A fresh egg, with a clean, smooth, brown or white shell, a pure, deep-yellow yolk and a translucent, firm white — this is the
Formation of the egg
Reproductive organs of the hen
The egg is formed gradually over a period of about 25 hours. Many organs and systems help to convert raw materials from the food eaten by the hen into the various substances that become part of the egg.
The hen, unlike most animals, has only one functional ovary – the left one – situated in the body cavity near the backbone. At the time of hatching, the female chick has up to 4000 tiny ova (reproductive cells), from some of which full-sized yolks may develop when the hen matures. Each yolk (ovum) is enclosed in a thin-walled sac, or follicle, attached to the ovary. This sac is richly supplied with blood.
The mature yolk is released when the sac ruptures, and is received by the funnel of the left oviduct (the right oviduct is not functional). The left oviduct is a coiled or folded tube about 80 cm in length. It is divided into five distinct sections, each with a specific function, as summarised in table 1.
Table 1: Functions of various different sections of the hen’s oviductthe yolk
|Section of oviduct||Approximate time egg spends in this section||Functions of section of oviduct|
|1 Funnel (infundibulum)||15 minutes||Receives yolk from ovary. If live sperm present, fertilisation occurs here (commercially produced table eggs are not fertilised)|
|2 Magnum||3 hours||Albumen (white) is secreted and layered around|
|3 Isthmus||1 hour||Inner and outer shell membranes are added, as are some water and mineral salts|
|4 Shell gland (uterus)||21 hours||Initially some water is added, making the outer|
white thinner. Then the shell material (mainly
calcium carbonate) is added. Pigments may also
be added to make the shell brown
|5 Vagina/cloaca||less than 1 minute||The egg passes through this section before|
laying. It has no other known function in the
Optimum vitamin nutrition of laying hens
The overall goal of the layer industry is to achieve the best performance, feed utilization and health of birds. All nutrients including proteins, fats, carbohydrates, vitamins, minerals and water are essential for these vital functions, but vitamins have an additional dimension. They are required in adequate levels to enable the animal to efficiently utilize all other nutrients in the feed. Therefore, optimum nutrition occurs only when the bird is offered the correct mix of macro- and micronutrients in the feed and is able to efficiently utilize those nutrients for its growth, health, reproduction and survival.
Vitamins are active substances, essential for life of man and animals. They belong to the micronutrients and are required for normal metabolism in animals. Vitamins are essential for optimum health as well as normal physiological functions such as growth, development, maintenance and reproduction. As most vitamins cannot be synthesized by poultry in sufficient amounts to meet physiological demands, they must be obtained from the diet. Vitamins are present in many feedstuffs in minute amounts and can be absorbed from the diet during the digestive process. If absent from the diet or improperly absorbed or utilized, vitamins are a cause of specific deficiency diseases or syndromes.
Classically, vitamins have been divided into two groups based on their solubility in lipids or in water. The fat-soluble group includes the vitamins A, D, E and K, while vitamins of the B complex (B1, B2, B6, B12, niacin, pantothenic acid, folic acid and biotin) and vitamin C are classified as water-soluble. Fat-soluble vitamins are found in feedstuffs in association with lipids. The fat-soluble vitamins are absorbed along with dietary fats, apparently by mechanisms similar to those involved in fat absorption. Water-soluble vitamins are not associated with fats, and alterations in fat absorption do not affect their absorption, which usually occurs via simple diffusion. Fat-soluble vitamins may be stored in the animal body. In contrast, water-soluble vitamins are not stored, and excesses are rapidly excreted.
It is now well recognized by the feed industry that the minimum dietary vitamin levels required to prevent clinical deficiencies may not support optimum health, performance and welfare of poultry. The reasons for this are manifold: The productivity of poultry farming continues to grow through genetic improvement of the breeds and through modifications in nutrition, management and husbandry, which considerably increase the demand for vitamins. Furthermore, intensive poultry production may generate a certain level of metabolic, social, environmental and disease stresses, causing sub-optimal performance and higher susceptibility to vitamin deficiencies. The contamination of the feed with mycotoxins and vitamin antagonists can limit or even block the action of certain vitamins. Any of these factors, ranging from the animals’ genetic background and health status to management programmes and the composition of the diet, can separately or collectively affect the need for each vitamin. As intake and utilization of vitamins from natural sources is unpredictable owing to differing contents of vitamins in the feedstuffs (dependent on growing climate and harvesting time of crops, processing and storage conditions of feed ingredients) and variable vitamin bioavailability, it is safer to cover the total vitamin requirement of poultry through dietary supplementation.
