TECHNICAL ARTICLES Broilers/Layers/Breeders

Specific Gravity Determination for Hatching Eggs

Eggshell colour has a major influence on the visual appeal of an egg, as consumer preferences throughout the world differ for coloured eggs. While this colour preference applies to table eggs only, shell colour questions have been raised concerning a relationship between darker eggshell colour and improved eggshell quality.

Eggshell colour in broiler breeders typically ranges from a light cream to a dark brown. The pigment found in the eggshell is a natural porphyrin. Multiple genes influence the pigment and eggshell colour is believed to be a sex-linked trait characteristic of hens. The link between shell colour and shell quality is unclear, therefore it is best to use other parameters such as specific gravity, shell thickness and shell strength (or breaking strength) to determine shell quality.

Specific gravity tests are a non-invasive method to determine eggshell thickness, and therefore eggshell quality. Specific gravity reflects the amount of shell present relative to the amount of albumen, yolk and membranes. It can be used to assess shell quality of hatching eggs as the hen ages or during periods of stress which may affect shell quality. Specific gravity is highly related to the incidence of breaks and cracks. The incidence of breakages is above normal if the specific gravity of a flock averages less then 1.080.

Specific gravity measurements have shown to decrease after approximately forty five weeks of age. While it is expected that specific gravity will decrease with age of flock, the test will allow one to quantitate the shell quality of a flock at any particular age. It has been shown that hatchability of thin shelled eggs (<1.080) was at least two percent less than thicker shelled eggs in hens of the same age and had a higher incidence of embryonic death.

The specific gravity of an object equals the weight of its own volume relative to the weight of an equal volume of water, when both are at the same temperature. The specific gravity of an egg is equal to the eggs density relative to water. An egg has four basic parts: yolk, albumen, shell membranes and shell. The specific gravity of all four parts of the egg are different (shell, 2.325; yolk, 1.032; albumen, 1.038, shell membranes, 1.075). Since the specific gravity of the shell is more than two times higher than the other parts of the egg, the percentage of egg that is shell has a major influence on the specific gravity of the whole egg. As the amount of shell increases, the specific gravity of the egg increases. Egg specific gravity, therefore, is a good indicator of percentage shell and is the reason a specific gravity test can be used to determine shell quality.

To perform the specific gravity determination, you immerse the egg in a series of increasingly concentrated salt solutions until the egg floats on the surface of one of the solutions.

Eqipment required:
1. Plastic container with sealable lid
2. Feed grade or all-purpose non-iodized salt
3. Hydrometer with a specific gravity range of 1.050 – 1.100
4. The day before the measurements are performed a minimum of 100 fresh eggs should be randomly selected from the flock. Avoid selecting only the better or poorer eggs. The eggs selected and the water to make the solutions should be placed in a room where the measurements will be performed, this will allow all materials to reach the same temperature overnight. On the day of the test, the salt solution/s should be made, use the hydrometer to determine the specific gravity of the salt solution (ie 1.080) and then lower the eggs into the solution. If the egg floats it’s specific gravity is <1.080.

Points to remember:
1. Specific gravity measurements should be taken within 24 hours after the eggs are laud.
2. The salt solution can be sealed and re-used, but must be rechecked for accuracy.
3. Specific gravity determinations should be performed the same way each time to compare scores.
4. The specific gravity readings will not be accurate if the temperatures of all materials have not been allowed to equilibriate.
5. Always obtain a random sample of eggs for the test.


Poultry Science Facts – M. J. Wineland (North Carolina State University)

Shell Quality and Colour Variation in Broiler Breeder Eggs – N. S. Joseph, N.A. Robinson, R.A. Renema and F.E. Robinson (Department of Agricultural, Food and Nutritional Science, University of Alberta)

Date published: 2003-10-20

C. Bowles