Thermally Aggregation Behaviors' Interfacial And Emulsifying Properties Of Soy Protein

Heat treatment, a common process in the food industry such as spray drying andthermization, has been widely applied to modify structural and functional properties of foodproteins. Protein denaturation is usually accompanied by heat-induced protein aggregation,which is believed to play an important role in the relationship between protein structure andits functional properties. However, to date, protein aggregation behavior have not beencharacterized to understand this mechanism. In this study, two aggregation behavior ofprotein, including amorphous aggregation and fibrillar aggregation, were triggerred byheating soy proteins at different pH and temperature. Heat-induced structural changes ofprotein as well as the effects of protein aggregation on interfacial and emulsifying propertieswere investigated, which will provide the theory basis for the development of soy proteinproducts. The main conclusions are as follows:1. Soy proteins were heated at neutral pH (pH7.0) to induce the formation of amorphousaggregates. Moreover, to explain the difference in aggregation behavior occurred at differenttemperature, the structural changes of protein were investigated using some technologies suchas small-angle X-ray scattering experiment. There were obvious differences in the unfoldingdegree, size and molecular interaction of heated proteins at90and120°C. Heat treatment at90°C induced an increase in surface hydrophobicity due to partial unfolding of protein,accompanied by the formation of aggregates linked by disulfide bond. In contrast, soy proteinheated at120°C exhibited more flexible conformation and high free sulfhydryl group, even ifsome exposed hydrophobic groups are involved in the formation of aggregates. Some free SHmay be buried in the interior of these aggregates, leading to the inhibition of disulfide bondformation. Moreover, the aggregation degree of heated protein increased with increasingprotein concentrations.2. We evaluated the influence of heat treatment (90and120°C) on interfacial properties(adsorption at the oil-water interface and dilatational rheology of interfacial layers) of soyprotein isolate using dynamic drop shape analysis combined with oscillating drop technique.The structural and interfacial properties of soy protein depended strongly on heatingtemperature (90and120°C) and aggregation degree of protein. The SPI heated at120°Cexhibited higher surface activity due to more flexibility and surface hydrophobicity withcompared to native protein, as evidenced by higher diffusion rate and faster increase ofsurface pressure. At long-term adsorption, a shorter time was needed to reach relativeequilibrium adsorption for heat-treated proteins because of their large size. In addition, a weakly dissipative viscoelastic system at the interface and primarily elastic in nature wereconfirmed for soy protein. Heat treatment at120°C led to rapid development ofintermolecular interactions in the adsorbed layer, as evidenced by faster increase ofdilatational modulus. It is speculated that multilayer formation occurred for heated protein at asurface pressure (π) higher than the equilibrium surface pressure. The increases in aggregationdegree of heated protein decreased the adsorption rate, surface pressure and surfacedilatational modulus, resulting in the rearrangement of adsorbed primary layer and multilayerformation.3. Native and heated proteins were used to stabilize the oil-in-water emulsion, and theeffects of ionic strength, aggregation degree of protein, and additional treatment onemulsifying properties were evaluated. The particle size, microstructure, and creamingstability of emulsion depended strongly on ionic strength. At100mM, heated proteinemulsion, especially protein heated at120°C, exhibited strong salt resistance, as evidencedby the reduction in flocculation degree. Heat treatment led to the increase in the surfaceprotein concentration, pH resistance, and freeze-thaw stability, which may be related to theformation of multilayer formation. Protein heated at120°C had better freeze-thaw stability.4. The amyloid-like fibrillation of soy β-conglycinin subunits (α, α' and β) and glycininsubunits (acidic and basic subunits) upon heating (0–20h) at85°C and pH2.0werecharacterized. The fibrillation of all subunits was accompanied by the progressive polypeptidehydrolysis. Soy β-conglycinin exhibits a higher capacity to form amyloid fibrils than glycinin.The hydrolysis behaviors, fibrillation kinetics and morphology of amyloid-like fibrilsconsiderably varied among α, α' and β subunits, which appeared to be associated with thedifferences in the amino acid composition and typical sequence of peptides. The disruption ofthe α and α' subunits, particularly their extension region, may play an important role inaffecting the rate of structural changes in fibril formation. Heating at pH2.0led to fibrilformation of acidic subunit of glycinin with the appearance of lag phase during fibrillation.Besides, the disruption of ordered structure of β-conglycinin fibrils occurred upon furtherheating (6–20h) due to extensive hydrolysis, which was called the―fibril shaving process‖.Overall, the fibrillation of β-conglycinin followed a multiple steps including polypeptidehydrolysis, assembly to amyloid structure, and growth into macroscopic fibrils with fibrilshaving process.5. The influence of fibrillation of soy protein on its interfacial properties (adsorption atthe oil-water interface and dilatational rheology of interfacial layers) were evaluated. Heattreatment at pH2.0led to the enhancement of protein surface activity and strength of protein adsorbed layers at the oil-water interface, as supported by the increase in pressure increasesand interfacial modulus of soy protein. The formation of multilayer was speculated accordingto the results of dilatational rheology of interfacial layers. Protein fibrillation increased theparticle size and pH resistance of emulsion, and decreased surface protein concentration, saltresistance, and freeze-thaw stability.