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Impact of gelation conditions and structural breakdown on the physical and sensory properties of stirred yogurts.

The objectives of this research were to understand structure-function relationships between the initial yogurt gels and stirred yogurts made from these gels. Yogurt gels were made from milk preheated at 75 or 85 degrees C for 30 min and incubated at 32, 38, or 44 degrees C. Yogurt gels were sheared at 5 s(-1) for 1 min to make stirred yogurts. Small amplitude oscillatory rheology and shear rate sweeps (flow curves) were performed to determine the dynamic moduli and apparent viscosity of yogurts, respectively. Confocal scanning laser microscopy was used to examine the microstructure. The sensory properties of stirred yogurts were determined using quantitative descriptive analysis. Yogurt gels made from high preheating temperatures and low incubation temperatures tended to have an increase in storage modulus, apparent viscosity values at structural breakdown, yield stress, and a decrease in the maximum in loss tangent values and permeability. During the shearing of intact gels, there was an initial increase in apparent viscosity in the low shear rate region, because of the resistance derived from the intact network; commonly used models for stirred yogurts could not adequately describe this type of viscoelastic behavior. Increasing milk preheating temperature and decreasing incubation temperature resulted in stirred yogurts with increased apparent viscosity, oral viscosity, and sensory mouth coating attributes. Storage modulus values of the initial gels were positively correlated with the apparent viscosity at 10 s(-1) (r = 0.61), oral viscosity (r = 0.77), and mouth coating sensory attributes (r = 0.72) of stirred yogurts. A significant negative correlation was observed between maximum in loss tangent values of initial gels and shear stress values (r = -0.68) of gels at structure breakdown, and in stirred yogurts, the oral viscosity (r = -0.85), and apparent viscosity at 10 s(-1) (r = -0.78). Permeability was negatively correlated with the oral viscosity (r = -0.84) and apparent viscosity at 10 s(-1) (r = -0.82) in stirred yogurts. These highly significant correlations demonstrated that the structure of the initial gel network and structural breakdown process had a major impact on the physical and sensory attributes of stirred yogurts. The presence of a yield stress and the elastic nature of gels at low strains resulted in commonly used models that could not adequately predict the behavior in the very low shear rate region, although they were able to predict the remainder of the flow profile.