Structure and physical properties of yogurt gels: effect of inoculation rate and incubation temperature

The effects of inoculation rates and incubation temperatures on the physical properties and microstructure of yogurt gels were investigated. A 2-factor experimental design with 3 replicates was used for data analysis. Yogurt gels were made with 0.5, 1, 2, 3, or 4% (wt/wt) inoculation rates and incubated at 40 or approximately 46 degrees C. Dynamic low amplitude oscillatory rheology was performed to monitor the formation of yogurt gels. Gel permeability and the amount of surface whey were determined. Gel structure was examined by confocal scanning laser microscopy. Higher storage modulus values were observed in yogurt gels made at higher inoculation rates and lower incubation temperatures. Gels made at higher inoculation rates and incubation temperatures exhibited higher yield stress and maximum loss tangent values, respectively. Permeability, pore size, and whey separation of yogurt gels increased with decreased inoculation rate and increased incubation temperature. An increase in the inoculation rate resulted in a decrease in the pH where the maximum in the loss tangent occurs, presumably reflecting less efficient solubilization of colloidal calcium phosphate (which is a slow process) and the need to attain a lower pH to complete the solubilization. Significant positive correlations were observed between whey separation and the value of maximum in loss tangent (r = 0.94) and permeability (r = 0.89). Whey separation was negatively correlated with storage modulus (r = -0.48). It was concluded that rearrangements of casein particles in the gel network and the rate at which the solubilization of colloidal calcium phosphate occurred were important driving forces for whey separation and weak gel.