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In commercial egg production, not just the number of egg per hen housed is an important parameter. Egg quality is certainly as important. 


By David Cavero Pintado


Egg quality: Main characters to be treated throughout the text.


Characteristics Heredity
Height of albumen 0,30 – 0,40
Yolk percentage 0,35 – 0,45
Blood and meat inclosures 0,05 – 0,10
Yolk colour 0,05 – 0,10
Double yolk eggs 0,15 – 0,25

Table 1. Heredity of the main characteristics related to internal egg quality



The consistency of the albumen is, together with the height of the air chamber, one of the indicators of the freshness of the egg. The consistency of the albumen decreases inexorably over time after oviposition, due to an increase in pH, which leads to degradation of the binding of the proteins ovomucine and lysozyme, which make the albumen increasingly fluid.

The quality of albumen also decreases significantly with the age of the birds, as well as with the appearance of outbreaks of certain diseases such as infectious bronchitis or Newcastle disease.

Figure 1. Albumen height is measured with the gauge of a micro meter, at approximately 1 centimetre from the yolk


The albumen is becoming more and more fluid as time goes by. Larger eggs have a higher albumen height, so it is common practice to make a small correction based on egg weight over albumen height. In this way, Haugh Units (HU) are obtained. These are calculated based on the following formula:

H.U. = 100 – log (h – 1,7w 0,37 + 0,76)

h (mm) = height of albumen density

w (grams) =  egg weight

The H.U. value decreases over time, with this decrease being more pronounced when the storage temperature increases  (graph 1).

Graph 1. Effect of storage duration (days) and temperature on Haugh Units (source: Coutts and Wilson, 2007)

“For consumer acceptance, table eggs should not go below 60 Haugh Units (H.U.)”

Particularly in hot climates, it is important to collect eggs several per day and to store these under refrigerated conditions.

Genetic improvement, together with adequate storage, makes it possible to maintain the height of the albumen over time. Given the moderate heritability of the albumen height, between 0.30-0.40, it is possible to improve this character through genetic selection. However, the contribution that can be made by genetic improvement is worthless, if the eggs are not subsequently stored under appropriate conditions (Figure 1).

In addition, the albumen height has a negative correlation with the hatching rate (Flock et al., 2007), although this can be solved by giving more specific weight to this characteristic in the male lines, so that a good hatching rate is ensured in the breeding hens and a good quality of albumen in the commercial egg.



Companies manufacturing egg products, value a high dry matter content in the egg.

Since the measurement of the percentage of dry matter in the egg is difficult, an indirect measure is used in the selection programme, such as the percentage of yolk.

The genetic correlation between both characteristics, percentage of solids and percentage of yolk, is around 0.9 (Icken et al., 2014). This is due to the fact that albumen has a lower dry matter content, approximately 12-14%, compared to bud, which has around 50-52%.

The heritability of this character is moderate, h² = 0.35-0.45.

There is a negative genetic correlation between egg weight and yolk percentage. Also due to its positive correlation with the hatching rate, it is a character with a higher specific weight in the selection of the female lines.

The percentage of yolk increases with the age of the hens and is higher in light breeds than in medium breeds (29% compared to 28% on average during the whole laying cycle). At a certain age, smaller eggs have a higher percentage of yolk.

Any management practice aimed at increasing egg weight will decrease the percentage of solids in the egg.

The percentage of yolk increases with the age of the hen, and is higher in light than medium breeds.



These are inclusions that can sometimes be found in the egg.

  • Blood Inclusions

They are normally found in the yolk and are often the result of a small haemorrhage in the ovary or oviduct or as a consequence of the breakage of small capillaries in the process of breaking the follicular wall during ovulation. Sometimes pigment residues can also be found in brown egg-layers which can be mistaken for blood spots.

  • Meat Inclusions

These are usually found in the egg white and are due to the shedding of dead cell tissue.

White vs. Brown

In white eggs the frequency of meat pieces is very low, but on the contrary in brown egg lines the incidence can be around 3-5% in the case of pieces greater than 2-3 mm.

It is a character with a low heritability, 0.05 – 0.10, and also has a rather high negative correlation (around -0.35) with the shell colour in brown eggs.

Furthermore the geneticist has the added problem of the lack of variation within this character, due to the low incidence of inclusions in the pure lines. Naturally these are selected to reduce the incidence of meat and blood spots, but this is not easy.

The selection is aimed at eliminating birds with a higher incidence of inclusions in order to keep the frequency of blood and meat spots at very low levels.


Factors that can aggravate the incidence op meat and blood inclusions 



The colour of the yolk is mostly a sensory quality for the consumer, whose preference varies greatly depending on the geographical area. The colour of the yolk is determined primarily by nutritional factors and the health of the birds, and is of low heritability.

The colour of the yolk mainly depends on the carotenoids that are given to the bird through the feed and therefore can easily be adjusted to market preferences through the diet, making it unnecessary to work this character through genetics.

The most widespread method of measuring the colour of the yolk is the use of the Roche scale. This is based on the colour contrast of the yolk with a range of 15 pre-set values, ranging on a scale from pale yellow to reddish orange. In Spain for example, depending on the area, values between 11 and 14 are required.

