Biophysical studies of milk protein interactions in relation to storage defects in high protein beverages.

摘要

Much like Ultra High Temperature (UHT) processed milk, UHT processed high protein beverages suffer from storage defects including gelation and sedimentation. Although these beverages are made with milk proteins, the food system is very different and much more complex from that of milk. Thus, one cannot assume that the mechanism of development of storage defects is the same as in milk. As such, the goal of this project was to investigate the factors affecting storage stability of high protein beverages.;A number of different protein products (alpha-lactalbumin, beta-lactoglobulin, calcium caseinate, WPC, WPI, MPC, MPI) and combinations of these proteins were UHT processed and their storage stability was observed. Unfortunately, the changes were too subtle to be detected by FTIR spectroscopy and we had to rely more heavily on visual observations. From these observations it was determined that altering the protein combinations did not prevent sedimentation of caseinates. However, the presence of beta-lactoglobulin changed the consistency of the sediment. Additionally, it was also noted from these trials that proteins alone are most likely not responsible for age gelation of high protein beverages since gelation was only observed when proteins were in the presence of all the other beverage components.;Finally, the interactions between beta-lactoglobulin and caseinates were analyzed using FTIR-spectroscopy and Surface Plasmon Resonance (SPR). FTIR spectroscopic analysis indicated that alpha- and beta-caseins exhibited a protective effect on beta-lactoglobulin during heating and changed the order of events in beta-lactoglobulin's unfolding. This change in sequence of events suggested that hydrophobic interactions are involved in the casein-beta-lactoglobulin interaction. SPR study of casein-beta-lactoglobulin interaction demonstrated that there was a specific interaction between these proteins in the order beta-casein> alpha-casein> kappa-casein> UHT calcium caseinate. Based on the above results, we propose a model scenario of protein-protein interaction in UHT high protein beverages.;As a first step, a method was developed to facilitate the study of milk protein solutions by Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR) Spectroscopy. Scanning the proteins in solution followed by subtraction of the contribution to the absorbance by water provided the most reliable and repeatable results. Next, the suitability of the data for various forms of spectral enhancement was assessed. Finally the method was applied in a series of experiments in order to determine if the data could provide numerical information. Thermodynamic data for heating of beta-lactoglobulin was obtained using three methods: transmission-FTIR spectroscopy in D2O, ATR-FTIR spectroscopy in D2O and in H2O. All techniques provided equivalent results, indicating that analyzing proteins in water by ATR-FTIR spectroscopy can be used to provide quantitative information.