Microrheology Of Typical Food Emulsions

Food emulsions are typically complex fluids consisting of multiple ingredients and multiple phases with structure and dynamics of various characteristic lengths and time scales.Their macroscopic properties,e.g.stability and texture,are closely related to the structure and rheological properties at the microlevel.Therefore,the understanding of the microstructure and the local dynamics properties of such systems is of great importance for food scientists and has attracted considerable attention.Microrheology is an emerging technique which probes the local viscoelastic properties and dynamics of soft materials by observing the motion of tracer particles embedded within them.It has great potential application in the study of complex fluids because of its advantages over traditional rheology,such as smaller sample requirement,wider measurable frequency range,and particularly,the ability to examine the spatial heterogeneities of inhomogenous samples at the microlevel.Inthispaper,by using particle tracking microrheology and bulk-rheology techniques,the effect oil phase concentration,droplet size and tracer particle size on the rheological behaviors of the typical food emulsions emulsified using Tween80 were studied.In addition,the microrheology and bulk rheology of soybean protein isolate(SPI)and SPI emulsion during the acid-induced gelation process were also studied.The relationship between micro-and bulk-rheology properties of emulsions were discussed.The main conclusions are as follows:1.Effect of oil concentration.At lower oil concentration,the mean squared displacement(MSD)of the tracer particles in emulsions stabilized by Tween80 is linear with lag time with a logarithmic slope eaqual to 1,behaving as in Newtonian fluids,while for at high oil concentration,MSDs are non-linear with logarithmic slopes smaller to 1,behaving as in viscoelastic fluids.The shear modulus from the microrheology show a transition earlier than by bulk rheology.The viscosity form both the microrheology and bulk rheology was fitted to the Krieger-Dougherty(K-D)model.The results show that the droplet undeformed volume fraction (?) is similar,while the criticaldeformationvolume fraction from bulk rheology is smalller than microrheology.The power-low index n PLindicating the shear thinning behavior of viscosity shows that,n PL from bulk rheology decreases gradually as oil concentrationincreases,but n PL from microrheology decreases much more sharply.These results show that the emulsion droplets behave as “hard spheres” when investigated using microrheology while behave as soft droplets when studied using bulk rheology.2.Effect of droplet size and tracer size.At the same oil concentration,the smaller the droplet size,the smaller MSDs,indicatingthestronger viscoelasticity.Bulk-rheology shows the same trends of visocosity with microrheology.For low oil concentration emulsion,the selectedtracer particle size has little impact on the microrheological results,but when the oil concentration increased to 42.5%~45%,at short times,the greater the tracer particle size,the weaker the viscoelasticity of the emulsion.3.Acid induced gelation of SPI solution and emulsion.At the initial stage of acidification,MSDsof tracer particels in both SPI solution and emulsion approximatelyhave a linear relationship with time.With furtheracidification,MSDsshow a platform,and the elastic modulus becomes dominate,showing gelation behavior;the SPI solution has the maximum elastic modulus nearthe isoelectric point but SPI emulsion has the maximum elastic modulus at p H higher than the isoelectric point.As the acidification process continues,the MSDs of tracer particles return to a approximately linear relationship with time,indicatingthe gel structure began to break down;but eventually the residue structuresof the solution and emulsion are different.The microscopic rheological results are consistent with the macroscopic rheological results and complement each other.