Abstract: This study aims to mitigate the deterioration of egg yolk quality caused by freezing-thawing treatment. The functional and physicochemical properties of frozen egg yolk samples thawed by four thawing methods such as standing at room temperature, immersion in warm water, ultrasonic and microwave were investigated and compared with those of fresh egg yolk. Furthermore, under various thawing conditions changes in protein structures were explored by Raman spectroscopy. The results indicated that natural thawing brought about the highest turbidity in egg yolk, the lowest C=C stretching vibration intensity of lipoprotein alkyl chains as indicated by Raman spectroscopy, and the highest percentage of β-sheet, demonstrating the highest degree of protein cross-linking. The protein solubility, emulsifying activity and emulsion stability of egg yolk thawed in warm water bath were significantly lower than those of egg yolk thawed by ultrasonic treatment (P < 0.05). Among the four thawing methods, the surface hydrophobicity of egg yolk thawed in water bath was the strongest. The disulfide bond content of proteins in egg yolk thawed by microwave treatment was significantly higher than that in egg yolk thawed by ultrasonic treatment (P < 0.05). The highest asymmetric C-H bending vibration intensity of aliphatic amino acids and the lowest proportion of α-helix structure were observed in the microwave-thawed sample, together with the formation of microgel particles. By contrast, the ultrasonic-thawed egg yolk was characterized by superior emulsifying activity and emulsion stability as well as the smallest particle size and the most concentrated distribution. Moreover, the hardness, cohesiveness and viscosity were low, being not significantly different from those of fresh egg yolk (P > 0.05). In conclusion, ultrasonic thawing is a suitable thawing method for frozen egg yolk.

Key words: egg yolk; thawing methods; functional properties; physicochemical properties; Raman spectroscopy