Introduction

For eggs, the consumer might equate good egg quality with eggs that have a dark yellow yolk when the egg is broken out. Or, that same consumer might equate a good quality egg as one which has a nice egg shaped shell without any calcium deposits on it and is not cracked. Grades are used to classify eggs into different levels or ranges of quality such as AA, A, or B grades. Most consumers buy eggs that have the grade classification of AA or A but for the most part they do not know what the distinction is between the two. It is important, however, for people involved in every aspect of the egg industry to know: 1) what egg quality really is, 2) what factors go into the determination of good or bad egg quality, and 3) how can egg quality be conserved.

What is Egg Quality

Egg quality is a general term that relates to various standards that are imposed on the eggs. These standards can be broken down into those used for determining the quality of the egg shell itself (exterior egg quality) and those standards which relate to the quality of the interior of the egg (interior egg quality). Some of these standards are based on subjective measures of egg quality and some are based on a more quantitative measure of egg quality. In general, exterior and interior egg quality standards are based on shell cleanliness, shell soundness, shell texture, shell shape, relative viscosity of the albumen, freedom from foreign matter in the albumen, shape and firmness of the yolk, and freedom from yolk defects. In order to classify eggs into the various grades used, an evaluation of all these items needs to be done.

What is Egg Shell Quality

There are many factors that affect the overall quality of the egg shell, but before discussing these factors, it is important to know what makes up the structure of the egg shell. The egg shell consists of about 94 to 97% calcium carbonate. The other three to six percent is organic matter and egg shell pigment. There are also as many as 8,000 microscopic pores in the shell itself. The outer coating of the shell itself consists of a mucous coating called the cuticle or bloom which is deposited on the shell just prior to lay. This protein like covering helps protect the interior contents of the egg from bacteria penetration through the shell. Egg shell quality is determined by the color, shape, and structure of the shell. Colors can range from white to tints to brown and egg shape can also vary.

Numerous factors affect the general functional quality of the egg shell. These factors affect the quality of the shell mostly prior to when the egg is laid. The thickness of an egg shell is determined by the amount of time it spends in the shell gland (uterus) and the rate of calcium deposition during egg shell formation. If the egg spends a short period of time in the shell gland, then shell thickness will be less. Also, the time of day when the egg is laid will also determine the thickness of the shell. In general, the earlier in the day or light portion of the photoperiod the thicker the shell will be. The amount or rate of calcium deposition will also affect the thickness of the shell. Some strains of birds may be able to deposit calcium for the egg shell at a faster rate than others. Another factor such as the age of the hen plays a role in determining the functional quality of the egg shell. As the hen ages, the thickness of the shell usually declines. Other egg shell quality factors such as the formation of abnormal ridges, calcium deposits, or body checks (ridges) are important considerations in determining egg shell quality.

A major problem which affects the thickness of the egg shell is related to the internal blood acid-base status of the bird. During exposure to warm environmental temperatures, the hen reacts by increasing its rate of breathing (increases panting rate) in order to help cool itself. This physiological occurrence causes the lowering of CO2 in the blood and produces a condition termed "respiratory alkalosis." The pH of the blood becomes alkalotic and the availability of Ca++ needed for the formation of the egg shell is reduced. This causes an increase in the production of soft-shelled eggs. Thus, egg shell quality is somewhat compromised during the warm summer months.

The asthetic quality of egg shells relate to the quality factors which the consumer can see, such as soundness of the shell, cleanliness of the shell, shape of the shell, and color of the shell. Several factors do affect asthetic egg shell quality. Factors such as wash water temperature which affects the incidence of "thermal" cracks, moisture condensation on the shell, refrigeration temperature, and mechanical handling of the egg all affect the asthetic quality of the egg shell.

Microbiological contamination of the inside of the egg is greatly affected by the ability of the egg shell to stop the invasion of micro-organisms and bacteria from entering the egg through the shell's pores. When the cuticle or bloom is deposited by the hen on the shell this acts as a barrier to keep bacteria from entering the egg. When eggs are washed, however, this removes most if not all of the cuticle from the shell surface. Thus, bacteria have an easier time entering the egg after washing. Even when the cuticle is removed, the two inner shell membranes help prevent bacteria from entering the egg. These barriers provide a good line of defense against invading bacteria.

Conserving Egg Shell Quality

Many factors can contribute to the conservation of good egg shell quality. If the egg producer manages his flock in the proper manner by providing the proper nutrition and environmental conditions, then high egg shell quality should be achieved. A specific procedure that a producer should do to assure the production of eggs with good egg shell quality is to avoid scaring the birds so that the egg spends the normal amount of time in the shell gland. A key item to remember is to not stress the flock in any manner. Although it may not be practiced, the use of a "ahemeral" lighting program has shown to be effective in causing eggs to spend a longer time in the uterus, thus, producing eggs with thicker shells. Nutritionally speaking, it is important for the producer to feed a ration properly formulated with the correct amount of calcium and phosphorus in the diet (usually 3.5-3.75% calcium, .45% phosphorus). The production of eggs with thick, strong shells usually occurs from young vs. older flocks. Thus, a producer should expect more shell breakage and eggs produced with thinner shells to occur with older flocks and those producing eggs a couple of months after they've been molted. It is also important for the producer to monitor the health of the flock. Since diseases such as Infectious Bronchitis and Newcastle cause egg shell abnormalities in the texture of the shell and shape of the shell, producers should continually monitor their flock for these diseases and follow management practices to avoid their flock from contacting these diseases.

