dependence of the mechanical behaviour and the type of interaction forces in acid casein gels were studied by dynamic rheological and permeability measurements. Gels were formed by quiescent heating of casein dispersions acidified in the cold to pH 4.6; they were investigated after an ageing time of 16 h or more. The dynamic moduli increased with frequency ω over an ω range of 10-3 to 10 rad s-1 indicating that a considerable proportion of the bonds in an acid casein gel have a relaxation time between 0.1 and 103 s. The ratio of the loss to storage modulus was independent of ω at 30°C but increased with decreasing ω at lower temperatures and decreased at higher ones. This indicates that acid casein gels become relatively more viscous over longer time scales at lower temperatures. The dynamic moduli strongly decreased with increasing measuring temperature. Since most types of interaction forces, except hydrophobic interactions which increase in strength with temperature in the range considered, are not strongly temperature dependent this points to an indirect effect, involving reconformation of the protein molecules and particles. The dynamic moduli as a function of the pH showed a small but significant maximum near the isoelectric pH. For the formation of a casein gel the ionic strength had to be at least about 0.1 M. Further raising of the ionic strength by addition of NaCl resulted for acid skim milk gels at first in a small increase of the storage modulus and upon further addition in a decrease. These results point to (mainly attractive) electrostatic interactions being important in acid casein gels, besides hydrophobic interactions and, probably, the ever present Van der Waals attraction, as well as steric and entropic effects related to protein conformation. Experiments with sodium caseinate gels in which part of the NaCl was replaced by CaCl2 point to a specific effect of Ca on gel formation and properties. © 1990.