Effect of Heat Treatment on the Quantitative Detection of Egg Protein Residues by Commercial Enzyme-Linked Immunosorbent Assay Test Kits

This study examined the changes in the solubility of egg proteins as affected by different heat treatments and compared the performances of three commercial test kits for the quantitation of protein residues in heat-treated samples. National Institute of Standards and Technology (NIST) whole egg standard reference material #8415 and Henningsen spray-dried whole egg powder were subjected to heating in the presence of water at 60 and 100 degrees C, autoclaving for 5 or 10 min, or dry heating at 60-400 C for 10 min. The amount of protein in the heated samples was assayed using the bicinchoninic acid total protein assay as well as egg-specific commercial enzyme-linked immunosorbent assay (ELISA) kits. Elevated heat resulted in a lower level of proteins extracted. Neogen's Veratox kit, which is reactive to multiple proteins in egg, greatly underestimated the amount of residual proteins in the boiled or autoclaved samples. Tepnel BioSystems' Biokits assay, which employs antibodies specific to a heat-stable marker protein (ovomucoid), registered a higher level of protein in these samples. Both test kits substantially underestimated the amount of residual proteins in samples dry-heated at temperatures >176 degrees C. The Morinaga test, using an improved extraction buffer, registered the highest level of protein in the heat-treated NIST samples but not the Henningsen samples. The underestimation by the commercial test kits was attributed to changes in the immunoreactivity of residual proteins after heat treatments and not the differences in the amount of protein extracted. These results suggest that thermal processing may affect the quantitative analysis of allergens and needs to be taken into account in the validation of commercial ELISA test kits.