Thermal and high pressure effects on the structural properties of condensed dairy based systems

Work in this PhD project focused on the structural properties of condensed dairy based systems subjected to thermal and high pressure treatments. Proteins under investigation were whey protein isolate (WPI), bovine serum albumin (BSA) and a mixture of immunoglobulins (Ig) extracted from WPI. The process of protein denaturation, their micro and macro molecular properties and glass transition phenomena were observed with thermal and high pressure processing using modulated and micro differential scanning calorimetry, small amplitude dynamic oscillation on shear and confocal laser microscopy imaging. Theoretical modelling of rheological properties further supported experimental observations of these materials undergoing vitrification. In the first part of this investigation, WPI and BSA at concentrations up to 15% (w/w), in the presence of sugar as co-solute at concentrations between 0 – 65% (w/w), produced mixtures at a maximum level of solids of eighty percent in formulations. <br><br>WPI and BSA thermal denaturation slows down with increasing concentration of the small-molecule co-solute recorded as the mid-point transition temperature (Tmid) of the endothermic event. The transition shifts to higher temperatures indicating that changes in solvent quality retard the thermodynamic processes of protein unfolding and aggregation. Tangible evidence of the changes in protein morphology is provided by microscopy showing protein aggregates of reduced size at high levels of added sugar. Distinct levels of network crosslinking are achieved in high-solid materials by manipulating the thermal regime of protein denaturation, e.g. heating to 85°C for 0, 30 and 60 min at a controlled scan rate of 1°C/min from ambient temperature (25°C). Subsequent controlled cooling at this scan rate exhibited identical molecular relaxations for the native (unheated) and the three thermally denatured protein/sugar systems producing a calorimetric glass transition temperature (<i>T_g</i>) of ~ -48°C. In contrast, network relaxation recorded with dynamic oscillation shows that increased network crosslinking accelerates vitrification phenomena, with the mechanical or network<i> T_g</i> varying from -40°C to 4°C for the native and denatured protein/sugar matrices with increasing intensity of the thermal treatment. <br><br>Concentrated immunoglobulin preparations, i.e. from 60 to 80% solids (w/w), were also examined at atmospheric conditions and following application of a high hydrostatic pressure. Immunoglobulins exhibit pressure stability throughout the experimental concentration range by conserving native conformation, which results in cohesive structure formation observed by thermomechanical analysis and infrared spectroscopy. Application of the free volume theory demonstrates that pressurized immunoglobulin preparations are able to form glassy systems upon cooling at subzero temperatures. This has been attributed to a reduction in polymeric free volume under pressure and the development of an efficient friction coefficient amongst tightly packed particles that link to form a three-dimensional matrix. Pressure treated assemblies of condensed immunoglobulins demonstrate viscoelastic behaviour matching that of the thermally treated counterparts, but retain bioactivity, which is largely lost with thermal treatment. The works suggests opportunities for the utilisation of high-pressure treated immunoglobulins in starch or dairy based formulations of functional foods in an effort to initiate replacement of thermally treated dairy powders with limited biofunctionality.<br>