Tunable Soy Protein-Steviol Glycoside Interactions And Their Relationships With Interface-Dominated Food Systems

It is essential to understand the complex surface behavior of protein-surfactant mixtures, which is closely related to the formation and stabilization of interface-dominated food systems, such as foams, emulsions, and suspensions. Recently, naturally occurring surface-active substances have attracted increased interest in the food fields. In this study, we first found that diterpenoid steviol glycosides(STE), a noncaloric natural sweetener in food products, can be developed into a new type of natural surfactants. The potentials of soy protein-STE mixtures as natural and efficient emulsifiers, foaming agents, and stabilizers for suspensions were then evaluated and investigated. The relation between bulk behavior, surface behavior and macroscopic functional properties was also discussed. These findings would provide the theoretical and technical supports for the development of natural ingredients in functional food applications. The main conclusions are as follows:1. The potential of soy glycinin(11S)-STE mixtures as novel foaming agents was evaluated. In the presence of intermediate STE concentrations(0.25-0.5%), the weak binding of STE with 11 S in bulk occurred by hydrophobic interactions, which could induce conformational changes of 11 S. Accordingly, the strong synergy in reducing surface tension and the plateau in surface elasticity for mixed 11S-STE layers formed from the weakly interacting mixtures were clearly observed. This effect could be explained by the complexation with STE, which might facilitate the partial dissociation and further unfolding of 11 S upon adsorption, thus enhancing the protein-protein and protein-STE interfacial interactions. These surface properties were positively reflected in foams produced by the weakly interacting system, which exhibited good foaming capacity and considerable stability probably due to better response to external stresses. However, at high STE concentrations(1-2%), as a consequence of the interface dominated by STE due to the preferential adsorption of STE molecules, the surface elasticity of layers dramatically decreased, and the resultant foams became less stable.2. The impacts of protein fibrillization modification on protein-surfactant interfacial interactions and foaming behavior were investigated. The nonlinear dilatational rheological behavior and microstructure of the air-water interface was studied by large amplitude(10-30%) oscillatory dilatational rheology and Lissajous plots. The presence of small peptides in 11 S fibril samples resulted in a faster adsorption kinetics than that of native 11 S. The addition of STE, affected the adsorption of 11 S significantly, whereas no apparent effect on the adsorption of the 11 S fibril system was observed. The rheological response of interfaces stabilized by 11S-STE mixtures also differed significantly from the response for 11 S fibril-STE mixtures. For 11 S, the STE reduces the degree of strain hardening in extension and increases strain hardening in compression, suggesting the interfacial structure may change from a surface gel to a mixed phase of protein patches and STE domains. The foams generated from the mixtures displayed comparable foam stability to that of pure 11 S. For 11 S fibril system STE only significantly affects the response in extension, where the degree of strain softening is decreased compared to the pure fibril system. The foam stability of the fibril system was significantly reduced by STE. These findings indicate that fibrillization of globular proteins could be a potential strategy to modify the complex surface and foaming behaviors of protein-surfactant mixtures.3. The formation and stabilization of emulsions stabilized by soy protein isolate(SPI)-STE mixtures were further investigated to evaluate their potential as emulsifiers. Results showed that the behavior of SPI-STE mixtures at the oil-water interface was similar to that at the air-water interface. With the addition of intermediate STE concentrations(0.25-1%), synergistic effects in interfacial tension decays and a plateau in the elasticity for mixed SPI-STE interfaces were clearly observed. The effects should be mainly attributed to the formation of SPI-STE complex, enhancing interfacial protein-protein and protein-STE interactions, thus resulting in the presence of a plateau in the elastic behavior. These interfacial properties were positively reflected in the emulsions prepared by mixtures. The emulsions exhibited a fine formation ability and long-term stability after 120 days, which was believed to be the better response to external deformations. At high STE content(2%), STE dominated the formation of interface mainly by the preferential adsorption of STE molecules.4. To strengthen the effectiveness of resveratrol(RES) as a natural antioxidant in food systems, this work attempted to enhance the water solubility of RES by utilizing the solubilizing properties of STE and investigated the effect of STE-solubilized RES(STE-RES) incorporation on the stability of SPI-based emulsions. The water solubility of RES increased with the increase of STE concentration up to its critical micelle concentration, suggesting the solubilization of hydrophobic RES in STE self-assembled micelles. STE micelles competitively adsorbed at the oil-water interface with SPI, forming a mixed SPI and STE interfacial layer, thus resulting in a decrease in particle size and evident enhancement in the physical stability of SPI-based emulsions. The oxidative stability of SPI emulsions was enhanced, which was believed to be mainly attributed to the targeted migration of RES to the interface during the adsorption of the STE-RES complex.5. The potential of denatured SPI-STE mixtures as novel stabilizers for producing stable nanosuspensions using RES as a model compound was evaluated. The RES nanosuspensions(RESN) were prepared by the nanoprecipitation-ultrasonication method. Below the CMC of STE(0.25-0.5%), the RESN stabilized by the SPI-STE mixtures exhibited outstanding physicochemical characteristics, such as small particle size(below 200 nm) and high yield/entrapment efficiency(above 97%), which should be ascribed to the higher surface activity of mixtures due to SPI-STE complexation and hence faster adsorption on particle surfaces, thus contributing to the decrease in particle size. Meanwhile, STE could function as a filler to fill the defects left by SPI, thus providing a more complete and compact coverage on particle surface. Accordingly, the RESN exhibited a remarkable storage stability and could maintain good redispersibility after direct freeze-drying. At high STE concentrations(1-2%, above its CMC), the formation of RESN was hindered due to the decreased surface activity of mixtures and the presence of STE micelles, which could cause particle surfaces incompletely protected. As a result, the resultant RESN became very unstable and highly susceptible to particle aggregation during storage.