More than ever before, the layer industry is currently facing the challenge to improve productivity in order to remain competitive in today’s cost-driven environment. Fortunately, high-performing layer breeds with improved performance pattern, optimized feed conversion capabilities and favourable health characteristics are available. But in order to allow the birds to perform up to their genetic potential, their nutrition and especially their vitamin supply needs to be optimized. In particular, B vitamins are required for efficient nutrient utilization, and together with vitamin A are important to support the hens’ metabolic activity for maintenance and high laying performance. Furthermore, both vitamins C and E improve the birds’ resistance to stress, and help sustain health and longevity. Specific benefits related to superior egg quality can be achieved if supra-nutritional levels of vitamin E are added to the feed of laying hens. And finally, considerable vitamin D activity is required in order to support an adequate skeletal development and to avoid leg problems of various origins.
The optimum vitamin supplementation levels are given in the table below.
|Vitamins (added to air-dried feed)||Replacement pullets||Laying hens|
|Vitamin A (IU/kg)||7 000–10 000||8 000–12 000|
|Vitamin D3 (IU/kg)||1 500–2 500||2 500–3 5001|
|Vitamin E (mg/kg)||20–30||15–302|
|Vitamin K3 (mg/kg)||1–3||2–3|
|Vitamin B1 (mg/kg)||1.0–2.5||1.0–2.5|
|Vitamin B2 (mg/kg)||4–7||4–7|
|Vitamin B6 (mg/kg)||2.5–5.0||3.0–5.0|
|Vitamin B12 (mg/kg)||0.015–0.025||0.015–0.025|
|Pantothenic acid (mg/kg)||9–11||8–10|
|Folic acid (mg/kg)||0.8–1.2||0.5–1.0|
|Vitamin C (mg/kg)||100–150||100–200|
|Hy•D® (25-OH D3) (mg/kg)||0.0693||0.0693|
1 Do not exceed 3000 IU D3/kg feed when using Hy•D®
2 Under heat stress conditions: 200 mg/kg
3 Local legal limits of total dietary vitamin D activity need to be observed
Source: DSM Vitamin Supplementation Guidelines, 2006; Optima Nutrición Vitamínica de los animales para la producción de alimentos de calidad, 2002
The nutritive value of the egg
The egg is one of the most complete and versatile foods available. It consists of approximately 10% shell, 58% white and 32% yolk. Neither the colour of the shell nor that of the yolk affects the egg’s nutritive value. The average egg provides approximately 313 kilojoules of energy, of which 80% comes from the yolk.
The nutritive content of an average large egg (containing 50 g of edible egg) includes:6.3 g protein0.6 g carbohydrates5.0 g fat (this includes 0.21 g cholesterol).
Egg protein is of high quality and is easily digestible. Almost all of the fat in the egg is found in the yolk and is easily digested.
Eggs contain every vitamin except vitamin C. They are particularly high in vitamins A, D, and B12, and also contain B1 and riboflavin. Provided that laying hens are supplemented according to the Optimum Vitamin Nutrition concept (see chapter ‘Optimum vitamin nutrition of laying hens’), eggs are an important vehicle to complement the essential vitamin supply to the human population.
Eggs are a good source of iron and phosphorus and also supply calcium, copper, iodine, magnesium, manganese, potassium, sodium, zinc, chloride and sulphur. All these minerals are present as organic chelates, highly bioavailable, in the edible part of the egg.
Internal and external egg quality
Quality has been defined by Kramer (1951) as the properties of any given food that have an influence on the acceptance or rejection of this food by the consumer. Egg quality is a general term which refers to several standards which define both internal and external quality. External quality is focused on shell cleanliness, texture and shape, whereas internal quality refers to egg white (albumen) cleanliness and viscosity, size of the air cell, yolk shape and yolk strength.
Internal egg quality
Internal egg quality involves functional, aesthetic and microbiological properties of the egg yolk and albumen. The proportions of components for fresh egg are 32% yolk, 58% albumen and 10% shell (Leeson, 2006).
The egg white is formed by four structures. Firstly, the chalaziferous layer or chalazae, immediately surrounding the yolk, accounting for 3% of the white. Next is the inner thin layer, which surrounds the chalazae and accounts for 17% of the white. Third is the firm or thick layer, which provides an envelope or jacket that holds the inner thin white and the yolk. It adheres to the shell membrane at each end of the egg and accounts for 57% of the albumen. Finally, the outer thin layer lies just inside the shell membranes, except where the thick white is attached to the shell, and accounts for 23% of the egg white (USDA, 2000).
Egg yolk from a newly laid egg is round and firm. As the egg gets older, the yolk absorbs water from the egg white, increasing its size. This produces an enlargement and weakness of the vitelline membrane; the yolk looks fl at and shows spots.
As soon as the egg is laid, its internal quality starts to decrease: the longer the storage time, the more the internal quality deteriorates. However, the chemical composition of the egg (yolk and white) does not change much.
