The Effect of Milk Fat, the Ratio of Casein to Water, and Temperature on the Viscoelastic Properties of Rennet Casein Gels

The objective of this study was to evaluate the effects of milk fat, the ratio of casein to water, and temperature on the viscoelastic properties of a model system for cheese based on dried rennet casein power. Casein gels were formed by mixing rennet casein, butter (as a source of milk fat), water, and some minor ingredients at 0.89, 0.58, and 0.38 ratios of casein to water; milk fat was added at 0, 12, and 24% at each ratio. Strain sweep, frequency sweep, melting behavior, and stress relaxation were used to characterize the viscoelastic properties of casein gels. The addition of milk fat reduced the linear viscoelastic region of casein gels. The complex modulus increased as milk fat content increased at temperatures ≤20°C, decreased at temperatures ≥30°C, and increased at all temperatures as the ratio of casein to water increased. Any particular value for the complex modulus could be obtained by a number of different combinations of the milk fat content, the ratio of casein to water, and temperature. The casein gels showed three distinctive zones of viscoelastic behavior over a temperature range of 5 to 70°C: rubbery solid, transition, and melt. The melting temperature and relaxation time of the casein gels increased as the ratio of casein to water increased; milk fat content had less effect on these gel properties. The casein matrix alone (0% added milk fat) showed a gradual softening with temperature starting at about 10°C. Thus, these composite gels represent a rather unique system by which the dispersed filler phase (milk fat) melts over the same temperature range as the underlying polymer matrix dissociates.