Hard yolk characteristics and mechanism of salted duck eggs

Hard yolk (HY), commonly found in cooked, salted eggs, has recently become a great concern in the commercial production of salted eggs because of its negative impact on the taste of yolk. Herein, the chemical and structural characteristics of HY and the underlying mechanism of HY were investigated. Although there were similar nutritional compositions between HY and non-hard yolk (NHY), HY exhibited higher hardness, springiness, gumminess and chewiness than NHY, suggesting the lower acceptability of HY. In HY, a large amount of fibril-like substance was wrapped around the surface of yolk particles, whereas this was not observed in NHY. HY demonstrated higher β-sheet content and surface hydrophobicity but lower α-helices than NHY, indicating that α-helices of yolk protein change into β-sheet. In addition, HY exhibited higher disulphide bonds than NHY (13.5% vs. 11%). A higher level of protein aggregates was observed in HY compared with that in NHY, whereas a reducing agent, dithiothreitol (DTT), decreased the level. Similarly, the addition of tea polyphenols as a reducing agent in the pickling solution reduced HY formation and improved the taste texture of salted eggs. The results of this study show that the occurrence of HY compromised the taste acceptability of salted egg. Disulphide bond–mediated protein aggregation plays an important role in mediating HY formation through which the reducing agent suppresses HY formation. •The hard yolk core negatively affects the texture characteristics and reduce total acceptability of salted duck egg yolk.•Formation of fibril-like substance prevented oil exudation and moisture loss, resulting in development of hard yolk core.•The protein aggregation through disulphide bond contributes greatly to the formation of fibril-like substance.•Addition of tea polyphenols protects free sulfhydryl against oxidation and suppresses hard yolk core formation.