Effect of pickling temperature and concentration of salt solution on lipid of duck egg yolk

Abstract: Salted duck egg is a kind of Chinese-style pickled egg with a long shelf life. Its pleasant and fragrant taste is preferred by most of the people in China and other Southeast Asian countries. Salted duck egg is normally made by pickling the duck eggs in 20.0%-30.0% NaCl solution at room temperature (30℃) for 30 days. Lipid is one of the important structural and functional compositions for duck eggs; it has a very important impact on the quality of the duck egg, and has close relationship with the nutrition value and flavor and texture of duck eggs. The lipid oxidation also plays a role in the human body health, and is closely related to many diseases of the human body. The prooxidant effect of NaCl will enhance the oxidation of duck egg lipid during pickling. In that case, the objective of this paper was to research the oxygenolysis of the lipid of duck egg. The effects of different pickling temperatures, concentrations of NaCl solution and pickling time on duck yolk lipid oxygenolysis were investigated by using the methods that determined yolk index, lipid content of yolk, saponification value of yolk, thiobarbituric acid(TBARS) value of yolk, fatty acids composition and free fatty acid content in this paper. The result showed that yolk index, lipid content, saponification value of yolk and free fatty acid content all significantly increased, and phospholipid content decreased with the increase of pickling temperature and time (P<0.05). As the concentration of salt solution increased, saponification value of yolk significantly increased (P<0.05), yolk index and lipid content of yolk didn't change obviously, and free fatty acid content decreased because of oxygenolysis. The increase of pickling temperature would accelerate lipid oxidation, and meanwhile could reduce the amount of final products of lipid oxidation. With the concentration of salt solution increasing, the TBARS value of yolk increased. Dynamics analysis showed that the activation energy of initial lipid oxidation reaction decreased as the concentration of salt solution increased, which accelerated lipid oxidation. The changes of fatty acid composition were that the saturated fatty acids content increased, especially those with small molecular weight, and the unsaturated fatty acids content decreased on account of oxygenolysis, in which the decrease of the amount of those with small molecular weight, e.g. myristic acid and palmitoleic acid, was more than those with large molecular weight, e.g. arachidonic acid and linoleic acid. The amount of the saturated free fatty acids and unsaturated free fatty acids both increased after pickling. The increase of the amount of the free fatty acids was because of the oxygenolysis of phospholipids and neutral lipid. The saturated free fatty acids were mainly made of short-chain fatty acids, e.g. caproic acid and octanoic acid, and had little in quantity for the medium-chain fatty acids. Palmitoleic acid and linoleic acid took a greater proportion in the unsaturated free fatty acids, and their contents increased during the pickling period. This paper may provide a theoretical reference for the effect of pickling condition on the oxygenolysis of the lipid of duck egg yolk.