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Brazilian Journal of Poultry Science

Print version ISSN 1516-635XOn-line version ISSN 1806-9061

Rev. Bras. Cienc. Avic. vol.2 no.2 Campinas May/Aug. 2000 

Distribuição do Tamanho de Poros em Cascas de Ovos Determinada pela Porosimetria de Mercúrio

Pore Size Distribution in Chicken Eggs as Determined by Mercury Porosimetry



Autor(es) / Author(s)

La Scala Jr N1
Boleli IC1
Ribeiro LT2
Freitas D2
Macari M1

1- Faculdade de Ciências Agrárias e Veterinárias - UNESP

2- Frango Sertanejo S.A.

Correspondência / Mail Address

Prof. Dr. Newton La Scala Júnior

Via de Acesso Prof. Paulo Donato Castellane s/n
14870-000 - Jaboticabal - SP - Brasil


Unitermos / Key words

porosidade em cascas de ovos, porosimetria de mercúrio

eggshell porosity, mercury porosimetry

Observações / Notes

The authors are grateful to FAPESP and CNPq for the financial support.


Neste trabalho foi aplicada a porosimetria de mercúrio na caracterização da porosidade de cascas de ovos de poedeiras com 28 semanas de idade. Aplicando-se a técnica de porosimetria de mercúrio, pudemos descrever as características associadas a porosidade de modo mais amplo, determinando uma distribuição do tamanho de poros nas cascas de ovos estudadas. Nossos resultados mostraram que a maioria dos poros nas cascas de ovos tem tamanhos entre 1 a 10 mm. Neste artigo introduzimos a técnica de porosimetria de mercúrio como uma nova ferramenta aplicada no estudo de cascas de ovos.



In this study we investigated the application of mercury porosimetry technique into the determination of porosity features in 28 week old hen eggshells. Our results have shown that the majority of the pores have sizes between 1 to 10 m m in the eggshells studied. By applying mercury porosimetry technique we were able to describe the porosity features better, by determining a pore size distribution in the eggshells. Here, we introduce mercury porosimetry technique as a new routine technique applied into the study of eggshells.




An egg constitutes a natural incubation chamber, providing the embryo contained within all the nutrients required for its development. The nutrients are deposited in the eggs by the female at the time of their formation (White, 1991; Shen et al., 1993). Furthermore, for a complete and normal development of embryos, eggs must present shells with the necessary structural properties to provide an adequate flow of oxygen, carbon dioxide and water vapour loss (Rahn et al., 1979; Rahn, 1981a,b). Pore size, in this context, has an important role in the establishment of the shell conductance and, consequently, in the determination of the respiratory rate of embryo. In current studies on the porosity of egg shells, the pore number and total area were determined (Rahn, 1981a,b; Cristensen et al., 1996), but the pore size distribution was not presented in terms of its occurrence in the egg shell. Accurate knowledge of the pore sizes in egg shells could contribute in a significant way to the evaluation of their functional quality and, therefore, the requirements for production of eggs with an appropriate shell structure. Baxter-Jones (1994) reported that an eggshell has 10.000 to 20.000 pores, but only a few of them can be penetrated by bacteria, suggesting that bigger pores are scarce in eggshells.

In this work, we have studied the porosity features of chicken eggs by applying a new technique, mercury porosimetry, for characterization of such kind of samples. The mercury porosimetry technique is a well known and mostly used in physics, chemistry and engineering (Van Brakel et al., 1981; Whittemore, 1981). The great advantage of this technique is that mercury porosimetry analyzes the pores in an ampler way, by determining the pore size distribution in the eggshell samples.



Our samples were eggs from 28 week old COBB hens obtained from a commercial hatchery. Measurements of the pore size distribution were obtained from egg shells without the inner membrane in two conditions: with containing the outer shell membrane and without it. In the second case, the egg shells were boiled in a solution of 5% NaOH for 10 minutes, in order to remove the membrane. After washing and drying, egg shell pieces weighting approximately 1.5 grams were submitted to mercury porosimetry analysis.

The mercury porosimetry technique is based on the determination of the mercury volume infiltrated in a pore, as a function of the external pressure applied in the space where the sample is placed. Figure 1a describes what happens when a non-wetting liquid is penetrating into a pore of diameter 2r. Due to the cohesion forces between liquid and surface wall, the liquid surface develops a characteristic curvature, defining a contact angle q. Such angle depends on the surface tension between the liquid and the material in contact.



