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Colloidal and Rennet Coagulation Properties of Concentrated Casein Micelles with Altered Mineral Equilibrium

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Title: Colloidal and Rennet Coagulation Properties of Concentrated Casein Micelles with Altered Mineral Equilibrium
Author: Zhao, Zhengtao
Department: Department of Food Science
Program: Food Science
Advisor: Goff, Douglas Jr
Abstract: The changes in mineral equilibrium in milk affect the structure of casein micelles, and as a consequence, their processing functionality. In this thesis, the changes in the physico-chemical properties of casein micelles were measured after NaCl addition or pH-modification as a function of concentration. The addition of NaCl led to the dissolution of colloidal calcium phosphate and the decrease of the surface charge. The light scattering properties were also affected as a function of concentration. The addition NaCl retarded rennet induced gelation, indicating a change in the surface properties of the micelles, and the inhibition was less evident at high casein volume fractions. The changes of the serum composition, rheological and the light scattering properties of the micelles were also studied as a function of pH before concentration. The pH modified concentrates were also studied for their rennet-induced coagulation behaviour. Acidification leads to the release of colloidal calcium phosphate, and a decrease of radius. After re-equilibration, casein dissociation occurs, especially for samples previously adjusted to pH<6.0. It was also determined that the re-equilibration temperature has no influence on the dissociation of the caseins. After concentration, more caseins are released to the serum phase for the samples with lower pH, especially at protein concentration >6%. The voluminosity of casein micelles decreased with reduced pH, and a maximum max value of 0.9 was found at pH 6.0. Acidification significantly decreased rennet gelation and a slight increase in the gelation time was found after concentration. The mixture of these modified and concentrated casein micelles with raw milk facilitated the gelation process and the elastic modulus of the final gels was much higher. However, when the mixtures were equilibrated against raw milk, no gelation occurred, due to the loss of colloidal calcium and micellar dissociation in the serum phase. These results clearly demonstrate that processing history of milk strongly affects the structure and the functionality of casein micelles. This project helps us better understand the changes of structure-function of casein micelles and the role of minerals during the processing.
Date: 2015-09

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