Structural analysis of proteins in thermally induced ovalbumin and egg white gels
Creators: name: Qin, Ling
Publisher: name: University of British Columbia
Year: 2009

description: Hen egg white proteins have been extensively utilized as ingredients in food processing because of their high nutritious value and unique functional properties. The ability to form a gel upon heating makes them key ingredients in many food products. Three factors including pH, ionic strength and protein concentration play very important roles in determining the structure and thus the properties of egg white gels. In this project, the relationship between the structure and functional properties of pure ovalbumin and egg white before and after gelation under different pH and ionic strength conditions were studied. Egg white is a complicated system containing many different kinds of proteins, among which ovalbumin is a major component. Therefore, the structure and function relationships of thermally induced pure ovalbumin gels were studied first as a representative of egg white gelation. Raman spectroscopy was the major technique used in this project to study the protein structure changes of ovalbumin and egg white after thermal gelation. Other structure and functional properties of ovalbumin and egg white proteins before and after heat-gelation were also studied, including gel strength, turbidity and solubility, as well as the charges of protein molecules, interchange between SH groups and SS bonds and protein hydrophobicity difference before and after heating. Depending on the conditions of the heating medium, ovalbumin and whole egg white formed transparent solutions, transparent gels, opaque gels or turbid gels after thermal treatment. Generally, the transparent solution and transparent gels were only formed by heating the protein solutions at pH distant away from the isoelectric point and very low salt concentration or even without salt. As the salt concentration increased, the turbidity of the gels increased as well and eventually the opaque gels were formed. The turbid gels were formed by heating ovalbumin or egg white solutions at pH close to their isoelectric point regardless of the ionic strength. The strength of the heat-set ovalbumin and egg white transparent and opaque gels was much higher than that of the turbid gels. The results of Raman spectroscopy showed that conformational changes of ovalbumin and whole egg white proteins upon thermal gelation involved the formation of stable intermolecular P-sheet structure. In other words, the heat-denatured ovalbumin and egg white proteins did not take on a random-coiled structure but rather retained a considerable amount of secondary structure, with the exposure of some hydrophobic regions to the solvent environment. The solubility results of gels indicated that thermally induced ovalbumin and egg white gels were mainly formed by noncovalent crosslinks such as hydrophobic and electrostatic interaction or hydrogen bonds. Formation of thermally induced disulfide bonds was also involved during gel formation, especially in egg white gels. The charges measured as the zeta potentials showed that the charges of ovalbumin and egg white protein solutions were high at pH 3.5 or 7.5 and low at pH 5.5. The sulfhydryl groups and disulfide bonds determined by Ellman's or NTSB reagent showed the decrease of sulfhydryl groups and increase in disulfide bonds during heating, indicating the sulfhydryl/disulfide interchange or the oxidation of the sulfhydryl groups. The investigation of hydrophobicity by ANS fluorescence probe demonstrated a large increase of hydrophobicity of ovalbumin and egg white proteins after heating. This result suggested that heating caused the exposure of some hydrophobic regions to the solvent environment and the hydrophobic interactions were the main reason for gelation.