Egg internal quality is a major issue in poultry, whether in layers or breeders, because, in the former, internal quality defines the egg nutritional quality and acceptance in the market. At the same time, in breeders, it is translated into better hatchability and chick’s quality.

The egg internal quality depends on different parameters that are related with the yolk and the albumen. As mentioned in other articles, yolk comes from a mature ovum, while albumen is synthesized in the magnum, and the protein membrane recovering it, in the isthmus.

1. Yolk quality
   1.1 Yolk color
   1.2 Nutritional quality
   1.3 Yolk resistance
   1.4 Hemorrhages and other imperfections
   1.5 Microbiological quality
2. Albumen quality
3. Conclusions

1. YOLK QUALITY

The yolk’s primary function is to supply food for the development of the embryo and, additionally, is the part of the egg that most stands out and, therefore, it is an important factor that determinates its acceptance by the consumers.

The next sections describe the main parameters that describe yolk quality and the factors that can make it vary.

1.1. YOLK COLOR

Despite not being related to nutritional quality, yolk color is important because of the importance it has to the consumer, whose preferences vary depending on the geographical area. For example, in Europe, consumers prefer orangish yolks; on the contrary, in the United States, yellowish yolks are more appreciated.

Yolk color depends, mainly, on nutrition, which means the amount of pigments that birds eat with their diets. This varies depending on:

• The amount of pigments in the diet: carotenoids (xanthophylls, such as lutein-zeaxanthin) present in different ingredients, such as corn, wild flowers, fruits, and others. Intensive poultry production makes supplementation with additional pigments necessary, such as canthaxanthin, to decrease the variations in the yolk color between animals and along the animal productive life.
• Pigments quality: pigments can deteriorate, especially if stored under inadequate conditions (for example, in contact with sunlight) or for too long. It is also advisable to add an antioxidant to the feed, to prevent their oxidation, and to mix them homogenously with feed so that all animals receive the same amount of pigments.
• Absorption in the gut: the capacity to absorb pigments in the gut will also affect yolk color. A greater absorption of pigments helps to obtain a more intense color with the same diet. For this, gut must be healthy (infection prevention) and have optimized its structure and physiology by using intestinal conditioner pronutrients.
• Pigment storage in the organism: in highly productive layers, it is recommended to start administering pigments weeks before laying.

1.2. NUTRITIONAL QUALITY

Yolk represents between the 30 and the 33% of the total egg weight and contains more fat, less water and less protein than the egg white. Moreover, it contains the greatest number of vitamins and minerals.

The main factors that can affect yolk’s nutritional quality are:

• Nutrient absorption in the gut: it is essential to have an optimal nutrient utilization so that they are transferred to the egg (yolk and albumen). Molecules from plant extracts, such as intestinal conditioner pronutrients, can be used to improve the physiology of the gut mucosa and optimize nutrient absorption. Also, it is important to maintain intestinal health by using products based on the cimenol ring that eliminate intestinal pathogenic bacteria.
• The liver: it is in charge of fat metabolization and protein synthesis, two of the main yolk components. Any problem affecting its physiological capacity, whether toxins or infectious causes, will negatively affect yolk and, therefore, chick quality. It is essential to protect the liver by using antioxidants, mycotoxin binders to prevent damage caused by fungal toxins, and liver conditioner pronutrients, capable of optimizing the liver physiology.
• The diet: an inadequate or poorly balanced diet, particularly regarding fat and protein quantity and quality, can decrease yolk quality.  On the contrary, it is possible to obtain enriched eggs by increasing nutrient concentrations above the standards, some of them are already commercialized (vitamin E and omega-3 enriched eggs). The success of this technique depends on multiple factors, such as the transfer efficacy of vitamins to the final product (A. Sahlin and J.D. House, 2019). Egg enrichment can be an interesting merchandising tool, as they can be directed to people with reduced intake capacity.

1.3. YOLK RESISTANCE

It depends on the strength of the vitelline membrane. In commercial eggs, yolk resistance is related with yolk quality and freshness, while, in fertile eggs, it is tightly related to the embryo’s nutrient absorption and, therefore, to hatchability and chick quality.

