Heat-Induced Changes Occurring in Oil/Water Emulsions Stabilized by Soy Glycinin and β-Conglycinin

Glycinin and beta-conglycinin are the two major proteins in soy protein isolate, and their emulsifying behavior was the subject of this study. These proteins form a thin layer of 30-40 nm when adsorbed at the interface. Microcalorimetric experiments showed that the thermal transitions of these proteins in the emulsion were very similar to those of the proteins in solution. The results also suggested that molecular rearrangements occurred during adsorption of beta-conglycinin, as an endothermic transition peak appeared at high temperature when this protein was present at the interface. In general, beta-conglycinin exhibited greater emulsifying activity than glycinin, confirming previous reports. Heating at 95 degrees C for 15 min caused a decrease in solubility of glycinin, and interactions between the oil droplets, with an increase in the apparent viscosity, shear thinning behavior, and droplet particle size distribution of the emulsions. While, similar behavior was noted in b-conglycinin after heating at both 75 and 95 degrees C. Furthermore, the order of processing affected the subunits' composition at the interface. Heating the solution before emulsification caused a higher protein load at the interface and with all of the subunits present. On the other hand, when heating was carried out after homogenization, the basic glycinin polypeptide and the beta subunit of beta-conglycinin were absent from the interface, suggesting that heat-induced complexes between these subunits formed and remained soluble in the unadsorbed phase.