Egg cuticles - The outermost layer of the eggshell

Weyhey! Welcome to a new section. I'll be utilising my several years of higher education in bringing to you information on chickens, sourced from research articles. I never knew there was a journal of British Poultry Science! So why research articles and not books? Well, articles go through a peer review process before publication. Yes, I've felt the pain of submitting an article, only for it to be rejected, the pain of reviewers asking for more tests, more corrections and data. All requested by people who work in the field and try and improve the quality of the article. 

I've contributed to book chapters, but the one's I have done are basically a review of the current published literature. I'm not so sure about stand alone books, does it just get reviewed by the editor? I mean, there are some books out there that just give bad information (anyone remember that paleo diet cookbook that got pulled from the shelves?). Anyway, I'll talk about a topic I find interesting and reference the articles that I use at the bottom.

To kick this one off:

The above egg was laid by Chocolat. The underlying dark colour of the shell can be seen, but there is an excessively thick coating over the top of it.  This led me to look up egg coatings and I found that chicken eggs have an antibacterial coating on the outside. Called the cuticle, this is formed of mainly proteins with an underlying mineral layer and a top layer of proteins.


There has also been talk that this cuticle layer (also known as bloom) has antibacterial properties. So the scientist in me had to go an look up some research papers. And yes! It does have antibacterial components that prevent bacterial from growing on the eggshell and entering through the pores.

So some information on the cuticle.

• It's normally about 5-10 microns thick, so you can't see it (the one laid by Chocolat is obviously thicker >.<)

• The cuticle fills the pores of the shells to about a 50 micron depth. Again preventing bacteria from entering into the egg and also regulating water evaporation through the pores.

• It is generally coated unevenly over the egg!

• Different hens put down different amounts of cuticle. The pattern of cuticle deposit is dependent upon the hen. 

• At the pores, there is a thicker cuticle coating

• Hens put down more cuticle on an egg before a "rest" day. For example, say a hen laid on days 1, 2 and 4. The egg laid on day 2 will be thicker than the egg on day 4 (I can't recall if Chocolat laid the next day, I don't think she did?)

• If you clean eggs and scrub them, this scrubbing action removes the cuticle (this was kinda obvious).

• Chicken poo on the eggshell also causes cuticle degradation (so just pick off the poo and wipe with a wet tissue to preserve the cuticle)

• When the cuticle proteins were extracted, Ovocalyxin-32 was the predominant protein. This is a protein only found in avian eggs (duck and geese eggs too). Interestingly it is similar to a skin protein. So the cuticle could be thought of as a chicken egg version of skin.

•  The cuticle layer possesses antimicrobial activity

• Proteins found in the cuticle layer that may have antimicrobial activity: Lysozyme C, ovotransferrin and ovocalyxin-32

• Proteins extracted from the cuticle were successful in reducing the bacterial growth of P. aeruginosa, B. subtilis and  S. aureus, but not E.coli D31 (this is an E.coli strain that is resistant to ampicillin)

• A concentrated lipid extract of the cuticle was able to suppress the growth of E.coli D31. The extraction method for proteins may have destroyed the lipids.

• The cuticle shows less degradation when kept at 5 degrees in comparison to 24 degrees (so keep your eggs in the fridge!)

I hoped you liked this insight into egg cuticles. Let me know if you want to have this kind of information.

As to why Chocolat laid an egg with such a thick cuticle layer? I have no idea, but she hasn't done one since. It was interesting, the white layer became transparent when damp. I'm not exactly sure to the cause of this, it's not dissolving the proteins as when it dries it becomes white again. I can only assume that the water changes the refractive index of the cuticle layer and makes it transparent.

And the information that it has some antibacterial activity, good, especially since I found that egg under the roost among the poo (Yes, I know I have to wash it or it destroys the cuticle layer!)

Hey, you can see the uneven coating of the cuticle in that photo!

Let me know if you like this and the references are below.

And don't forget to cuddle your chickens, I mean, they put down antibacterial coatings for your eggs and all that!

References

Cuticle, shell porosity and water uptake through hens’ eggshells (1984) Sparks, N & Board R, British Poultry Science, 25(2):267-276
 (I couldn't get the full article for this, only the abstract)

Proteomic analysis provides new insight into the chicken eggshell cuticle (2012) Rose-Martel, M; Du, J & Hincke, M; Journal of Proteomics, 75(9):2697-2706

 Microstructure of matrix and mineral components of eggshells from white leghorn chickens (Gallus gallus)  (1996) Dennis, J; Xiao, S; Agarwal, M; Fink, D; Heuer, A & Caplan, A; Journal of Morphology, 228(3):287-306

Factors affecting the cuticle of the egg as measured by intensity of staining (1975) Ball, R; Logan, V & Hill, J; Poultry Science; 54:1479-1484

Antimicrobial activity of lipophilic avian eggshell surface extracts (2010) Wellman-Labadie, O; Lemaire, S; Mann, K; Picman, J & Hincke, M; Journal of Agricultural and Food Chemistry; 58:10156-10161

Antimicrobial activity of cuticle and outer eggshell protein extracts from three species of domestic birds (2008) Wellman-Labadie, O; Picman, J & Hincke, M; British Poultry Science; 49(2):133-143