ABSTRACT
Nowadays, acid-induced milk protein gels are used extensively in the dairy industry and are the base of many food products. Therefore, a deeper fundamental understanding of the effects of process parameters and system components on the mechanisms of gel formation and final gel properties in real and model systems is important for researchers and industries. The present study was designed to investigate the effects of casein-to-whey protein ratio, incubation temperature, and heating temperature on viscoelasic properties of the final gel in a model system. Samples of solutions with different casein-to-whey protein ratios [2.1:0.9, 2.4:0.6 and 2.7:0.3] were prepared and refrigerated overnight. This was followed by heating the samples at different temperatures [70, 80, or 90°C] for 15 minutes using a water bath and cooling to incubation temperatures [25, 35 or 45 °C]. In the next step lucono-sigma-lactone was added and the samples were transferred to a rheometer. The gelation process was monitored by the time sweep oscillatory test. A second-order polynomial model was fitted to the experimental data and the optimum response was determined using the Minitab15 software. Incubation and heating temperatures influenced significantly [P<0.001 and P<0.01, respectively] the G'200 value [storage modulus value of the final gel measured 200 minutes after addition of GDL]. The casein-to-whey protein ratio had no significant influence on the G'200 value. Increasing the incubation temperature led to a decrease, whereas increasing the heating temperature led to an increase, in the G'200 value. The maximum value of G'200, 268/93 Pa, was obtained at the presence of 2.6152% [w/w] casein and at incubation and heating temperatures of 25 °C and 90 °C, respectively. Smaller quantities of casein and higher incubation and heating temperatures shortened the length of time required for gel formation. Finally, increased incubation temperatures and decreased heating temperatures led to increased the damping factor [tan sigma]. The effects of different process parameters on the viscoelastic properties of dairy gels can be determined and characterized by oscillatory rheometry. The findings of this study show that incubation temperature is the most effective factor, whereas casein-to-whey protein ratio has no significant effect on the final gel strength