Assuming a cylindrical shape of pores it is possible to relate the pressure (p) under which mercury is introduced into a pore with the pore diameter d=2r. In a pore with a circular cross section of radius r, the surface tension of a liquid in the capillary acts as repulsive force pushing the liquid out of the pore. This force is given by:


where s is the surface tension of the mercury surface. The force is directed into the capillary direction and has a magnitude of:


where q is the contact angle with which mercury penetrates the pores. In the equilibrium state the capillary force is balanced by the external pressure p that pushes the liquid into the pore with a force of magnitude:


When both forces are equalized, this state is determined by the Washburn equation (Washburn, 1921):

The equation above indicates that at lower pressures mercury penetrates into the bigger pores (bigger diameters d) and at higher pressures mercury penetrates into the smaller ones (smaller diameters d). In the case of mercury in contact with the great majority of materials, q and s have values of approximately 1350 and 480 mN/m, respectively. Hence, it is possible to determine the pore radius using the equation above by knowing the right pressure when mercury starts to penetrate into the pores.

In practice, the volume of mercury introduced into the pores is determined by the variation of the mercury level in a tube connected with the sample space (Drake, 1949). Figure 1b presents a schematic drawing of the experimental setup. The samples are placed in a closed cell called Penetrometer, and evacuated. After a low vacuum level has been reached (~ 3 kPa), the cell is filled with mercury and pressure is increased continuously. As pressure increases, mercury starts to penetrate into smaller and smaller pores, and the instrument senses the intrusion volume by means of changes in the mercury level (Drake, 1949). If a variation in the mercury level occurs when pressure increases from pi-1 to pi this is understood as an indication that pores with diameter

exist in the sample. It is important to notice that the variation in the mercury level is proportional to the number of pores with that diameter in the sample. Therefore, the pressure with which mercury penetrates in the sample determines the pore diameter and the incremental volume introduced determines the relative number of pores with that diameter in the sample. The incremental volume introduced versus pore diameter gives a convenient means to see distribution characteristics. In this work, a commercial model Pore Sizer 9310, produced by Micromeritics Instrument Corporation (Norcross, GA USA), was applied into the porosity analysis of egg shells.

In order to confirm the results obtained by mercury porosimetry we have also performed electron microscopy analysis of the samples. The studied chicken egg shells surfaces and outer shell membrane of the eggs were examined by scanning electron microscopy. Egg shells were broken into 0.3 x 0.5 cm by fracturing the shell with a scalpel. The shell samples were submitted to critical point drying and, after sputter - coating with gold, they were examined and photographed using a JEOL-5410 scanning electron microscope.



Figure 2 presents a typical result obtained when the mercury porosimetry technique is applied into the characterization of chicken eggs containing the outer membrane. As one can see, when we plot incremental volume introduced versus pore diameter, at least one peak appears indicating the presence of pores in the sample. A logarithm scale was chosen for clarity, and the experimental results (squares) are described by a quite symmetric polynomial regression line (solid line). In the inset of the same figure is seen an amplification of the graph in the range from 0 to 8 mm. Our experimental result is fitted by a Gaussian function (Normal distribution) centered in 3.5 mm and having a width of 2.1 mm, revealing that 68% of the sample pore sizes is between 1.4 and 5.6 mm. Such results certainly indicates a large array of the pore sizes in the sample, and this feature can not be determined by the routine techniques applied to egg shell analyzes (Rahn, 1981a,b; Cristensen et al., 1996).



The porous structure of the studied egg shells are presented in Figures 3 to 6, in a set of 4 electron micrographs. In Figure 3, it is possible to see the pore channels in the egg shell, connecting the inner and the outer sides of the egg shell. The outer shell pores and the vesicular holes are presented in Figures 4 and 5, respectively.









By comparing the pore sizes obtained by mercury porosimetry and the electron micrographs, it is clear that the peak observed in Figure 2 does not correspond to the pores and holes observed in Figures 3 to 5. In order to confirm the relationship of the huge peak observed in Figure 2 with the web of the outer membrane (Figure 6), mercury porosimetry was applied to study the eggshell without membranes. This result is presented in Figure 7, where it is possible to see that the huge peak positioned around 3.5 mm (Figure 2) has vanished. This is an indication that the web membrane influenced the results presented in Figure 2. It is important to notice also that, in Figure 7, a set of peaks can be observed having diameters from 0.2 to above 5 mm and these correspond to the shell pores and vesicular holes already presented in Figures 3, 4 and 5.




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