The Haugh unit (pronounced “how”) is a measure of internal egg quality. Developed by Raymond Haugh in 1937, the Haugh unit is a measure of egg protein based on the height of its egg white (albumen) and the weight of the egg.
For this test, an egg is weighed, then broken onto a flat surface and a micrometer is used to determine the height of the egg white immediately surrounding the yolk. The height, together with the weight determines the Haugh Unit (HU) rating which ranges between 0 and 130. The higher the number, the better the quality of the egg as fresher, higher quality eggs have thicker whites. Eggs with a Haugh unit above 67 are considered Grade A.
Grading stations often test Haugh units to assess egg quality and shelf life. If there is an issue identified by measuring the Haugh unit, the grader will often work with the farmer directly to address it.
Factors that can influence egg white quality, and therefore the Haugh unit measurement
- The age of the bird
- The breed of bird
- How quickly eggs are collected and placed in the cooler following lay
- How long the eggs have been stored. Over a 10 week period, there is a significant change in Haugh unit, with average values decreasing by almost 15%.
It has been found that Haugh units have not been greatly impacted by bird nutrition, the barn environment or hen housing.