Can those inconspicuous little holes on the eggshell actually cause big bugs on the dining table?

As the saying goes, flies don't bite seamless eggs, but did you know that seamless eggs can also "breathe". The eggshell is covered with countless tiny pores, which allow the egg to exchange weak gases with the outside world. However, it is these pores that have become the "culprit" for wild bird eggs to be more susceptible to mold and contamination by pathogenic bacteria such as Salmonella and Proteus mirabilis.

The porous structure of the eggshell: a breathing channel for life

The eggshell is composed of an outer eggshell membrane, a calcareous eggshell, and an inner shell membrane. The outer shell membrane has two layers: an outer inner eggshell membrane and an inner albumen membrane. These membranes are composed of fine fibers interwoven into a mesh structure [1]. As the place where life is conceived, its outer shell is not completely closed. The eggshell is covered with countless tiny pores, which are important channels for bird eggs to exchange gases with the outside world. They ensure that the embryo can obtain sufficient oxygen during the incubation process and discharge the carbon dioxide produced by metabolism. It is these pores that allow wild bird eggs to maintain vitality while also facing external threats.

Image source: Pixabay

Why are wild bird eggs more likely to grow mold and be contaminated with pathogenic bacteria?

Wild bird eggs are more susceptible to mold growth and pathogen contamination than farmed eggs mainly because of their unique wild incubation environment and storage conditions. In the complex and changing environment of the wild, although the eggshell membrane can effectively block direct penetration of mold, mold hyphae can still use the pores on the eggshell as an invasion route [2]. They first grow on the inner eggshell membrane, then destroy the inner eggshell membrane and the albumen membrane, and invade the albumen to reproduce. At the same time, a wide variety of pathogens are widely distributed in the natural environment, posing another major threat to wild bird eggs. In particular, wild bird eggs laid near water and in dark and humid mountain forests have a significantly increased risk of infection due to the large variety of microorganisms in the surrounding environment, including Salmonella and Proteus mirabilis. When the newly laid eggs come into close contact with the surrounding humid environment, these pathogens have the opportunity to enter the eggs through the pores of the eggshell. In addition, the storage conditions of wild bird eggs are usually loose and lack the strict management enjoyed by farmed eggs. In an environment with high humidity, mold and bacteria can quickly invade eggs even at lower temperatures, further increasing the risk of infection[2].

How to prevent it?

By understanding the risks posed by the porous structure of eggshells, we can take appropriate preventive measures. Compared with wild poultry eggs, it is safer to choose quarantined domestic eggs because modern chicken farms have strict control over the breeding environment, feed hygiene, egg storage and transportation. When storing eggs, keep the environment cool, ventilated and dry. Place them upright with the big end facing up and the small end facing down. The air chamber at the big end of the egg can prevent the yolk from floating up and sticking to the eggshell. Make sure to cook them thoroughly before eating, because high temperatures can kill most bacteria, including Salmonella[3].

The tiny pores on the eggshell are both a breathing channel for life and an entrance for external threats. In this era of pursuing health and quality, understanding and preventing these risks can help us protect the health of ourselves and our families while enjoying the deliciousness and nutrition brought by poultry eggs.

[1] Shi Xuefeng, Zheng Jiangxia. Research progress on microbial contamination and antimicrobial defense mechanism of eggs[J]. China Poultry, 2021, 43(12): 78-84. DOI: 10.16372/j.issn.1004-6364.2021.014.

[2] Bin Dongmei, Zhong Jinfeng. Sources and spoilage of microorganisms in eggshells and eggs[J]. Shanghai Animal Husbandry and Veterinary Newsletter, 2006, (03): 45-47.

[3] Xing Xianguo. Salmonella, the culprit of diarrhea caused by half-cooked eggs[J]. Food and Health, 2023, 35(08): 20-21.

Source: Chongqing Science Writers Association

Author: Yan Jing, public health physician at Chongqing Yongchuan District Center for Disease Control and Prevention, and Zou Jingbo, chief laboratory technician

Audit expert: Li Hanbin

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