Fabrication, Characterization And Application Of Soy Protein-based Pickering Stabilizers

In recent years, Pickering emulsions stabilized by food-grade particles have attracted increasing interests in the food industry, due to their great potential to be applied in functional food formulations. As compared with non-protein Pickering stabilizers, those from food protein have the following advantages:(1) they do not need any chemical modification;(2) they are compatible with high-pressure emulsification. However, the development of protein-based Pickering stabilizers is very limited. Based on this, the present work was to investigate the potential of soy proteins to be developed into a kind of effective Pickering stabilizer for oil-in-water emulsions, as well as the potential applications of these emulsions in the encapsulation and delivery of lipid soluble bioactive compounds.First, the effects of a pre-heat treatment on the emulsifying efficiency of soy proteins(including soy protein isolate, SPI and soy glycinin, SG) and properties of the resultant emulsions were investigated. As expected, the preheating significantly improved the emulsifying efficiency of soybean protein. Irrespective of the preheating, at a specific oil fraction(?) increasing the protein concentration(c) resulted in a progressive decrease in droplet size; increasing the energy input level of emulsification improved the emulsification efficiency. Besides, the preheating treatment significantly improved stability of the resultant emulsions against coalescence at comparable conditions. On the other hand, it was indicated that the emulsion stability against creaming was more favorable at higher ? and/or c values. The improved creaming stability was largely associated with the formation of gel-like network in the system. This could be further corroborated by the observation that for the emulsions stabilized by preheated soy proteins, increasing the energy input level of emulsification led to enhanced creaming stability. All the observations suggested that these emulsions stabilized by soy proteins, especially those preheated, exhibited similar behaviors to many classical Pickering emulsions. To prove this point, dynamic light scattering and atomic force microscopy analyses confirmed that most of particles in unheated or preheated soy proteins were present in the nanoparticle form with sizes of ~100 nm; the nanoparticles in heated soy proteins showed a strong integrity of structure that involved both hydrophobic interactions and disulfide bonds. This indicates that soy proteins exhibited a high potential to be formulated as a kind of effective Pickering stabilizers, through a simple heating treatment.Next, adsorption and packing of these Pickering nanoparticles at interface were characterized. At low c values, these Pickering emulsions were dominated by limited coalescence, from which the surface coverage and packing model could be estimated. The results indicated that these Pickering nanoparticles were packed at the interface at a relative low surface coverage of 32.5-41.7%. For these nanoparticles, the adsorption at the interface was highly dependent on the c. On the other hand, it was indicated that the presence of increasing concentrations(0-500 mM) of NaCl was remarkably favorable for their emulsification performance, adsorption and even interfacial packing at the interface, suggesting the improvement of emulsification and interfacial properties by electrostatic screening.Last, the potential of these Pickering emulsions to perform as an encapsulation and delivery system was investigated. Rheological and microstructural observations indicated that the rheological behavior and microstructure of these Pickering emulsions could be well modulated by variation in the c, ? and even the energy input of emulsification. The rheogical behavior of these emulsions was mainly determined by the extent of droplet flocculation. When the extent of droplet flocculation in the system was high enough, a gel-like network could be formed. In the case of gel-like emulsions, e.g., stabilized by preheated SG, the gel stiffness progressively increased with increasing the c and/or ?, and the presence of appropriate concentrations of NaCl was favorable for the gel-network formation. Furthermore, it was interestingly indicated that these highly flocculated or gel-like Pickering emulsions exhibited good oxidative stability, improved stability and a sustained-release behavior of encapsulated bioactives(e.g., β-carotene).In summary, soy proteins exhibit a great potential to be developed into a kind of effective food-grade Pickering stabilizers. These Pickering stabilizers exhibited excellent emulsification and interfacial properties, and could be applied to produce Pickering emulsions with unique characteristics, e.g. enhanced oxidative stability and sustained-release behavior. The findings are of importance not only for the fabrication of a kind of novel food-grade Pickering emulsions, but also for understanding the molecular mechanism of emulsifying properties of soy proteins.