Study On The Emusifying Properties Of Soy Protein-Polysaccharide Complexes

Proteins have been used as emulsifiers in food products for many years.The stability of protein contained oil-in-water emulsions depends strongly on the density and structure of the protein adsorption layers on drop surface.The protein adsorption layers prevent the drop-drop coalescence by stabilizing the emulsion films.However,protein-stabilized emulsions are highly sensitive to environmental stresses such as pH,ionic strength and temperature.When the pH approaches the isoelectric point of protein and/or salt concentration is higher in emulsion,the electrostatic repulsion of the protein adsorption layers decreases and therefore coalescence and creaming happen.When emulsion is subjected to heat treatment,for pasteurization or sterilization purposes,aggregation happens because of the denaturation of protein which holds the droplets together.Proteins of plant origin do not function effectively at acidic pH environments since they precipitate at pH values close to their isoelectric point.Most foods and beverages are acidic. Therefore,the poor emulsion stability at isoelectric point limits protein application in food and beverage industries.Among proteins of vegetable origin,soy protein is an abundant byproduct of soybean-oil industry and has good functional properties for food processing because of the high nutritional value and the contribution to food texture and emulsifying properties.However,to the best of our knowledge,the long-term stable emulsions stabilized by soy protein at acidic conditions have not been reported.This thesis studied the emulsifying properties of acid soluble soy protein(ASSP)-dextran conjugates at neutral pH.Two natural biomacromolecules,ASSP and dextran,are used to prepare the conjugates through the Amadori rearrangement of the Maillard reaction.The effects of the reaction time with different weight ratios of ASSP to dextran(WR) were investigated and the resultant conjugates were characterized by SDS-PAGE,turbidity,and light scattering.The conjugates can greatly improve the solubility/dispersibility of ASSP in the pH range of 4-6.Ultrasound emulsification was used to prepare emulsions.The effects of environmental stresses,such as pH,ionic strength,heating and storage,were investigated by light scattering.The emulsions prepared from WR 1:9 conjugate having submicron droplets are stable against heat treatment,long-term storage,the changes of pH and ionic strength.The emulsions were characterized withζ-potential and atomic force microscope. The bulky hydrophilic chains of dextran on the droplet surface greatly decrease the movement of the droplets as they were subjected to the electric field,which leads to the reduction of theζ-potentials.AFM result reveals that the ultrasonication can destroy the original aggregates in the conjugate solution and induces the formation of new aggregates. In the ultrasonication process,the increases of surface activity and aggregation of protein result in a formation of stable oil-water interface films.It was further verified that using protein-polysaccharide conjugates and ultrasonication emulsification is a versatile method for preparing stable emulsions.We also investigated the emulsification capability and stability of WR 1:9 conjugate at acidic pH.Compared with the emulsifying at neutral pH,the conjugate exhibits poorer emulsification stability.The reason is unknown at this stage.