Abstract
The aim of this work was to study the gelling behaviour and gel properties of selected mixed protein systems, namely soya protein isolate with whey protein isolate, and deamidated wheat protein with salt-soluble meat proteins. Initially the four protein isolates used (soya isolate PP 500E, Bipro whey isolate, soluble wheat protein, and salt-soluble meat proteins) were characterised for protein composition, surface and exposed hydrophobicity and thermal stability. On heating, the three commercial isolates did not show any conformational transitions by Differential Scanning Calorimetry (DSC) but the salt-soluble meat proteins denatured at around 55°C. Studies of the individual protein isolates using phase contrast and transmission electron microscopy (TEM) as well as small and large deformation rheological techniques, showed that the most fine-stranded gels - those of Bipro whey isolate and soluble wheat protein - had high storage and shear moduli as well as high fracture shear stress and strain values but minimal syneresis. In comparison, gels with a more open network structure - those of the salt-soluble meat proteins - had a lower storage modulus and water-holding capacity. The gels with a randomly aggregated structure - those of soya isolate PP 500E - had low storage modulus but high shear modulus values and low water-holding capacity. Studies on the mixed protein gelling systems, using similar rheological and microscopic techniques, showed that soya isolate PP 500E - Bipro whey isolate mixtures phase separated with a phase inversion at a 5:1 ratio. Thus, due to differences in the gelling mechanism of the two isolates, Bipro whey isolate formed the continuous phase even when it was the minor component by weight. Soya isolate PP 500E gelled below its minimum gelling concentration (< 14 % w/w) by the addition of small amounts (1 % w/w) of Bipro whey isolate which were sterically limited into small inclusions. The resultant increase in the effective concentration of Bipro whey isolate caused the gel to be more elastic than the typical soya isolate PP 500E colloidal aggregated gel. Mixtures of the 11S globulin - glycinin - (which was 70 % (w/w) of soya isolate PP 500E), isolated from the native soya bean, and B-lactoglobulin (which was 70 % (w/w) of Bipro whey isolate) phase separated. However, mixtures of these native proteins formed larger phase domains when compared with the commercial isolate mixtures. By using isolates which had been processed to different degrees this phenomenon could be exploited to gain the optimum phase dimensions for a product. Gels formed in the autoclave (120 C, 2 atm. ), from low ratios of soluble wheat protein (1 % w/w) to salt-soluble meat protein (19 % w/w) with added calcium (20 mM), had higher values of shear modulus and storage modulus, when compared with those obtained from salt-soluble meat protein gels (20 % w/w) with added calcium. The mixed gel was also more elastic and less prone to syneresis than salt-soluble meat protein gels. Electron microscopy indicated that the soluble wheat protein was involved in binding the fibrous network strands of the salt-soluble meat protein gel. In general, for both the single and mixed protein systems studied, the rheological behaviour of gels were directly related to their structure. The addition of small amounts of one protein to another had the greatest effect on the structure and rheological properties of the resultant mixed gel. From this it was concluded that if a protein is present in small amounts it will be dispersed throughout the system forming pockets of concentrated protein dispersions thereby increasing gel strength. When more of the dispersed protein is added it self-associates and this is likely to de-stablise the system.