The Structural Basis of Egg Shell Translucency and its Role in Food Safety of Table Eggs

Eggshell translucency is the appearance of lighter coloured regions of the shell visible when an egg is viewed over a light source. Some research has been conducted into the causes of translucency in the past but there has been little work recently and the cause of translucency remains uncertain. This study aimed to identify the structural cause of eggshell translucency and determine if translucency correlates with bacterial penetration of eggs.

Experiments confirmed that translucency is caused by a build-up of moisture in the structures of the shell because translucency disappears when shells are dried out but reappears following rehydration of the shells. The presence of the shell membrane does not appear to be integral to the appearance of translucency because removal of shell membrane and rehydration of egg shells showed no difference with or without membrane present.

Observation of the mammillary layer of eggshells under the scanning electron microscope revealed some features that were correlated with a high incidence of translucency such as type B mammillary bodies and cubic cone formations. The size and density of the mammillary bodies did not correlate with translucency score.

A CT scanner was used to provide high resolution details of shell structure. While these images showed several underlying features common to translucent shells, there was no consistent single feature that could be associated with a high incidence of translucency. However, counts of the number of pores per unit area that passed directly through the shell, branched internally or branched externally showed differences among translucency score categories (see Table). The number of straight pores was inversely related to translucency score whereas the number of externally branching pores was directly correlated. However, studies of egg shell conductance and pore density for a range of translucency scores showed no statistically significant differences although shell conductance tended to increase with translucency score whereas pore density tended to decrease.

Table 1 - Types of pores in shells of different translucency scores(4 = highest incidence)

Whole egg and agar eggs studies were conducted to investigate the ease with which Salmonella Agona could penetrate washed and unwashed shells of different translucency scores. For whole eggs, penetration was more likely at 37°C than at 20°C incubation and washed and unwashed eggs were not different. For the agar egg studies, unwashed eggs had a higher penetration rate than washed eggs. There was a tendency for penetration to occur more often in eggs from translucency scores 2 and 3. Egg shell translucency is more likely caused by a combination of factors including pore branching, mammillary layer ultrastructure and perhaps the density of the shell calcite. 

ACKNOWLEDGEMENT: This research was conducted within the Poultry CRC, established and supported under the Australian Government's Cooperative Research Centres Program. Dr. Matt Tighe, Rebecca Haling and Richard Flavel assisted with the CT scanning.