(this article starts in the fruit shell)

No matter gruel, rice, moon cakes, zongzi or green dumplings, there is no mistake in matching salted egg yolks with red and bright oil. What did salt do to duck eggs, making salted egg yolks so fragrant and oily that immortals exist?

Oil is always there, but in different ways

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first of all, the oil of salted egg yolk certainly does not appear out of thin air during the pickling process. The egg yolk itself is rich in lipids, and the lipid content in the duck egg yolk is about 30% [1].

then why doesn't fresh egg yolk run out of oil? The key lies in the way grease exists.

in the egg yolk, the oil is not free on the surface, but is uniformly dispersed in the aqueous solution in the form of small oil droplets-that is, it forms an emulsion system. It is the proteins and phospholipids in the egg yolk that help stabilize the oil dispersion, which act as emulsifiers. Lipids, phospholipids and proteins together make up the structure of lipoproteins: hydrophobic oils are hidden inside, and the surface is mainly exposed to the relatively hydrophilic side of proteins and phospholipids.

also contains a lot of oil, but the appearance and texture of the emulsion and the separated oil are very different. In particular, the greasy feeling of the oil-in-water emulsion such as fresh raw egg yolk is much lower than that of the separated oil.

add salt and reheat to make an oily egg yolk

fresh egg yolk can be regarded as an emulsion with uniform dispersion of fat, while the salted egg yolk on the table has a lot of oil separated from it, indicating that the original emulsion structure has been destroyed, that is, "demulsification".

the first factor causing the destruction of emulsion structure is naturally the salt that enters the duck egg during curing. On the one hand, salting will reduce the moisture in the egg yolk, on the other hand, it will also change the solubility of protein. With the extension of curing time, the raw egg yolk will gradually solidify and separate oils will begin to appear.

(Salted Duck Egg changes during pickling, the upper row is raw eggs and the lower row is heated. K. M. Lai et al)

however, the heating steps after pickling are also very important for making an oily egg yolk. Heating can further destroy the structure of lipoprotein and release more oil. Some experiments show that after pickling with 25% salt water for 35 days, the oil leakage rate of raw Salted Duck Egg's egg yolk is about 20%, and cooking it can increase the oil leakage rate to about 40% [1].

by the way, just boiling fresh eggs can also increase the oil leakage rate of egg yolks, but the degree is far less obvious than that of cooked salted egg yolks, and can not achieve the effect of oil flow visible to the naked eye.

Why are they duck eggs?

Why are duck eggs always used to pickle salted eggs, while salted eggs are much less? This is because the duck egg yolk has better texture and oil production effect after pickling. The composition of egg yolk is slightly different from that of duck egg yolk, and the structure and pore size of the two eggshells are also different, so the salting effect is also different [2].

Cooking emulsified eggs

eggs are not often used to pickle salted eggs, but egg yolk is a "natural emulsifier" often used in cooking. Extra fat is added to the raw egg yolk and stirred to encourage them to disperse into small oil droplets.

many baking recipes emulsify the melted butter with egg yolk to make it easy for the oil to be evenly mixed with other ingredients. But the most classic example of this is mayonnaise, a thick sauce that relies entirely on egg yolks to emulsify and season vegetable oil. Mayonnaise has a high fat content of more than 70%, but because it is still a "water-in-oil" emulsion system, small oil droplets are uniformly wrapped in the water phase, so you will not see an oil slick and taste as greasy as pure vegetable oil.

(using a blender to stir vegetable oil and egg yolk to make mayonnaise | James Strange)

A few years ago, there was a news that mayonnaise was "made into a bowl of oil" by heating mayonnaise in a microwave oven, which was also a process of breaking milk. Microwave heating destroys the structure of the emulsion and separates the oil.

(mayonnaise separated by oil after microwave heating | screenshot of Weibo)

unpopular salted egg white

people are obsessed with the taste of salted egg yolk, not only eating it directly, but also making it into yolk moon cakes, yolk zongzi, Egg-Yolk Puff, baked pumpkin yolk, but no one always wants to eat Salted Duck Egg's egg white. On the one hand, it must be due to the lack of greasy aroma and rich texture of egg whites, and on the other hand, it is also because salted egg whites are too salty.

Experimental data show that after 35 days of pickling, the salt content of Salted Duck Egg's egg yolk is about 2.25% and 3.29%, while the salt content of egg white soars to 8.23% Mel 11.61% [1]. Egg yolks are far less salty than egg whites for two reasons: first, salt spreads more slowly to egg yolks; second, egg yolks have low water content and high lipids, and salt is difficult to dissolve in grease.

salted egg yolk is far more marketable than salted egg white, but the traditional pickling method is carried out with whole duck eggs, which inevitably leads to the problem of how to deal with the excess egg white. In order to avoid waste, there are also some techniques for separating duck egg yolks and pickling them separately.

in your mind, which way to eat salted egg yolk can make the best use of it?

Resources:

Https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0182912

https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1365-2621.1998.tb15668.x