In a newly laid egg the albumen pH lies between 7.6 and 8.5. During storage, the albumen pH increases at a temperature dependent rate to a maximum value of about 9.7 (Heath, 1977). After 3 days of storage at 3 °C, Sharp and Powell (1931) found an albumen pH of 9.18. After 21 days of storage, the albumen had a pH close to 9.4, regardless of storage temperature between 3 and 35 °C (Li-Chan et al, 1995).
Heath (1977) observed that when carbon dioxide (CO2) loss was prevented by the oiling of the shell, the albumen pH of 8.3 did not change over a 7-day period of storage at 22 °C. In oiled eggs stored at 7 °C, albumen pH dropped from 8.3 to 8.1 in seven days (Li-Chan et al, 1995).
Increases in albumen pH are due to CO2 loss through the shell pores, and depend on dissolved CO2, bicarbonate ions, carbonate ions and protein equilibrium. Bicarbonate and carbonate ion concentration is affected by the partial CO2 pressure in the external environment.
In newly laid eggs, the yolk pH is in general close to 6.0; however, during storage it gradually increases to reach 6.4 to 6.9. Egg quality preservation through handling and distribution is dependent on constant care from all personnel involved in these activities. The quality of the egg once it is laid cannot be improved, so efforts to maintain its quality must start right at this moment.
The decrease in internal egg quality once the egg is laid is due to the loss of water and CO2. In consequence, the egg pH is altered, resulting in watery albumen due to the loss of the thick albumen protein structure. The cloudy appearance of the albumen is also due to the CO2; when the egg ages, the CO2 loss causes the albumen to become transparent, compared with fresh eggs.
To minimize egg quality problems two things are important: frequent egg collection, mainly in the hot months, and rapid storage in the cool room. The best results are obtained at a temperature of 10 °C. There are six main factors affecting internal egg quality: disease, egg age, temperature, humidity, handling, and storage.
Disease: Newcastle disease and infectious bronchitis produce watery albumen, and this condition may persist for long periods after the disease outbreak has been controlled (Butcher, 2003). Egg age: eggs several days old show weak and watery albumen, and the CO2 loss makes the content alkaline, affecting the egg flavour. Temperature: high temperatures cause a rapid decrease in internal quality. Storage above 15.5 °C increases humidity losses. Humidity: high relative humidity (RH) helps to decrease egg water losses. Storage at an RH above 70% helps to reduce egg weight losses and keeps the albumen fresh for longer periods of time. Egg handling: rough handling of the eggs not only increases the risk of breaking the eggs, but also may cause internal egg quality problems. Storage: eggs are very prone to take on the odours of other products stored with them; separate storage is therefore advised.
The variables mentioned above are particularly important to ensure that a 1-week-old egg, properly handled, can be as fresh as a day-old egg kept at room temperature.
If the egg is properly handled during shipment and distribution, it will reach the consumer’s table with adequate freshness.
External egg quality
Poor eggshell quality has been of major economic concern to commercial egg producers, with estimated annual losses in the USA of around 478 million US dollars (Roland 1988). In Australia in 1998, the impact was of the order of 10 million Australian dollars per year. Information obtained from egg grading facilities indicates that 10% of eggs are downgraded due to egg shell quality problems. Based on values for the UK, Germany and the USA, it has been estimated that the incidence of broken eggs ranges between 6 and 8% (Washburn, 1982). In Mexico in 2005 it was estimated that the egg industry lost between 30 and 35 million US dollars, based on average figures of 2.5% broken eggs and 4% weak shells. These losses occur only between laying and packing, not taking into account losses in transit to the end consumer (DSM Mexico, 2005, unpublished data).
To maintain consistently good shell quality throughout the life of the hen, it is necessary to implement a total quality management programme throughout the egg production cycle.
It has been always recognized that the hen has the most extraordinary method of obtaining and depositing calcium (Ca) in the entire animal kingdom. An egg has an average of 2.3 g of calcium in the shell, and almost 25 mg in the yolk (Etches, 1987). A modern hen laying 330 eggs per cycle will deposit 767 g of calcium; assuming a 50% calcium retention rate from the diet, the hen will consume 1.53 kg of calcium per cycle.
Exterior egg quality is judged on the basis of texture, colour, shape, soundness and cleanliness according to USDA (2000) standards. The shell of each egg should be smooth, clean and free of cracks. The eggs should be uniform in colour, size and shape.
There are five major types of shell problems in the egg industry: 1. cracks due to excess pressure; 2. cracks due to thin shells; 3. body-checks; 4. pimpled or toe holes, and 5. shell-less eggs.
When a producer complains about an increase in downgrade eggs, the first thing required is to determine which types of problems have increased. In a processing plant with 97% A-quality eggs, a typical distribution of the different types of shell problems (downgrade) might be 2.13% stains, 0.85% blood spots, 0.85% meat spots, 61% pressure cracks, 9.8% thin shell cracks, 6.8% body-checks, 13.6% pimpled and 5.1% toe holes. If the percentage of any type of shell problem is abnormally high, then that is the problem needing attention.