One of the egg shell quality problems that often occurs is the production of eggs having body checks. These are eggs which are cracked in the uterus during shell formation, then the egg shell forms on top of the crack. Robert Bastian reported in a newsletter published by the University of Georgia (Commercial Egg Tips) ways for producers to reduce the problem of body checks. His recommendations were to:

• avoid overcrowding of hens in cages which produces body checked eggs because hens contact themselves and the sides of the cage,
• use flocks that are of a relatively young age because older flocks produce more body checked eggs, and
• use a lighting program which is no longer than the longest natural light in open houses.

Since it was discussed previously that heat stress has a negative effect on egg shell quality , it is important for producers to try to maintain thermoneutral temperatures in the layer house. Sometimes that is an impossible task during the summertime, however, the use of efficient ventilation and possible bird misting systems may help to avoid serious heat stress conditions.

There are several procedures that need to be followed in order to conserve the asthetic quality of egg shells. The frequent gathering of eggs will help prevent the accumulation of dirt and stains on the shell. In addition, when eggs are washed the temperature of the wash water should be about 20°F warmer than the eggs. This will help prevent the occurrence of thermal cracks or "blind checks" as they are sometimes called. Table 1 showed a linear correlation between the incidence of thermal checks and the difference between egg temperature and wash water temperature (DeKalb, 1977). Today's in-line commercial gathering of eggs has helped reduce the incidence of checked and cracked eggs, but producers should still be aware of problems in the collection system that could damage eggs.

As far as procedures followed to conserve micro-biological contamination of egg shells, appropriate washing and egg processing techniques should be followed. The use of the proper sanitizing agent is very important to maintaining egg shell cleanliness and free from bacteria contamination.

What is Interior Egg Quality

Interior egg quality relates to the functional, asthetic, and microbiological contamination factors of the albumen (white) and yolk. Good interior egg quality is essential to consumers who use eggs in many common baking and cooking items. As soon as the egg is laid, its quality begins to decline. As time of storage increases, the overall egg quality as measured by conventional grading standards declines. Even with eventual breakdown of interior egg quality, the chemical composition of the whole egg, albumen, and yolk does not change very much (Table 2).

In addition to their nutritional value, eggs (albumen and yolk) have numerous important functional and asthetic properties. These were described in Mountney and Parkhurst (1995) and are:

• Eggs act as a leavening agent in baked goods (texture of breads and cakes).
• Eggs act as a binding agent to hold other ingredients together.
• Eggs act as a thickening agent (custards, puddings, etc.).
• Albumen acts as a smoothing agent to give icings a desirable texture.
• Egg yolks contain Lecithin (a natural emulsifier) which helps keep fat and other ingredients in a uniform suspension.
• Eggs are used as a clarifying agent which are used to remove extraneous materials from beverages, coffee, etc.
• Eggs are used as a coating for cakes, rolls, cookies, etc.
• Egg yolks add color and richness to foods.

In addition to the previously mentioned functional and asthetic qualities of eggs, both the albumen and yolk tend to take up flavors and odors of surrounding food items or environment.

Microbiological contamination quality and factors of albumen and yolk are very important. Egg white contains a low concentration of the enzyme lysozyme. This enzyme has been shown to have the capability of breaking down the cell walls of some bacteria. Egg white also has a high pH which acts as a retardant for bacteria growth. Other enzymes are also found in egg yolk (peptidase, catalase, amylase, etc.) which help to keep it free from bacteriological contamination. In addition, the egg yolk has a coating called the vitelline membrane which also protects it. Thus, egg albumen and yolk have many defense mechanisms which help prevent microbiological contamination.

Conserving Interior Egg Quality

A number of items need to be mentioned about methods to maintain higher interior egg quality and prevent the breakdown of egg albumen and yolk. It has been shown that an egg will lose on the average of about 2% of its weight when held at 50°F vs. 5% of its weight when kept at 70°F after 20 days of storage. Thus, the main method utilized to maintain high interior egg quality is to store eggs at a relatively cool temperature (45°F or lower). Also, it is important to maintain a relatively high relative humidity (70-80%) in the egg storage area. Since eggs lose moisture due to increased storage time and temperature, the high humidity environment will slow this process down. The use of low storage temperatures will help maintain the good functional properties of both albumen and yolk, reduce the loss of carbon dioxide through the shell from albumen, and help maintain albumen pH. The flow of water from the albumen to the yolk which occurs at high temperatures will be retarded if eggs are stores at a low temperature. Other treatments such as storing eggs in sealed containers, i.e., styrofoam cartons will help reduce the loss of carbon dioxide and moisture.

The process of oiling eggs will also help maintain interior egg quality. The practice of oiling eggs prior to washing the eggs is not a good idea, however, if this is done, the oil gives the eggs a coating of protectant like an umbrella and bacterial contaminants will have an easier time of penetrating the shell. So, it is important to oil eggs after they have been washed. Also, when washing eggs, the temperature of the wash water should be at least 20° warmer than the egg to avoid bacteria from entering the egg. Since it has been previously mentioned that egg albumen and yolk will pick up flavors and odors of the surrounding environment, it is important not to store eggs in a place where they could pick of strong aromas from other food.

There are also a few other items to consider to maximize the conservation of interior egg quality. It has been shown that albumen quality can be influenced by breeding as well as the occurrence of blood spots in eggs. It has also been shown that the use of cottonseed meal in the ration may cause yolks to have a brownish tint. Thus, egg producers wanting to produce eggs with high interior egg quality with no yolk discolorations should be aware of the strain of bird used as well as the type of ration fed.

Summary

In summary, it is important for egg producers and marketers to realize the factors which contribute to egg shell and egg interior quality. Knowing what causes a reduction in egg shell and interior quality is important, but equally important is knowing how to correct the cause to conserve good egg quality.

Further Information

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Source: Illinin PoultryNet - University of Illinois - August 2003