NUTRITIONAL AND HEALTH ATTRIBUTES OF EGGS

From the Part VI, Chapter 28 of the HANDBOOK OF POULTRY SCIENCE AND TECHNOLOGY


INTRODUCTION

Eggs contain everything that a bird needs to initiate and complete all of the processes of embryonic development. Birds invest in the reproductive process up front, transferring all the protein and energy needed to allow the embryo to reach the point of hatch. This is all packaged in a sturdy shell that is constructed in a way that allows for gas exchange while maintaining a defense that keeps pathogens out (Figure 1).
As an important part of the human diet, eggs are one of the few foods that are consumed throughout the world. It surely could be a timeless chicken-or-egg debate; however, without doubt, eggs are a perfect food, containing almost every nutrient essential to sustaining life—thus their role as total life support for the embryonic chick. The original jungle fowl were domesticated near modern-day India around 3200 b.c. Evidence of laying egg consumption from domesticated chickens is found in Chinese and Egyptian records dating back to 1400 b.C. Additionally, there is archaeological evidence of egg consumption going back to the Neolithic age. Romans first brought domestic fowl to northeastern Europe and England, and they first appeared in North America with Columbus’s second voyage in 1493 (Katz, 2003).
The protein in egg white is of such high quality that it has become the standard against which other proteins are judged. Egg yolk contains a great number of vitamins and minerals, including vitamins A, B12, D, and E, plus riboflavin, folic acid, iron, zinc, phosphorus, selenium, and choline. It is one of the few sources of vitamin K.

However, shell egg consumption in the developed world has decreased or leveled off over the last three decades, due largely to consumer perceptions as to its cholesterol content. Other reasons for the decline include changing eating patterns [e.g., breakfast (when eggs have traditionally been eaten) is the meal most often skipped] and a demand for “heat and serve” grocery products that can be eaten on the move (Elkin, 2006). The shell egg market has turned around in recent years and slow growth is currently being experienced, with new emphasis on the egg’s positive attributes. Specialty eggs, such as omega-3, brown eggs, free-run, and organic eggs, are in greater and greater demand by consumers.
Consumers are increasingly interested in functional foods that can prevent or ameliorate chronic diseases. “Designed” egg concepts, such as eggs enriched with omega-3 fatty acids, which reduce the risk of cardiovascular diseases; with essential antioxidants such as lutein for eye health; and with vitamins such as folate for the prevention of neural tube defects in babies, represent an important milestone for the egg industry (Sim and Sunwoo, 2006).

TABLE EGG PRODUCTION

Producing an egg involves the conversion of genetic, environmental, and nutritional cues into a cascade of signals from the neuroendocrine system. These signals must be integrated and responded to by the organs and tissues primarily involved in reproduction, which will in turn produce more signals for both local and distant activities. The resulting eggs produced are the net result of a bird’s attempt to coordinate the demands that its body and environment have placed on it. Specific feed ingredients, hen age, and flock management decisions can directly affect the egg environment. Minor dietary ingredients are not always preferentially deposited in the yolk or egg. This opens the door to creating an “enriched” environment within the egg. An understanding of how some feed ingredients affect the egg environment directly can contribute to the improved success of table egg enrichment programs.

Control of Egg Production

The reproductive system of the laying hen is made up of the ovary, hormonal control centers, and support structures. The system includes the hypothalamus, the anterior pituitary, the ovary, the oviduct, the liver, and the skeletal system. The onset of sexual maturation begins when gonadotrophin-releasing hormone (GnRH)–producing neurons acquire the ability to release GnRH, a small protein hormone. Increasing day lengths are perceived by the bird, and if its nutritional status is adequate to support a reproductive effort, maturation of the reproductive tract begins. The GnRH will travel a short distance to the anterior pituitary, where it stimulates the release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH). These hormones trigger synthesis of estrogen and androgen hormones from the small follicles of the ovary. As follicular maturation progresses, the response to LH increases, whereas the response to FSH decreases (Calvo and Bahr, 1983). On the ovary, steroidogenesis of the small follicles, particularly estrogen production, triggers the sexual maturation process. Increased concentrations of estrogen in the blood are associated with visible external changes, such as reddening and enlargement of the comb and wattles, a prenuptial feather molt (feather drop), and a widening of the pubic bones to permit egg passage. In the liver, estrogen stimulates production of egg yolk lipids. Yolk formation is stimulated by estrogen and modulated by some of the metabolic hormones. The oviduct also enlarges to its mature size and grows capable of secreting egg albumen.
Follicles destined to ovulate are recruited from a pool of immature follicles. The ovary contains thousands of these tiny follicles. More follicles than are needed are recruited for development. This ensures that a constant supply of good-quality follicles are available to enter the rapid growth phase, where yolk deposition occurs. Excess small follicles are reabsorbed by a hen through a process of follicular atresia. The rapid yolk deposition phase is the hen’s greatest energetic investment in yolk formation. Follicles that reach this stage are highly likely to be ovulated. Continuous recruitment of replacement follicles from the immature follicle pool on the ovary creates continuity in yolk deposition requirements.
A hen will normally maintain five to eight large yellow follicles on the ovary. The growing follicle is transferred into the rapid growth phase weighing between 0.6 and 0.7 g and requires 7 to 11 days to pass from this point through to ovulation (Grau, 1976). In contrast, the process of egg formation in the laying hen takes between 24 and 27 h from the moment of ovulation to the moment that an egg is laid.
The ovulatory cycle of the hen is 24 h. Follicular maturation occurs independently, and if the most advanced follicle reaches the point of maturation (ability to secrete progesterone into the bloodstream rather than first having it converted to estrogen) during the 10-h portion of the day during which this can be detected, ovulation will be triggered.

