Formation Of Dairy Protein Microgels And Their Properties

The structure and functional properties on dairy protein at conventional conditions had already been studied, but a specific pattern of aggregation of milk proteins leading to stable microgels was identified in unconventional conditions by controlling the degree of aggregation. The morphology of the small fine stranded microgels with an average diameter lower than10μm in length were observed. Therefore, the project was aimd to study the preparation of microgels (heat-temperature, heat-time, pH, the concentration of protein) and the stability of microgels (thermal stability, acid stability), exploring the differences between microgels aggregation pathway and the routine of aggregates. In addition, we studied the differences between microgels and normal protein aggregates by analyzing the applied force (the surface hydrophobicity, free sulfhydryl groups), the kinetic and thermodynamic parameters (order of reaction, rate constant, aggregation pathway), properties (turbidity, Zeta-potentials, DSC, SDS-PAGE and rheological properties) and interface properties (foaming and emulsifying properties). Our main results were as follows:The preparation of microgels and its stability Transmission electron microscopy (TEM) was applied to investigate the morphology of microgels. We use network structure as the evaluation standard, which prepared by heating4wt%(protein basis) of SMP solution with pH6.2at90℃for25min, exploring the influence of pH, temperature, the concentration of the protein and heat-treatment time on the aggregation of whey proteins and caseins. We found that the best suitable condition to form microgels is:Heating1wt%(protein basis) of SMP solution with slightly acidic pH conditions (pH5.7-5.9) at90℃for25min. Microgels in pH6.2-6.7range is stable at the room temperature; microgels, in neutral environment, were stable at90℃within5min.Applied force The surface hydrophobicity and free sulfhydryl groups were determined. Compared with normal protein aggregates, the results indicated that microgels have higher surface hydrophobicity, which were2454.14±20.04and2380.667±8.11, respectively. The free sulfhydryl of microgels and normal protein aggregates were reduced from the initial5.17±0.03μmoL SH/g to4.14±0.03μmoL SH/g and4.81±0.08μmoL SH/g, respectively. It can be seen from the result that the surface hydrophobicity and disulfide bonds have played an important role in the progress of forming microgels. The protein aggregates from different aggregation pattern owned different values of surface hydrophobicity and the free SH, especially, the values of microgels were between soluble and insoluble proteins aggregates at the same heat-induced conditions.Kinetic parameters for whey protein with casein micelles The amount of whey protein associated on casein micelles was determined, and the results indicated that there are obviously differences in process of forming microgels and conventional protein aggregation. The apparent reaction rate constant kn [(gL-1)1-nS-1] of whey protein at pH6.2is increased by approximately4times than conventional milk protein aggregation in the formation process of microgels, At the same time, the reaction order of whey protein was increased from1.36to1.64as the pH of the milk was decreased from pH6.7to6.2. In addition, we proposed a model of microgels aggregation pathway: the microgels could be prepared by self-limitation formation of intermediate aggregates of whey proteins and casein micelles.Properties The turbidity, Zeta-potentials, DSC, SDS-PAGE and rheological properties were determined. Compared with the normal protein aggregates, the results indicated that microgels have higher turbidity, denaturation temperature and lower Zeta-potential. After heating, the dispersion were centrifuged to obtain different levels of protein aggregations, which was analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis and rheological properties, and the results indicated that the quantity of whey protein of microgels was in the middle of soluble proteins and insoluble proteins aggregates and microgels have good rheological properties, e.g. apparent viscosity and flow behaviors.Interface properties The foaming and emulsifying properties were determined. Compared with the conventional milk protein aggregation, the results indicated that microgels have lower foam property, to the contrary, there is obviously promotion in foaming stability with silent period prolonged. Compared with the solution protein, the foam stability of microgels was higher by approximately4times when the foam films silent6h. The same behaviors of microgels in the emulsification properties, compared with the normal protein aggregates, the emulsifying stability of microgels was improved by1.5times when the solution silent30min. It showed that the characteristics of the molecular structure of microgels are beneficial to improve emulsion stability and foaming stability of milk protein.