Effect of Ultrasound-Assisted Chitosan Treatment on Emulsification Properties of Liquid Egg Yolk and Structure of Egg Yolk Proteins

In order to develop special liquid egg yolk for making of egg tart, the effects of ultrasound-assisted chitosan treatment on the emulsification properties of liquid egg yolk and structure of egg yolk proteins were investigated in this study. The emulsification properties of fresh egg yolk, egg yolk added with chitosan, heated egg yolk, heated egg yolk added with chitosan, ultrasound-treated egg yolk and ultrasound-assisted egg yolk added with chitosan were evaluated by measuring emulsifying activity, emulsion stability and particle size, as well as microscopic observation. Egg yolk protein structure was characterized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), Fourier transform infrared (FTIR) spectroscopy, and fluorescence spectroscopy. The results showed that compared with the untreated samples, ultrasound-assisted chitosan treatment increased the emulsifying activity of liquid egg yolk by 29.51% and the emulsion stability by 9.47%, decreased the particle size by 50.01% and the interfacial tension by 16.67%, and smoothened the interfacial tension curve. The combined treatment reduced the size of emulsion droplets and increased the dispersivity of egg yolk particles as observed by scanning electron microscopy (SEM). The intensity of protein bands in SDS-PAGE was decreased after the combined treatment, which resulted in protein unfolding and exposure of hydrophobic amino acid residues and increased the surface hydrophobicity of liquid egg yolk increased by 12.91%. Moreover, the relative contents of α-structures and β-structures in proteins were reduced after the treatment and the relative content of random coil was increased by 5.26%, indicating that ultrasonic-assisted chitosan treatment led to more disordered and loose protein structure, and the increase of hydrophobicity could promote the emulsifying properties. In summary, this study demonstrates the great potential of ultrasound-assisted chitosan treatment to enhance the emulsification properties of liquid egg yolk for baking purpose.