After peeling some preserved eggs, the egg whites will have many off-white crystals that resemble pine flowers. Rumor has it that preserved eggs with pine flowers are more nutritious, while others say they are contaminated by heavy metals. Which is true?

How are preserved eggs made?

Preserved eggs are a uniquely Chinese seasoned egg dish. Traditionally, they are made by mixing medicinal ingredients with rice bran and mud, then wrapping them around duck eggs. After storage, the eggs change: the yolk solidifies into a jelly-like consistency, while the egg whites turn a translucent dark brown.

Why do preserved eggs solidify?

Because of these white, needle-like crystals, preserved eggs are also called pine flower eggs. The most basic method of making preserved eggs is to place duck eggs in an alkaline solution (lime mud or sodium hydroxide) and rice bran and store them for a period of time.

The principle is that alkali penetrates the egg, denaturing the protein and forming a gel. This is the visible coagulation of the egg white and yolk in preserved eggs. The alkaline substance then breaks down the protein to produce amino acids, which are the source of the unique flavor of preserved eggs and inhibit bacterial growth.

Pine flower crystals are amino acid salts.

Continuing the above process, the amino acids produced by protein breakdown encounter the alkaline substance that penetrates through the pores of the eggshell, gradually forming amino salts. Amino acid salts are insoluble in the egg white. Therefore, they crystallize in a specific geometric shape. These amino acid crystals are what we see as pine flower crystals.

However, not every preserved egg will produce pine flower crystals, leading to rumors that preserved eggs with pine flower patterns are more nutritious. This is not true. At best, they are just more beautiful preserved eggs, and their flavor and nutritional value should be the same as those of ordinary preserved eggs.

Why is the yolk dark green?

As mentioned above, the primary ingredients used in the production of preserved eggs are alkaline substances, commonly including sodium hydroxide, wood ash, lime, sodium carbonate, and natural baking soda. The action of alkalinity denatures and gels proteins, which in turn breaks them down to produce amino acids.

The brown color of the white and dark green color of the preserved egg are both due to the effects of alkalinity. The yolk produces hydrogen sulfide due to the alkalinity, which combines with added iron ions to form ferrous sulfide, giving it a dark green color.

Are there concerns about heavy metal contamination in preserved eggs?

Why are there rumors of heavy metal contamination in preserved eggs? This is because pH, temperature, and the environment significantly influence the production process of preserved eggs. Therefore, many manufacturers add heavy metals (such as lead oxide and copper oxide) to stabilize the production process and ensure consistent quality.

However, if excessive amounts of heavy metals are added or the egg’s pores are too large, excessive levels of heavy metal residues can occur.

Generally speaking, people should not consume preserved eggs in large quantities. Even if they eat one per meal, there is no concern that it will be harmful to their health.