Casein Micelles from Bovine Milk: Ethanol Induced Changes in Hydrophobicity and Interaction with Native Whey Proteins

• Main Author: Trejo, Raymundo
• Format: Others
• Published: Trace: Tennessee Research and Creative Exchange 2009
• Subjects: Food Science
• Online Access: http://trace.tennessee.edu/utk_gradthes/566

Caseins, in the form of micelles, are the most abundant milk protein. The nature of these micelles is still not fully understood and several models have been proposed. The first chapter discusses this topic, along with the importance of milk proteins to the food industry, and their allergenic properties. In the second chapter the changes in the hydrophobicity of dissociated casein micelles are explored. As new applications for milk proteins are discovered, it becomes more important to understand their physicochemical properties when subjected to different treatments. It has been reported that casein micelles disassociate when heated in the presence of ethanol. The changes to the hydrophobicity of milk proteins during that process were evaluated by utilizing the fluorescent hydrophobic probe 1-anilinonaphthalene-8-sulfonic acid (ANS). The results showed that the fluorescence intensity of ANS decreased when the samples were heated. Using the information obtained from the fluorescence spectroscopy it was possible to infer that the hydrophobicity of the milk proteins also decreased when the casein micelles disassociate in the presence of ethanol. The third chapter deals with the association of native casein micelles and whey proteins. This association was thought to only be consequence of whey protein denaturation and interaction with k-casein through S-S linkages. Size exclusion chromatography (SEC) was used to isolate native casein micelles from raw skim milk samples with a pH value of 6.8, 6.0, and 5.5. In a separate experiment, casein micelles were precipitated by lowering the pH of milk to ~4.6, and the vi whey was removed and substituted with protein free serum (PFS). Then, the casein micelles were re-suspended in PFS at a native pH. SDS-PAGE was conducted on the samples utilizing silver staining of total proteins for higher sensitivity of detection. Whey proteins were found in association with the casein micelles at all pH values. Samples of both native pH and re-suspended casein micelles were tested with gel electrophoresis under native and reducing conditions. The bands for the whey proteins were only visible under the reducing conditions. These findings demonstrate that whey proteins are also part of the native casein micelle structure.