Molecular understanding of high pressure effects on the structural properties and microbial/enzymatic inactivation in condensed globular proteins

Globular proteins are biological macromolecules used extensively in traditional and novel product development via thermal processing. High pressure has the potential to act as a preservation agent for foods and has been applied to protein systems at low levels of solids (< 20%, w/w) for research on gelation or emulsification as well as proteolysis and enzymatic inactivation. However, the effect of high pressure on the molecular properties of globular proteins at high levels of solids (up to 80%, w/w) is not characterised to date. Therefore, the main objective of this Thesis was to illustrate the structural and functional properties of condensed globular protein systems (bovine serum albumin, soy glycinin, ovalbumin and whey protein) subjected to high pressure treatment in relation to:<br>• Stability of secondary conformation against high pressure treatment<br>• Changes in the thermomechanical glass transition of pressurized protein systems <br>• Microbial kill and enzyme inactivation in high solid protein systems following application of high pressure<br>Thermomechanical and X-ray diffraction examination of all globular proteins argues for glassy behaviour at subzero temperatures in high solid preparations (e.g. 80%, w/w) recorded experimentally and modelled theoretically. That was discussed based on the assumption that free volume decreases linearly with temperature, and the glass transition temperature (Tg) was defined as the point where the thermal expansion coefficient undergoes a discontinuity, with thermal motions becoming extremely slow.<br>MicroDSC and deconvoluted FTIR spectra argue that disulphide bonds are involved in the pressure stability of globular proteins, with BSA, which consists of a sequence of seventeen disulphide bridges, maintaining native morphology. Soy glycinin with twelve disulphide linkages shows considerable molecular unfolding at low to intermediate levels of solids (up to 60%, w/w), whereas whey protein with two disulphide bonds in the beta-lactoglobulin molecule is the most affected globular protein under pressure.<br>Additional effects were noted based on the concept of surface hydrophobicity of the globular molecule. Ovalbumin with one disulphide linkage has a hydrophobicity value (So) of about 100, which is within the range of BSA (So ~ 2200) and whey protein (So ~ 35). Its extent of denaturation falls between that of BSA and whey protein indicating that both phenomena, i.e. disulphide linkage and surface hydrophobicity combine to produce the observed behaviour in phase morphology of globular proteins in relation to high pressure treatment. <br>Once the structural and mechanical properties of globular proteins under high pressure were studied, our efforts turned into the survival rate of foodborne pathogens (Staphylococcus aureus, Bacillus cereus and E. coli) and the proteolytic activity of degradative enzymes from Pseudomonas fluorescens strains 73 and 113, with a view to approximating product applications. Use of high pressure (600 MPa for 15 min at room temperature, which are conditions of industrial relevance) resulted in considerable reduction of foodborne pathogens. In accordance with the structural studies, soy glycinin with the highest water holding capacity exhibits increased protease and microbiological activity, as compared to BSA and ovalbumin systems.<br><br>