Understanding aqueous solution behaviour of bovine casein using a solvent and salts.

摘要

Proteins are complex macromolecules with subtle structures that can be modified by environmental changes. Proteins respond to their environment in different ways; consequently, the functional properties of a particular protein is a function of its primary, secondary, tertiary and quaternary structures as well as the prevailing solution conditions. It is our goal to understand how different proteins with differing structures behave under a wide range of solution conditions (i.e. temperature, pH, solute concentrations).; Bovine caseins exist as micelles of colloidal dimension in milk, but as caseinate when commercially purified. Both micellar casein (MCN) and sodium caseinate (NaCNT) were chosen as model protein systems because of their structural complexity and functional versatility. Ternary phase diagrams of these two forms of casein were constructed to characterize their solution behaviour at various pHs (6.0, 7.0 and 8.0) and temperatures (4, 22, 40 and 60°C) using ethanol and (NH4)2SO4 as solutes. Casein micelles and sodium caseinate formed a large variety of morphologies. The fine structure of various casein morphologies were investigated by scanning electron microscopy (SEM), and were related to their rheological parameters (G', G″ and tandelta) measured by dynamic mechanical analyzer (DMA). The ethanol-induced gels produced by casein micelles were more transparent and more elastic than was observed with sodium caseinate. It is believed that this was a result of micelles gels forming via a self-destabilization mechanism whereas sodium caseinate gels formed through a demixing/aggregation mechanism. The salt behaviour of the two caseins also differed significantly: sodium caseinate formed a salt-induced gel whereas micellar casein only produced as paste or precipitate. The aqueous salt solution behaviour of casein was also examined in detail since it was observed in the TPD studies that the addition of moderate (NH4)2SO 4 concentrations led to micelle dissociation. Dynamic light scattering (DLS) analysis indicated that no submicelles were formed in this disintegration process. Intrinsic viscosity measurement using rolling ball viscometer and secondary structure measurement using circular dichroism (CD) showed that caseins retain their secondary structure even in this dissociated state.; The solution behaviour of caseins (MCN vs. NaCNT) was compared with that of other previously studied proteins, i.e. ovalbumin, beta-lactoglobulin, bovine serum albumin (BSA) and type B gelatin, which were examined at pH 7.0 and three different temperatures (20, 40 and 60°C). The solution behaviour of MCN is more reminiscent of globular proteins like ovalbumin whereas sodium caseinate more closely resembles BSA. The resulting large database of solution behaviour information that has been amassed in this project will ultimately lead to a more global picture of food protein functionality. These studies of model proteins will also help us develop more efficient methods to separate or combine proteins with other ingredients to create innovative foods.