In this article we will briefly focus on determining the internal quality of the egg. We will focus on tests to determine the condition of the yolk. To do this, it is necessary to study the quality of the yolk and, in particular, to assess the yolk colour and yolk index.

Internal egg quality: yolk colour

The colour of the yolk is a very important aspect valued by the consumer at the time of purchase. The colour of the yolk is mainly due to xanthophylls and to a lesser extent to carotenes, among other pigments.

• Carotenes give a paler colour to the yolk, while the addition of xanthophylls gives a more intense colour.
• In industrial poultry farming it is more common to find paler coloured yolks, but this does not mean that they are less nutritious.
• To evaluate the colour of the yolks, a pattern called Roche fan is used.

Internal egg quality: yolk index

The yolk index relates the height and diameter of the yolk as a percentage. This indicates a higher quality of the egg, so the higher the index, the fresher the egg will be as the yolk will be more compact and will have lost less moisture.

Calculation of the yolk index: Yolk height divided by yolk diameter.

The normal yolk index is approximately 0.40- 0.42. When this figure is exceeded, it means that the yolk has dropped because the yolk membrane has become more elastic and has lost more water or moisture.

If to determine the quality of the yolk we have assessed the colour according to the Roche scale and the yolk index as a measure of egg freshness, to determine the protein quality we must talk about the Haugh units.

Egg Protein Quality: Haugh Units

Haugh Units represent an accurate and objective measure to determine the internal quality of the egg, especially with regard to protein quality.

Haugh Units (HU) relate the height of the dense white and the weight of the egg in the following formula:

“A” represents the height of the dense white and “P” the weight of the egg in grams.

It must be considered that, once the egg has been opened, the Haugh Units decrease as time goes by. The effect of temperature must also be taken into account since a difference of 10 degrees means a decrease of 1.5 Haugh Units.

Values of 50 Haugh Units represent an unacceptable egg quality for the consumer, from 70 onwards it is acceptable and values between 90 and 100 represent an excellent egg quality.

What do we need to calculate the Haugh Units?

• Micrometer, balance, and calculator.
• Before proceeding, ensure that the temperature is between 7 and 15ºC.
• Weigh the egg and record the weight.
• Crack the egg without damaging the dense white and pour it carefully.
• Measure the height of the dense white using the micrometer from the longest part and take two readings per egg in places where there are no bubbles and no chalazas.
• Finally, calculate according to the above formula.