Effects of metal ions on the physico-chemical, microstructural and digestion characteristics of alkali-induced egg white gel

Preserved egg is an alkaline egg product and traditionally relied on the addition of certain heavy metal ions to ensure its quality. To solve the possible safety problem of the heavy metal ions (Cu2+) in preserved egg and improve the pickling process, the effects of alternative ions (K+, Ca2+, Zn2+ and Fe3+) at various levels (0%-0.133%) on the gelation behavior and digestion characteristics of egg white (EW) proteins under strong alkali treatment were investigated. Results showed that multivalent ions promoted the particle size of alkali-induced EW aggregates. As the concentration of K+ increased, the soluble protein content, water-holding capacity, hardness and springiness of alkali-induced egg white gels (EWGs) were superior to those of Ca2+, Zn2+ and Fe3+. EWGs had larger turbidity and rougher microstructures when Ca2+ or Fe3+ was added. The primary intermolecular forces involved in the formation of EWG were ionic bonds and disulfide bonds. Fourier transform infrared spectroscopy (FTIR) analysis showed that strong alkali and metal ions induced the secondary-level structure to interconvert. Adding metal ions increased the gel digestibility. Electrophoresis results showed no significant difference among various bands of the soluble part of EWGs. Principal component analysis showed that four principal components were extracted, explaining 88% of total variance. It was concluded that low concentration of multivalent ions improved the performance of gels. K+ had a significant enhancement on gel properties, while Ca2+, Zn2+ and Fe3+ were more conducive to the formation of polymers, thereby affecting the properties of alkali-induced EWGs by forming or destroying intermolecular forces.