Yolk Production and Egg Formation

The enrichment ingredients provided to hens to support the creation of enriched eggs must be transferred to an egg in either the yolk or the albumen. The egg is approximately 58.5% albumen, 31% yolk, and 10.5% shell. These values vary with age and bird strain. The proportion of yolk in the egg increases as the bird ages, at the expense of albumen. A normal egg weight for a laying hen would be in the range 50 to 65 g. The egg contains a large amount of nutrients for the purpose of supporting the growth of the embryo. However, this concentration of easily digestible nutrients also makes it an ideal food source. The albumen contains primarily protein (and 88% water), whereas the yolk contains both lipids and proteins (2 : 1 ratio). The high fat content allows more energy to be packed into a smaller package without the concomitant association with water as happens with proteins (Speake and Thompson, 1999). Most of the constituents of yolk are derived from the blood plasma. Key ingredients such as yolk lipids and vitellogenin are constructed in the liver and transported to the ovary, where they are transferred to the growing ovum. More than 90% of fatty acid synthesis occurs in the liver of poultry, for example (Leveille et al., 1975). The liver processes dietary fat, constructs other fat from carbohydrates, and packages them for release into the bloodstream. As such, the yolk production and transportation mechanisms are critical components in support of the reproductive effort of hens.
Estrogen stimulates the range of structural adaptations which ensure that yolk–very low density hypoprotcin can be diverted to the growing ovarian follicles (Speake and Thompson, 1999). One of the roles of estrogen is to stimulate production of specific apoproteins for the creation of a unique very low density lipoprotein particle destined for yolk deposition. The yolk materials are deposited in the ovarian follicles in a circadian pattern, with distinct, successive layers of yolk deposited around the yolk core. During ovulation, the mature ovarian follicle ruptures along the stigma, a linear avascular area on the follicle, and the ovum is released from the ovary.

The yolk is protected by the yolk membrane, which is protein-based and forms as a meshlike structure. The infundibulum, which is the uppermost region of the oviduct, uses its thin, lightly muscularized tissue to engulf the ovum and funnel it into the oviduct (Figure 2). The yolk passes through in 15 to 30 min. The perivitelline membranes of the yolk and the chalazae layer of the albumen are applied in the infundibulum. The chalazae are the white coiled albumen structures that hold the egg yolk in the middle of the egg.
The ovum takes 3 to 4 h to pass through the magnum, where egg albumen is released, due in part to mechanical pressure from the moving ovum (Moran, 1987). The thick albumen from this region was formed previously and the walls of the magnum contain about a 2-day supply. Shell membranes are added to the forming egg during the 1.5 h it needs to pass through the isthmus. As the egg cools, the two shell membranes separate at the large end of the egg, forming the air cell (Figure 1), which will continue to grow due to moisture loss from the egg during storage.  Final “plumping” occurs when fluid is added to the albumen in the shell gland. A crystalline calcium carbonate and glycoprotein matrix is formed on the shell membrane from glands in the surface of the shell gland secreting sodium bicarbonate, sodium chloride, potassium chloride, and calcium chloride in liquid form. Chicken eggs require roughly 20 h in the shell gland, followed by a period of a few seconds to pass through the vagina to complete the oviposition process (Burke, 1984). Management of pullets and laying hens must often be strain-specific to optimize the frame size and body mass needed for adequate early egg size, long-term persistency, and prevention of skeletal disorders. Selection of layer strain, nutrient intake, and the rearing photoschedule can affect laying hen productivity. The goal is to create a bird large enough to begin to lay with a good egg size and to maintain a high rate of lay for the entire production year without burning out or suffering the effects of calcium depletion.

