The egg is one of the major animal products. You would have eaten an egg before, just like many other people but have you ever wondered how it is formed? In addition, what is the nutritional importance of the egg? Is the egg only eaten at home or also useful for industries?
These and some other issues concerning the egg formation are the focus of this article. It is important to know what the egg is and its importance domestically and industrially.
What is an egg? An egg is an oval or round object laid by a female bird, reptile, fish, or invertebrate, usually containing a developing embryo, which it nurtures and sustains. It is a reproductive cell useful in multiplying or increasing the numbers of animal species.
Eggs are produced or laid by different species of poultry such as chickens, turkeys, ducks, guinea fowls, quail, etc. Chicken eggs are the most commonly consumed eggs and they lay both fertilized and unfertilized eggs.
Fertilized eggs develop from the embryo into the chick, which hatches after twenty-one days. Unfertilized eggs are mostly eaten and are referred to as table eggs because they contain no embryo that will develop into a chick.
In general, eggs are composed of several parts (Figure 3): they have a hard outer shell, which would help protect a growing bird embryo (if the egg is fertilized); and inside the shell are two main components the albumen or “white” of the egg and the yolk.
There are also some thick milky white membranous structures on the inside of the shell (shell membranes). An egg consists of the yolk (30 – 33%), albumen (~ 60%), and shell (9 – 12%).
Eggs are a source of nutrition for people and have been described as one of the most perfect food sources for man.
In developed countries, table eggs are sorted into different grades of sizes. In general, eggs come in newly laid, small, medium, large, and extra-large but specific grades are used by different countries.
However, in most developing countries, no specific grading system is applied to eggs for sale, and eggs are sold subjectively.
Read Also: Factors Affecting Hatchability of Poultry Eggs
Egg Formation and Production Process
The egg is formed through the hen’s (female chicken) reproductive system (Figure 4). The formation of an egg takes just over a day, from ovulation to oviposition.
Ovulation is the release of an ovum (yolk or egg not fully formed) from a ruptured follicle (structure in the ovary where the ovum develops). The ovum drops into or is captured by the infundibulum.
Ovulation generally occurs about half an hour after the previous egg has been laid. Oviposition is the laying of the egg.
Fertilization occurs in the infundibulum, which stores sperm for seven to fourteen days if the hen was inseminated (introduction of sperm by the male chicken (cock) at mating).
Formation occurs as the egg travels down the oviduct (made up of magnum and isthmus) and is encased in the various layers that make up a chicken egg.
Although two sets of ovaries and oviducts are present during embryonic development, only the left set fully develops in chickens. The mature ovary will have several follicles in different development stages at any one time.
The largest follicle is the one that is ovulated to produce an egg. The ovary will normally produce one mature yolk in about 24 hours.
The formation of the chalazae and perivitelline membrane (the membrane surrounding the yolk) occurs in the infundibulum of the oviduct. It will take approximately 15 minutes for the yolk to pass through the infundibulum to the magnum.
The albumen is deposited when the yolk is in the magnum. The magnum is the longest part of the oviduct and the yolk will take 2-3 hours to pass through the magnum.
The albumen has several functions, which include a nutritional source for a developing embryo, a cushion to protect the yolk against mechanical injury, a bactericide to prevent infection, and a template for the deposition of the shell membranes.
Next, the developing egg passes into the isthmus, where the inner and outer shell membranes are formed. This process takes approximately 1½ hours. The egg then enters the shell gland or uterus, where it will spend 18 to 21 hours.
During that time, the albumen takes up electrolytes and water through a process called “plumping” (or simply filling up). The shell, which consists of roughly 95% calcium carbonate and 5% organic material, is also formed in the shell gland.
The hen’s calcium requirements are highest at this stage of egg formation. Once the shell is completely formed, a protective coating called the cuticle is laid down over the shell.
The laying of the egg, or oviposition, is induced by the effect of hormones, which contract the uterus forcing the egg out through the vagina and cloaca with the large end (broad end of the egg) of the egg coming out first before the small end (narrow end of an egg). Though the egg travels through the oviduct’s small end first, it is laid large end first.
Read Also: Causes of Egg Production Reduction in Poultry Farms and Ways to prevent them
1. Egg Production
The egg is produced commercially by a specialized type of chicken called layers. These chickens are produced by male and female chickens called breeders.
The fertilized eggs are hatched by machines called incubators that automatically provide the conditions necessary for the eggs to hatch. The chickens are hatched after 21 days and are called pullets.
