Beware! A family of four was poisoned by eating preserved eggs. What happened? Don’t miss this step before eating...

Expert in this article: Li Lin, PhD in Food Science, Professor of Zhongshan College of University of Electronic Science and Technology of China, Food Safety Expert of Zhongshan City

Everyone is familiar with preserved eggs. Many people like to use them to mix with tofu, which is called cold preserved egg tofu.

It tastes good, but there are also endless news reports about preserved eggs being poisoned.

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Some people may ask, why do preserved eggs smell strange but taste delicious? What is the cause of "preserved egg poisoning"?

Why is the preserved egg solid even without being heated?

Preserved eggs were created in the early Ming Dynasty and have a history of more than 500 years. They are transparent and elastic, with a fresh and smooth taste, and unique color, aroma and taste.

The production process of preserved eggs varies slightly from place to place, and the traditional and modern recipes are also different, but the chemical principles behind them are the same. The gel properties come from the moderate denaturation and hydrolysis of proteins under alkaline conditions.

During the production process, whether using wood ash, quicklime or traditional soapberry, they are actually alkaline substances. These alkaline substances will penetrate into the eggshell during the pickling process, causing the protein in the egg to denature and slightly hydrolyze in an alkaline environment, changing its original free-flowing state and eventually coagulating and precipitating.

The beautiful pine branch-like pattern ("pine flowers") on the preserved eggs is neither a standard feature of high-quality preserved eggs nor a crack in the eggs. Instead, it is the product of protein hydrolysis - fibrous magnesium hydroxide hydrate crystals formed by the combination of amino acids and magnesium ions in protein.

Where do the differences in the colors of preserved eggs come from?

Although preserved eggs are crystal clear, some are dark brown while others are golden and translucent.

The reason why preserved eggs are colored is that the protein in the egg white releases hydrogen sulfide after hydrolysis, which reacts with the iron and zinc in the egg yolk after penetration to produce various "colored substances".

Traditional preserved eggs are dark green in color because lead-containing yellow lead powder was used in the past. The sulfur-containing protein in the eggs combined with the lead-containing substance to produce dark blue-gray lead sulfide.

Modern preserved egg production has abandoned yellow lead powder. The production of brown is related to the reaction between hydrogen sulfide and iron and zinc on the one hand, and the Maillard reaction on the other hand. Under alkaline conditions, the Maillard reaction speeds up, making the color of poultry eggs darker. If the pickling time is short, the poultry eggs will appear golden yellow, while if the time is long, the egg white will become brown. The dark green color around the yolk is because iron sulfide is formed in the yolk.

Why do preserved eggs smell strange but taste delicious?

Preserved eggs are made in an alkaline environment, so when you peel off the shell of the preserved egg, you will smell an alkaline smell. At the same time, protein hydrolysis occurs during the pickling process, producing volatile substances such as hydrogen sulfide and ammonia, which makes it smell a bit strange.

A study conducted by the Ministry of Agriculture's Food Quality Supervision, Inspection and Testing Center in 2009 found that compared with fresh eggs, the content of cysteine ​​and arginine in preserved eggs is significantly lower. This is mainly because the alkali-unstable sulfur-containing amino acid cysteine ​​and basic amino acids (such as arginine and lysine) are destroyed or derotated during the production of preserved eggs, producing lysine, H2S and NH3. At the same time, poultry egg protein will also produce some umami amino acids and flavor peptides during hydrolysis, which improves the taste of preserved eggs.

In 2022, Nanchang University isolated five umami peptides from preserved eggs. These short peptides will bind to umami receptors to produce a delicious taste.

What exactly is “preserved egg poisoning”?

Because preserved eggs are related to lime and lead, many people believe that eating preserved eggs can lead to chronic lead poisoning.

This is mainly related to the yellow lead powder used in making preserved eggs. The main component of yellow lead powder is lead oxide, which is mainly used to prevent excessive alkali penetration and cause hydrolysis of egg white.

But in fact, since my country issued the food lead content standard in 2009, the preserved egg production industry has improved its production formula and abandoned the more harmful yellow lead powder.

In 2015, my country revised the national health standards for preserved egg production, requiring the use of lead-free processes and stipulating that lead content <0.5mg/kg can be labeled "lead-free". This dose is not harmful to the human body.

So, what exactly is “preserved egg poisoning”?

This is mainly related to the quality of the eggs themselves. As a common foodborne pathogen, Salmonella is everywhere. It can contaminate eggs through feces, while the egg white and yolk wrapped in the eggshell can be contaminated through the ovary or through the eggshell.

Although the marinade used to make preserved eggs is not suitable for bacterial growth, studies have shown that preserved eggs made from duck eggs contaminated with Salmonella are intactly solidified, and their sensory indicators such as color and smell are no different from normal preserved eggs. The sensory characteristics of preserved eggs will not be affected by bacterial proliferation.

This shows that as long as the fresh eggs are contaminated, no matter how high or low the bacteria level is, they can quickly reproduce, grow, and survive in the preserved eggs and remain infectious for a long time.

At present, the detection rate of Salmonella in fresh eggs and their products in my country is 3.9%-43.7%, and the pathogenic bacteria that cause the preserved egg food poisoning epidemic is mainly Salmonella typhimurium.

Salmonella is not resistant to high temperatures. It can be killed if the temperature at the center of food reaches 70°C for 5 minutes. Ordinary cooking and heating can effectively kill Salmonella.

Due to the dietary habits in some areas, preserved eggs are not heated when consumed, but are eaten directly after being mixed with cold salad. As a result, both the preserved eggs containing Salmonella and the Salmonella introduced during the processing cannot be controlled, ultimately making preserved eggs more likely to cause food poisoning than other egg products.

Will I be able to enjoy eating preserved eggs in the future?

Regarding preserved eggs, whether fried, stir-fried or steamed, as long as they are fully heated and eaten in time, you can fully enjoy this food.

Many people are curious why most preserved eggs are made from duck eggs?

In fact, whether you use chicken eggs or quail eggs, you can make beautiful preserved eggs as long as you follow the relevant process. Duck eggs have a higher fat and cholesterol content than other eggs, which makes the processed preserved egg yolk more delicate and taste better.

The sodium content of preserved eggs is very high (320 mg Na per egg, equivalent to 16% of the daily recommended intake). Therefore, although preserved eggs are delicious, most people still need to limit their daily intake.

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