Table Egg Quality

The egg industry was built around the concept of marketing eggs into the consumer market while retaining their original quality. Quality is a broad term that captures aspects of physical characteristics, flavor, and odor. A high-quality egg can command a higher price for the producer and will maintain its quality longer during postmarketing. Egg grading is done commercially to classify egg quality using established standards. Quality is determined using external factors such as cleanliness, shell condition, color, and shape, as well as internal factors such as albumen thickness, yolk condition, air cell size, and the presence of defects such as meat or blood spots. Even poor-quality eggs deliver the same nutritional quality.
The storage of eggs is part of the commercial table egg marketing process. The maintenance of egg quality is strongly affected by storage conditions from point of lay to point of consumption. Moisture loss through the shell pores contributes to increased air cell size and reduced quality. This process can be modulated by the temperature, relative humidity, and ventilation of the egg environment. Incorrect temperature and humidity will increase CO2 loss from the egg, which results in the breakdown of albumen structure.
Thin, runny albumen is a sign of poor egg quality. Young flocks lay eggs with high-viscosity albumen (Lapao et al., 1999). Albumen pH rises with storage and hen age. The albumen proteins are broken down by enzymes, degrading the structure and releasing the protein-bound water. The albumen gradually liquefies with storage, and in older hens, storage can cause more rapid deterioration of albumen quality (Lapao et al., 1999).
The freed water and thin albumen can cross the yolk vitelline membrane, increase yolk mass, and put pressure on the yolk membrane (Moreng and Avens, 1985). This is an important issue in the eggbreaking industry, where adequate separation of yolk from albumen is required. Stabilizing the albumen or slowing the rate of deterioration during egg storage can optimize long-term egg quality. Degradation of egg quality can be slowed with good antioxidant levels. The primary defense mechanisms within the embryo are a group of three enzymes (superoxide dismutase, glutathione peroxidase, and catalase) which convert free radicals produced by cellular respiration into less harmful alcohols (Ursiny et al., 1997). A second level of defense consists of the natural antioxidants: Vitamin E, the carotenoids, ascorbic acid, and glutathione protect the developing chick (Surai, 1999). The secondary level of defense is more important in the table egg because there are no growing tissues.
Several Brazilian studies examined the effects of an organic form of selenium on albumen quality and consistency and noted an improvement in albumen height with organic selenium (Rutz et al., 2005). Payne et al. (2005) reported the opposite effect, demonstrating that other factors may also influence the results obtained. Whereas Monsalve et al. (2004) did not report a difference in vitelline membrane strength due to either an inorganic or organic selenium source, they did report a positive effect of increased selenium concentration on membrane strength. Results like this may indicate a protective effect on the cellular membranes of the magnum under certain conditions.  Rutz et al. (2005) theorized that the indirect mode of action of organic selenium here may be through enhanced function of the selenium-dependent GSH-Px antioxidant system. If the secretory cells and tubular glands of the magnum are able to function more effectively, more protein will be secreted into the lumen of the magnum, resulting in a more viscous egg white (Butts and Cunningham, 1972). Whereas egg-handling and storage conditions can influence how long an egg may stay fresh, there are also factors that can affect egg quality even before it is laid. Bird genetics, age, nutrition, environmental temperature, and disease status are some of major factors affecting egg quality.

Genetics 
Breed choice can be an important part of the fit of a company to its target market. Breed comparisons reveal strains with increased incidence of defective shells or poor early egg size. Hens can vary in their eggshell color, shape, yolk, and albumen quality attributes. Positive traits such as superior feed conversion or albumen quality can compensate for some negative traits, however. Some hen-based quality traits are difficult to reduce in time within a strain, such as blood or meat spots in brown eggs, because they are difficult to visualize with standard candling methods. Other breed-based traits, such as immune response, can be an integral part of a strategy to produce Ig-Y eggs.

Hen Age 
Hen age has the greatest influence on egg composition. The size of the pullet at photostimulation determines probable future egg size, with bigger eggs coming from bigger birds. Yolk size increases with age at a faster rate than albumen does, thereby increasing the proportion of yolk in eggs from older hens. Beginning egg production later through delayed photostimulation will result in larger eggs at the start of production (Robinson et al., 1996). As hens age, egg size increases while shell quality goes down. Big eggs in older hens will tend to have a lower percentage of shell than in younger birds, resulting in poorer shell quality due to a thinner shell. Rate of egg production also has a big influence on egg size and composition. High-producing hens sometimes have both smaller eggs and poorer shell quality because they are not able to keep up with the magnitude of the demands of their rate of egg production. The time of day can affect shell quality, as chickens generally lay bigger eggs in the morning than in the afternoon. The specific gravity of the afternoon eggs will be greater, suggesting that these eggs have a better quality shell than those of the morning eggs. A hen that is not laying as well will not have as many large yellow follicles on its ovary. These follicles are often larger and contribute to greater egg size. In general, the yolk/albumen ratio is fixed, meaning that yolk size directly influences how much albumen is laid down as it travels down the oviduct (Williams et al.,2001).

