| The overall objectives of this thesis were to determine the effects of processing variables on the physico-chemical properties of yogurt gels including whey separation and to investigate how the structure of the initial yogurt gel network and structural breakdown process affect the physical and sensory properties of stirred yogurts made from these gels.; Yogurt gels made from milk with low heat temperature, inoculation rate, and high incubation temperature had low storage modulus and yield stress values while these gels had high maximum in loss tangent, permeability, and whey separation. The acidification rate probably affected the solubilization of colloidal calcium phosphate within casein particles. The high maximum in loss tangent and permeability values indicated the possibility of extensive rearrangements of protein particles in the network; this suggestion was supported by the presence of large pores in micrographs. The susceptibility of the network to rearrangements was correlated with enhanced whey separation.; There was an increase in apparent viscosity values at the structural breakdown point in yogurt gels made from milk with a high heat temperature and low incubation temperature. The resistance derived by the intact network was probably responsible for the observed increase in apparent viscosity of gels in the very low shear rate region. The physical properties of the initial gel greatly influenced the characteristics of stirred yogurt, e.g., gels with high storage modulus and low permeability values had high viscosity and mouth coating attribute. Image analysis of the microstructure of gels yielded parameters, including area of pores and normalized variance, which indicated that there were extensive structural changes in yogurt gels incubated at 46°C during gel formation. There were positive correlations between area of protein aggregates and storage modulus, yield stress, oral viscosity, and apparent viscosity while star volume of pores were positively correlated with permeability (porosity).; This study suggests that yogurt gels are dynamic systems that are prone to rearrangements. The physical properties of yogurt gels could be explained using a model for casein interactions that emphasized a balance between attractive (e.g., hydrophobic attractions and cross-links contributed by calcium phosphate) and repulsive (e.g., electrostatic repulsion) forces. |
