Properties and microstructure of transglutaminase-set soy protein-stabilized emulsion gels

The development of cold-set protein-stabilized emulsion gels has attracted increasing interests, due to their potential to be applied as a kind of release-controlled carriers, especially for labile lipid-soluble bioactive compounds. This work aimed to elucidate the importance of changing oil volume fraction (ø = 0.2-0.6) for the formation and properties of cold-set soy protein isolate (SPI)-stabilized emulsion gels, induced by microbial transglutaminase (MTGase). The gelation process, mechanical properties, water-holding capacity (WHC) and microstructure of the correspondingly formed gels at various ø values were evaluated. The results indicated that increasing ø progressively increased the storage modulus (gel strength) and WHC, from about 200 to 8000 Pa and 52 to 88%, respectively. The high ø dependence of the gel strength and WHC was closely related to the microstructure of the formed gels. The network of the emulsion gels at high ø values (e.g. 0.6) was particulate in nature with coarse strands mainly composed of compact 'aggregated' oil droplets, while that at low ø values was filamentous or fine-stranded with oil droplets incorporated. The enzymatic treatment resulted in a progressive increase in amount of the proteins entrapped within the network, with highest extent observed at ø = 0.5. The entrapped proteins involved the biopolymers of β-conglycinin subunits or glycinin acidic polypeptides, covalently cross-linked by the enzyme, together with glycinin basic polypeptides. The gelling mechanism at high ø values could be largely related to aggregation or coagulation of protein-coated oil droplets, while that at low ø values, the gels was more like enzyme-set protein gels. These results would be of great help for the understanding of the gelling mechanism and the development of enzyme-set protein-stabilized emulsion gels.