For several years this trait was selected on the basis of conventional challenge experiments, administering feed with a high percentage of rapeseed and then smelling the eggs, to detect which birds suffered from the problem of fishy odour.

Selection based on these tests had limited success, as only birds carrying both recessive alleles showed this problem. Birds carrying a single recessive allele were unnoticed.

However, with advances in molecular genetics, it was possible to identify the mutation causing this problem in the FMO3 gene (Honkatukia et al., 2005), where an adenine nucleotide is substituted for a tyrosine nucleotide, making it possible to identify birds carrying this mutation and not to select birds with the recessive allele.

Today some breeding companies have completely eliminated this problem of fishy smell and birds can be fed feeds containing up to 12-14% of rapeseed without any negative effect on production or problems of undesirable smells or tastes.

Egg preservation

On the other hand, in order to preserve eggs free of bad smells and tastes, both in sales outlets and in consumers’ homes, it is important to keep eggs refrigerated, preferably in their original packaging and away from products with strong smells.

“Advances in molecular biology have made it possible to identify birds that are capable of laying eggs without a fishy smell, being fed a high percentage of rapeseed”


Especially at the beginning of laying it is not uncommon to find eggs with two yolks.

The incidence of double-yolked eggs increases if early stimulation is carried out, either by light or early change to laying feed, with a high calcium content.

Double-yolked eggs occur when two ovulations have taken place simultaneously or within a very short space of time or due to a delay in the passage of one of the yolks through the oviduct.

These double-yolked eggs are significantly larger than normal, on average about 25-30 g more than a single-yolked egg.


  • In the case of hatching eggs the hatching rate will be much lower than normal
  • In the case of eggs for consumption, the problem of packaging arises, as they are so large that they are not suitable for the cartons or cases that are usually used ( Figure 2)

According to a study by Dr. Flock (1984), approximately 50% of the hens did not have any double-yolked eggs and only 5% of the hens had a high incidence of double-yolked eggs.

The average incidence of double-yolked eggs in the first 12 weeks of laying was around 2% (with peaks of 3-4%), with virtually no incidence after 40 weeks of age.

The heritability of this character in this study ranged from 0.18 to 0.26; values that have been confirmed in later studies (Wolc et al., 2012).

Based on these heritability values, it would be possible to select the double-yolk character. However, given the distribution of this trait, it would be easier to select to increase the incidence of these eggs than to reduce it.

Fortunately, there is a negative correlation between the number of double-yolked eggs and the number of eggs produced, which is around -0.30. In breeding programmes, by counting only saleable eggs in the selection nuclei, indirect selection is being made to reduce the incidence of double-yolked eggs, because as mentioned above, birds with high production potential are less likely to have double-yolked eggs.


There is no doubt that eggs are one of the most balanced foods in the human diet and contain very high quality protein. It contains the essential amino acids in proportions similar to human requirements and offers a wide range of vitamins.

It is also one of the foods of animal origin with the least amount of unsaturated fat. Fatty acids account for about 4 g, of which 65% are unsaturated and 35% are saturated (Institute of Egg Studies, 2009).

“Due to a misinterpretation of the studies carried out in the 1970s, eggs have had an undeserved bad reputation for years, the consumption of which was associated with an increased risk of cardiovascular problems.”

However, the effect that dietary cholesterol has on blood cholesterol in healthy people is minimal and instead depends largely on other factors including: genetic predisposition, body weight and lifestyle (Institute of Egg Studies, 2009).

In conclusion and following the American Heart Association’s statement, reasonable egg consumption, one egg per person per day, does not pose a risk for cardiovascular disease.

Due to the (undeserved) bad reputation of eggs in the seventies, many researchers have studied the possibility of reducing the cholesterol content in eggs through genetic selection, and moderate heritability has often been observed. However, it should be noted that selection programmes were effective in achieving higher cholesterol content, but not so effective in reducing it.

It should not be forgotten that cholesterol is necessary for the development of the embryo in the egg, so it does not seem reasonable to think that it would be possible to reduce this cholesterol content below a certain minimum level that would allow the development and birth of a chick.

According to the American Heart Association a reasonable egg consumption (one per day) does not pose any risk of cardiovascular disease



As explained, genetic improvement of laying hens concentrates on improving a significant number of characteristics related to the internal quality of the egg.

For the production of top quality eggs it is important to follow all management practices recommended by the genetics companies.

Not only must the feed composition suit to the birds, but also shape and particle size of the feed must be homogeneous.

The supply of quality feed and water must be guaranteed, so that the birds have an adequate intake of nutrients.

It is key that farms have a programme for monitoring and recording egg quality defects, so that problems can early be detected and appropriate corrective action can be taken well in time.

Flocks must be healthy and free from disease for good quality eggs.

Proper vaccination programmes tailored to the area of production, coupled with rigorous biosecurity programmes, are a vital tool in maintaining excellent egg quality.


  • A good genetic base
  • Good handling and feeding of the birds
  • Optimal bird health and good biosecurity
  • Good conservation and handling of eggs








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