Date of Award
Doctor of Philosophy
Food Science and Technology
Federico M. Harte
Qixin Zhong, John R. Mount, Juan L. Jurat-Fuentes
A definitive structure of the native casein micelle structure continues to elude researchers. Data and images obtained via cryo - transmission electron microscopy of isolated native casein micelles allowed for the reconstruction of a three dimensional model of the micelle; which contains water filled cavities (ca. 20 to 30 nm diameter), channels (diameter larger than ca. 5 nm), and several hundred high density nanoclusters (6-12 nm diameter) within the interior of the micelles.
The SDS polyacrylamide gel electrophoresis of isolated micelles (pH 6.8, 5.0) showed that whey proteins were found associated with the isolated micelles regardless of the pH of the milk sample. However, the introduction of the hydrophobic peptide Valinomycin (MW= 1111.32; LogP=-2.084) into raw skim milk did not result on the peptide being found associated with isolated casein micelles.
Taking advantage of the heat, ethanol, and pH mediated micelle disassociation, it was possible to improve the solubility of caseins in fluid milk via esterification. The esterification of casein resulted in a shift on their isoelectric point; which in turn resulted in a better solubility at low pH values. Caseins esterified at a pH 3 and 4 had a higher solubility than those esterified at other pH values and non-modified skim milk powder in acidified juice with a pH equal or below 3.5.
By adding a chill step (3-5 oC/pH ~5.2) to the processing of non-fat yogurt, it was possible to improve the whey holding capacity of the product while also producing a firm product without the need for added stabilizers or gums. In the conditions of the chill step, beta casein and calcium migrated out of the micelles. This resulted in a reduction on the size of the micelles. A possible explanation for the results would be that once the temperature returned to 47 oC, the liberated caseins self-associated into small micelles and together with the smaller original micelles formed a tight gel matrix.
Casein is fundamentally important to the dairy industry. By gaining an understanding of its structure and properties, it is possible to develop new applications and enhance their performance in dairy products.
Trejo, Raymundo, "Casein Micelles from Bovine Milk: Native Structure, Interactions, and Practical Applications of their Structural Modification. " PhD diss., University of Tennessee, 2012.