Factors affecting the vitelline membrane quality are:

• Genetics and age: the membrane is less resistant in older hens.
• Freshness: the membrane is more resistant in fresh eggs. As time goes by, yolk diameter increases and it becomes flat. Preserving the egg under the recommended storage conditions will slow down the loss of membrane strength.
• Nutrition: antioxidant deficiencies, such as vitamin E, C and selenium, affect its quality.
• Stress

1.4. HEMORRHAGES IN CHALAZAE AND OTHER IMPERFECTIONS

Chalazae are composed of high-density proteins in charge of maintaining the yolk in the middle of the egg. Hemorrhages between chalazae and yolk can be due to pathologic, genetic or nutritional (biotin and magnesium deficiencies) causes. The presence of these imperfections can decrease egg fertility and cause the consumers’ refusal, although it does not pose a risk for human health.

The presence of blood or meat stains, whether in the yolk or the albumen, result from a hemorrhage of small blood vessels broken during ovulation and is not related to food safety issues. Meat stains are, in fact, degenerated blood stains.

Their presence depends on several factors:

• Genetics: the trend to produce blood stains is heritable.
• Nutrition: vitamin A, mineral, antioxidant deficiencies or a low feed and water quality can cause the presence of stains in the yolk and the egg white.
• Toxics: mycotoxins, heavy metals, among others.
• Health: stains can be caused by different infectious agents, such as Salmonella Enteritidis.
• Stress: it can be related to management and environmental factors, such as the lighting program. Moreover, any kind of stress can affect the immune system and cause immunosuppression and increase animal’s susceptibility to infections.

1.5. MICROBIOLOGICAL QUALITY

Some disease with vertical transmission have their origin in infected ova; such is the case of the avian infectious toxic hepatoenteritis caused by SH₂-producing bacteria, which is transmitted from breeders to chickens through infected ova and affects chick quality, as it increases first-week mortality and decreases weight at the end of the productive period.

 

2. ALBUMEN QUALITY

The egg white, also called albumen, consists of the 60-65% of the total egg weight, and is the egg part with more protein, a key component in egg composition. It is a source or nutrient for the development of the embryo during incubation, so its quality will have a direct effect on the hatching percentage and chick weight and health.

Albumen quality is measured, mainly, using Haugh Units. To determinate this value, use a micrometer to measure the height of the albumen at 1 cm from the yolk and the egg weight, and use the following formula:

Haugh Units = 100*Log[H+7.57-1.7*P0.37)]

Where H = albumen height (in millimeters) and P = egg weight (in grams)

Albumen thickness is related to the following factors:

• Storage: Haugh Units decrease with time, depending on the storing conditions (temperature, humidity). This is because, with time, the egg losses water and CO₂ and the albumen becomes watery and transparent.

To avoid the loss of egg quality, eggs should be collected frequently, particularly during high temperature periods, and preserve them under in cold (10áµ’C) and humid environments (ideally above 70% humidity, to avoid water loss).

• Genetics: Haugh Units also vary depending on the breed and production line. Additionally, this article already mentioned the effect of genetics on parameters such as blood and meat stains in the yolk and the albumen.
• Age: as the hen gets old, albumen viscosity decreases.
• Nutrition: a higher concentration of protein decreases Haugh Units. On the contrary, supplementation with vitamin C and E has a positive effect on this parameter.
• Health: different infectious causes can affect egg quality and Haugh Units. For example, Newcastle disease and infectious bronchitis decrease albumen viscosity, even once the outbreak has been controlled (Butcher, 2003).

3. CONCLUSIONS

Egg’s internal quality is an important factor to take into account because it is tightly related to the product’s acceptance in the market (layers) and viability and chick quality (breeders).

There are multiple factors that can affect yolk and egg white quality, such as the management (including collection, transport and storage), animal health, nutrition and genetics. Having them in mind is essential to solve possible problems and optimize egg quality.

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