Study On The Interactions Between Particles In Protein Solution And Their Applications In Food

Food is a complex system composed of multiple components such as proteins and polysaccharides.In Principle,the interactions between the components in food regulate the architecture and mechanic properties of products.The complexity of components in food dramatically increases the difficulty of studying the interactions between these components.Since different level structures?such as metal salts,polysaccharides,colloidal particles,etc.?are common in food,especially heavy metals as the big issue in food safety,and colloidal particles serving either as one of the fundamental units in food,plus proteins as a critical component in foods due to its versatility,the pictures of the interactions between heavy metal ions/colloidal particles and protein could shed light on the theoretical works and food process design.Meanwhile,the relative high concentration of polysaccharides in food emphasizes the importance of interactions between polysaccharides and protein in order to rationalize food process design.Based on this realization,this study has been carried out from the following aspects:Firstly,the heavy metal salts YCl₃ and?-lactoglobulin?BLG?were picked up as a model to study the interactions between heavy metals and food protein.Salt concentrations of YCl₃,pH,the dilution factor and its order have been probed and their influences on the interactions between YCl₃ with BLG were studied by means of static light scattering,fluorescence spectroscopy,synchronous fluorescence,ultraviolet spectroscopy and Zeta potentiometry.The second virial coefficient?A₂?and the molecular weight?M_w?of the protein at different concentrations of YCl₃ were measured by static light scattering.The results show that the addition of YCl₃ to A₂ changes from greater than 0 to less than 0.As the salt concentration increases,the absolute value of A₂?A₂<0?decreases,indicating that the intermolecular force of YCl₃ is changed from repulsive to attractive,and the attraction decreased with the further increase of salt concentration.Fluorescence data indicates that the amino acid residues of the protein molecule are hydrophobic in the microenvironment,and the site of action is located near the tyrosine residue.The circular dichroism structure showed that the secondary structure of the protein almost kept intact,which indicated that the binding of YCl₃ to BLG was a specific positional binding.The decrease of the r value of the ultraviolet second-order derivative spectrum further verified the interaction between YCl₃ and BLG,indicating that the environmental polarity where the amino acid residue stayed was slightly reduced.The protein surface portion Y³⁺can be desorbed to some extent by adjusting the pH and the concentration ratio between YCl₃ and protein.Then,the polysaccharides?Ficoll-70,Dextran-70?and the proteins?BLG,BSA,lys, OVA,pepsin?were used to establish a crowded environment for protein molecules due to the existence of polysaccharides.The interactions between these two types of biopolymers were characterized ultraviolet spectroscopy,fluorescence spectroscopy,synchronous fluorescence,and fluorescence quantum yield.The results of UV spectroscopy indicated that due to the crowding effect of polysaccharides the microenvironment of the amino acid residues of the protein molecules changed and the environment of polarity was slightly enhanced while the results of fluorescence spectroscopy,synchronous fluorescence and quantum yield showed the protein conformation changed.With the increase of polysaccharide concentration,the polarity of the microenvironment of amino acid residues increased,although the extent of these changes depended on the protein species.Then DWS diffusion wave spectroscopy?DWS?technique was applied to monitor the fluid behavior of the polysaccharide-protein mixture system from the microscopic point of view.The electric field autocorrelation fucntions,mean square displacement?MSD?,viscoelastic modulus?G^',G”?,complex modulus?G^*?,complex viscosity,etc.of the mixed system of polysaccharide and protein were attained.Lastly,three kinds of colloidal particles?polystyrene microspheres,polystyrene microspheres-carboxyl group?and common proteins?BLG?BSA?lys?OVA?in food were used,and the viscosity of protein solution was measured by dynamic light scattering and static light scattering.The results of dynamic light scattering and Zeta potential indicate that the colloidal particles interact with the protein.Under neutral conditions,colloidal particles with more negative charge on the surface of the aggregate with the positively charged proteins.The results of static light scattering showed that there was no significant difference in the radius of rotation R_g with increasing protein concentration.Selecting the appropriate colloidal particles to measure the decay time?of the Brownian moving particles in the system by dynamic light scattering in combination with the Stokes-Einstein equation,the viscosity?of the solution was obtained.In order to verify the feasibility of the method,the viscosity of the pure water was measured in accordance with the literature values.The protein solution viscosity values obtained by microrheology were real and effective compared to ones obtained by normal techniques of viscosity determination in several protein solutions.