The pullets are raised in poultry houses for about five months before they start laying their first eggs. As mentioned earlier, it takes about 24 hours to lay an egg. Therefore, ideally, all chickens in a flock should lay an egg a day.
However, this is not perfectly so because of differences in the time that individual birds started to lay for the first time, but most lay every day within a time range. About 70-80% of the birds in a flock lay eggs every day.
The number of eggs laid by chickens reaches a maximum at about three months after the first layer and continues optimally for nine months, although at a decreasing rate.
However, birds can still be kept in lay for 12 to 18 or 24 months with a specific management procedure called molting, where birds are forced to lose feathers by dietary manipulations for a period after which they start laying increasingly again but not like before.
Read Also: How to Select the Best Quality Egg Laying Chickens at Point of Lay (P.O.L)
2. Egg Quality
Egg quality refers to all the attributes of the egg that contributes to its value. This is very important to the producer, the traders, and the consumer because consumers will not buy or give low value to poor-quality eggs.
The importance of egg quality may not be readily obvious in developing countries because of the absence of a grading system for egg quality.
For example, the United States Department of Agriculture (USDA) has quality grades that range from AA, A, to B, in decreasing order of value; and eggs are sold with proper markings or labels indicating these.
Nevertheless, even in developing countries, eggs that have misshapen shells, watery or spotted yolk, and albumen will be rated low or avoided by consumers.
Numerous methods have been used to study the quality of eggs. These methods involve the evaluation of each component that makes up the egg. The most common methods can be divided into external and internal qualities according to the following:
1. External Quality refers to a shell’s appearance (e.g. shape), cleanliness, and strength. Appearance is important because the shell is the first thing you notice about an egg.
Cleanliness is important because the shell is the egg’s first defense against bacterial contamination; the cleaner the shell, the easier it can do its job. Strength influences the egg’s ability to remain intact until it is ready for use.
The shell is made up of 94 percent calcium carbonate and accounts for about 12 percent of the weight of a large egg.
The shell quality is measured as breaking or crushing strength, the amount of shell as a percentage of egg weight, and shell thickness (measured with a micrometer gauge). Shell thickness is adjudged as the simplest, most rapid, and most adequate measure of shell strength.
2. Internal Quality: refers to the appearance and consistency of the egg’s contents, which is determined easily by breaking the egg into a glass plate for examination. Methods that evaluate the internal quality consider the thickness of the albumen, color, and thickness of the yolk in various measures.
The most common of these measures are the albumen index, yolk index, Haugh units, and yolk color. Haughs unit measures the quality of the albumen. As an egg age, both its white and yolk deteriorate and internal measures of quality give lower values.
Albumen and Yolk Index = (Albumen or Yolk height in mm / Albumen or Yolk width in mm) x 100
The following describes how the Haugh unit is determined. An egg is weighed, then broken onto a flat surface (breakout method), and a micrometer is used to determine the height of the thick albumen (egg white) that immediately surrounds the yolk.
The height, correlated with the weight, determines the Haugh unit rating. The higher the number, the better the quality of the egg (fresher, higher-quality eggs have thicker whites).
Haugh Unit = 100 log10 (h – 1.7w0.37 + 7.6).
Where;
h = observed height of the albumen in millimeters w = weight of egg in grams.
The Haugh Units are expressed in a figure. The Haugh unit value ranges from 0 to 130.
If the Haugh Units found are 75 and 90+, the internal quality/freshness of the sample egg is excellent, however, if the Haugh Units are between 60 and 30 or even less the internal quality/freshness can be considered poor or very bad.
Yolk color is measured by the use of a color fan (a device that has a spectrum of colors of different shades of yellow to orange) or a digital spectral machine to determine the specific color of the yolk.
The color of an egg yolk is directly influenced by the quality of the chicken feed. Egg yolk color is generally improved with high-quality feed with a large component of yellow, fat-soluble pigments, such as the carotenes in dark green plant material.
Although much emphasis is put on the color of the egg yolk, it does not reliably reflect the nutritional value of an egg.
Some of the natural pigments that produce a rich yolk color are xanthophylls without much nutritional value, rather than the carotenoids that act as provitamin A (precursors to the formation of vitamin A) in the body.
A diet rich in vitamin A itself, but without A-provitamins or xanthophylls, can produce practically colorless yolks that are just as nutritious as any richly colored yolks.
In conclusion, the exploitation of the natural attribute of the chicken to lay an egg a day is a very important step in ensuring the availability of food with a high nutritional value.
The portability of the egg and its high nourishing value will be significant in contributing towards the alleviation of global quality food shortages.
Read Also: Management Guide for Layers for Better Egg Production
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