Nutrition 
Manipulating the dietary protein is one of the most effective ways to alter egg weight. As dietary methionine content is increased, for example, there is an almost linear increase in egg weight (Leeson and Summers, 2001). Varying the dietary fat source and inclusion rate can also alter egg weight, with changes primarily in the albumen weight. These changes may relate to estrogen metabolism and its influence on oviduct function, as higher plasma estrogen concentrations were linked to higher egg weights (Whitehead et al., 1993).
In vitamin-enriched eggs, for example, factors affecting egg levels of specific vitamins include diet, breed and age of hen, level of egg production, and stage of the production cycle (Naber, 1979, 1993). According to Naber (1993), the most important factor influencing egg vitamin content was the dietary inclusion level of each vitamin. Increased dietary energy content can increase the percentage of yolk in an egg (Spratt and Leeson, 1987). The adjustment of combinations of specific dietary ingredients (such as methionine, choline, folic acid, and vitamin B12) has been used successfully to alter egg size without affecting the rate of lay (Keshavarz, 2003). Feed additives are one of the best ways to modify egg composition (as with omega-3 polyan-saturated fatty acid (PUFA)–enriched eggs) and add value-added ingredients. Yolk color and shell quality are both easily modified through feeding. Some minerals and water-soluble vitamins can be altered by changing dietary inclusion rates, although many of them are limited to the capacity of the carrier molecules and binding proteins that aid in their transfer into the yolk. Off-flavors and factors contributing to reduced shelf life can also be introduced inadvertently through the diet. Proper management of calcium metabolism is essential for laying hens to maintain high levels of production. Skeletal calcium deficiency and poor longterm shell quality are both management and animal welfare concerns in laying hens. Numerous products are on the market, from vitamin D metabolites to products enhancing calcium and phosphorus management by the hen. Simple changes such as using large-particle calcium sources contribute to improved shell quality and bone health by allowing the particles to be retained in the gizzard longer, thereby making it available for a longer portion of the day.

Environmental Temperature 
Egg size declines in the hot summer months. Declines in water consumption due to water being too warm or cold can also limit egg size. With heat stress, birds pant and shell quality can drop due to a reaction of calcium with carbon dioxide, resulting essentially in calcium being breathed out.

General Chemical Composition of Eggs

The weight and composition of a table egg is dependent on heredity, age, season, diet, and other factors. A typical White Leghorn egg usually weighs from 53 to 63 g with an average of 55 g. Board (1969) has shown that in addition to water (74%), the main chemical compositions of hen egg are 11.8% lipids, 12.8% proteins, and small amounts of carbohydrates and minerals. Most of the proteins are present in the egg white and the egg yolk, amounting to 50% and 44%, respectively; the eggshell contains the rest of the proteins. The yolk accounts for slightly over one-third of the edible portion, but it yields three-fourths of the calories and provides all or most of the fat in whole eggs.
The yolk comprises 48% water, 16% protein, 32.6% fat, and some minerals and vitamins. The white consists of 88% water, 10% protein, and some minerals. The amount of lipid in the egg white is negligible (0.01%) compared with the amount present in the yolk. The shell makes up 11% of the weight of an egg, and approximately 98% of the shell consists of calcium.
Carbohydrates are a minor component of hen eggs. Their average content is about 0.5 g per egg, 40% of which is present in the yolk (Sugino et al., 1997). Carbohydrates are present as free and conjugated forms which are attached to  proteins and lipids.

Glucose accounts for about 98% of the total free carbohydrate in the white. The content of carbohydrate in egg yolk is about 1.0%; 0.7% of it consists of oligosaccharides bound to protein, composed of mannose and glucosamine; the remaining 0.3% is free carbohydrate in the form of glucose. About 94% of the minerals are in the eggshell fraction; the rest are distributed in egg white and egg yolk. Most of the minerals are in conjugated form, and only a small portion is present as inorganic compounds or ions (Romanoff and Romanoff, 1949). Calcium represents over 98% of total mineral in the shell; other inorganic components include phosphorus, magnesium, and trace contents of iron and sulfur. Egg yolk contains 1.1% minerals (Board, 1969), phosphorus being the most abundant. More than 61% of the total phosphorus of egg yolk is contained in phospholipids. The major inorganic components of egg white are sulfur, potassium, sodium, and chlorine.

By Yuan Ren, Jianping Wu, and Robert Renema (Department of Agricultural, Food and Nutritional Science, University of Alberta, Canada) in the 'HANDBOOK OF POULTRY SCIENCE AND TECHNOLOGY' Volume 1: Primary Processing. Editor Isabel Guerrero-Legarreta, Ph.D., Consulting Editor Y.H. Hui, Ph.D. and Associate Editors. Wiley (A John Wiley & Sons Inc. Publication), Hoboken, NJ, U.S.A, 2010, p.536-544. Adapted to be posted by Leopoldo Costa.