Regulation of milk protein solubility by a whey-derived proline-rich peptide product

The effects of a bovine whey peptide product enriched in proline (wPRP) on the solubility of milk proteins were tested under ambient conditions or following heat treatment at 75 and 100 °C, for 1 and 15 min, followed by post-incubation storage at either ambient temperature or 4 °C for up to 7 d. wPRP promoted solubilisation of milk proteins in a concentration-dependent manner without heat treatment and also after heat treatment at 75 and 100 °C, and the effect was enhanced after storage under either ambient or refrigerated storage conditions. Interactions of wPRP and milk proteins were monitored by particle size analysis and tryptic digestion and specifically linked with solubilisation of αS1 casein (αS1-Cn), which supported observed changes in milk protein solubility. The results suggested that wPRP preferably prevented or reversed physical versus covalent protein aggregation, with the relaxation of hydrophobic interactions at 4 °C providing an additive effect. This application of wPRP represents a novel approach to stabilisation of dairy proteins following thermal processing with industrial usefulness yet to be explored.

Comparison of casein micelles in raw and reconstituted skim milk

During the manufacture of skim milk powder, many important alterations to the casein micelles occur. This study investigates the nature and cause of these alterations and their reversibility upon reconstitution of the powders in water. Samples of skim milk and powder were taken at different stages of commercial production of low-, medium-, and high-heat powders. The nature and composition of the casein micelles were analyzed using a variety of analytical techniques including photon correlation spectroscopy, transmission electron microscopy, turbidity, and protein electrophoresis. It was found that during heat treatment, whey proteins are denatured and become attached to the casein micelles, resulting in larger micelles and more turbid milk. The extent of whey protein attachment to the micelles is directly related to the severity of the heat treatment. It also appeared that whey proteins denatured during heat treatment may continue to attach to casein micelles during water removal (evaporation and spray-drying). The process of water removal causes casein and Ca in the serum to become increasingly associated with the micelles. This results in much larger, denser micelles, increasing the turbidity while decreasing the viscosity of the milk. During reconstitution, the native equilibrium between colloidal Ca and serum Ca is slowly reestablished. The reequilibration of the caseins and detachment of the whey proteins occur even more slowly. The rate of reequilibration does not appear to be influenced by shear or temperature in the range of 4 to 40 degrees C.

Thermal stability, mechanical properties and content of bacterial protein layers recrystallized on polyelectrolyte multilayers

The influence of the temperature on the surface topology, layer thickness and density of recrystallized bacterial S-layers from Bacillus sphaericus CCM2177 on polyelectrolyte multilayers in contact with liquid water was investigated. A quartz crystal microbalance with dissipation (QCM-D) was used to monitor the build-up of the polyelectrolyte multilayer and the adsorption of S-layer protein (1600 ng cm-2). The critical temperature (55 °C) at which the S-layer loses its 2-D structure was obtained from atomic force microscopy (AFM) and confirmed by neutron reflectometry (NR) experiments. The process of S-layer denaturation was found to be irreversible. Aggregates of denatured S-proteins resist lower loads than the crystalline nanostructure formed from folded S-proteins. The combination of the QCM-D results with the scattering length density and film thickness (14 nm) obtained from neutron reflectometry studies permitted the estimation of the density of adsorbed S-protein together with the bound water (M = 1.16 g cm-3), the dry protein scattering length density (2.02 × 10-6 Å-2) and the S-protein mass density (1.48 g cm-3). The results confirmed that S-proteins form very loosely packed layers on polyelectrolyte multilayers incorporating a water volume fraction of around 68%.