Egg quality

J.R. Roberts

663

Egg quality is influenced by a range of factors, including strain of bird, age of bird, nutrition, moult status, water quality, general stress, heat stress, disease, housing, production system, environmental contaminants and use of proprietary products designed to improve egg quality.

Improved understanding of the way in which the egg and egg shell are formed, includes knowledge of the proteins comprising the organic matrix of the shell has assisted with diagnosis of the causes of egg shell quality problems and with genetic selection for good quality. On-going technological advances have led to improved in-line monitoring of egg-shell quality. Egg quality is also important for food safety as eggs are periodically implicated in cases of food-borne illness.

Why egg quality is important

Commercial eggs can be broadly divided into table eggs and hatching eggs and egg quality is important for both. For the table egg, eggshell quality affects the visual appearance of the egg and the integrity of the egg, until the consumer uses it. It is also related to the microbiological quality of the egg. For fertile eggs, hatchability depends on the quality of the eggshell which needs to be intact, clean and free from bacteria and other microorganisms, permeable enough to allow gas exchange to occur but not too permeable or the egg will dry out too quickly. 

The shell of a hatching egg must also allow the chick to hatch out from the egg. Egg internal quality is also important in both table and hatching eggs. For the hatching egg, the contents must contain all the nutrients required by the developing chick.

For a table egg, the consumer prefers yolk of a particular colour and has a preference for a particular range of albumen viscosity. There is a known association between albumen viscosity and egg freshness, with fresh eggs having a more firm or viscous albumen. The internal composition of the egg also contributes to the microbiological safety of the egg.

The process of egg formation

Formation of the hen’s egg commences with the ovulation of the yolk from the left ovary into the left oviduct. The yolk constitutes part of the oocyte, which has developed from a primary oocyte as the result of deposition of proteins and lipids produced in the liver and transported to the ovary in the blood. The ovulated yolk is captured by the infundibulum of the oviduct where the developing egg remains for about 20 minutes and acquires the two outer layers of the perivitelline membrane including the chalazae. In breeder birds, fertilization occurs in the infundibulum.

The egg then moves into the magnum where it remains for about 4 hours during which time the egg white or albumen is secreted. The egg spends approximately 1 hour in the isthmus where the inner and outer shell membranes, consisting of a network of fibres made up of proteins and glycoproteins, are deposited onto the egg. The longest period of time, 19 hours is spent in the uterus (shell gland) where the eggshell is formed. The terminal region of the isthmus, the red isthmus also known as the tubular shell gland, is where water and electrolytes enter the egg by a process known as “plumping” and the formation of the mammillary cores commences over a period of about 5 hours. The first crystals of calcite form at the sites of organic aggregates present on the surface of the outer shell membrane and grow multi-directionally to form the mammillary bodies which join to form the compact palisade layer.

The eggshell consists of an organic matrix in addition to the inorganic calcium carbonate. There is evidence that the synthesis of the organic matrix, which is composed of proteins, glycoproteins and proteoglycans, controls the deposition of the shell. The surface crystal layer is deposited on the palisade layer, followed by the cuticle, which appears to play an important role in protection of the egg from bacterial ingress.

Measurement of egg quality

Over the years, egg quality has been tested in a variety of ways. Currently, in countries such as Australia, many commercial table eggs are processed using sophisticated equipment which automatically detects cracks, inclusions and dirt on the outside of the egg. Such automated equipment frequently includes the step of egg decontamination by washing. However, some producers use less sophisticated, more manual equipment. In addition, large processing floors also have a quality assurance facility which uses equipment which is more similar to that used in research laboratories. A range of testing equipment is available from companies such as Technical Services and Supplies (U.K.), Nabel Co., Ltd (Japan), Orka Food Technology (China) and Ovobel (Belgium). In the laboratory, measurements of egg shell quality include shell colour (measured by shell reflectivity or spectrophotometry), egg weight, shell breaking strength and deformation to breaking point, shell weight and shell thickness. Egg internal quality is measured as albumen height, Haugh unit and yolk colour. The extent of cuticle coverage of an egg can be determined by staining the cuticle with a histological dye and then measuring the extent of staining with a hand-held spectrophotometer. The quality of construction of an eggshell may be assessed by studying the ultrastructure of the mammillary layer of the eggshell following the removal of the shell membranes.

Factors affecting egg quality

a) Strain
As the result of genetic selection, different strains of laying hen vary in eggshell quality, egg size and production and there are clear differences between modern commercial birds and traditional breeds of laying fowl. Selection for increased production may result in reduced eggshell quality. Knowledge of the heritabilities of selected egg quality traits and the use of molecular genetic and marker technologies enable targeted selection programs by breeder companies.

b) Age
Egg-shell quality and albumen quality generally decrease as birds grow older. As hens age, egg size generally increases, eggs become more elongated, shell colour becomes lighter, shell weight generally increases (but not necessarily in parallel with increases in egg weight leading to decreased percentage shell), shell thickness may stay the same or decrease and egg shell breaking strength decreases. Shell ultra structural quality also decreases in eggs laid by older hens. Albumen height and Haugh Unit decrease with increasing hen age.

c) Nutrition
Optimal hen nutrition is vital in ensuring good egg quality. The pullet diet has an important effect on hen performance by influencing sexual maturity as well as body weight and composition at the onset of lay. The protein level of the feed during the growth phase influences body weight. In mature hens, egg weight is influenced by the quantity of protein consumed. In addition, the amino acid composition is important with methionine being the main limiting amino acid followed by threonine, valine and lysine. The energy content of the diet affects feed intake.

Management of the supply of calcium as pullets come into lay is critical because the calcium level of the feed and the form of calcium influences feed intake and egg weight. Feed too low in calcium may result in overconsumption leading to increased levels of fat in the body. 

Typically, a pre-layer diet with approximately 2.5% calcium is introduced 2-3 weeks before the onset of lay. The consensus appears to be that 50-70% of calcium should be in the form of coarse particles (2-5 mm diameter) and the remainder in powder form. Dietary phosphorus levels are important for bone formation and the ratio of calcium to phosphorus can affect the absorption of calcium from the gastrointestinal tract. The model of Kebreab et al. (2009) illustrates the interaction between these two minerals.

Vitamin D is essential for calcium metabolism and use of the vitamin D metabolite 25-hydroxyvitamin D3, which is converted into the biologically active form of vitamin D3 in the bird, can improve shell quality under some circumstances. Adequate levels of vitamin C are essential for normal good health and may also help to alleviate the effects of stress and it has been suggested that vitamin E assists under conditions of heat stress.

Use of non-starch polysaccharide-degrading enzymes has been shown to improve egg quality under some circumstances. Phytase supplementation is used in poultry diets to release phytate-bound phosphorus and reduce phosphorus levels in effluent. Phytase supplementation has been shown to improve eggshell quality and there is evidence for a synergistic effect between phytase and xylanase.

Contaminants such as mycotoxins have the potential to reduce production and eggshell quality. There were problems in the past with hens possessing an inherited gene accumulating significant amounts of trimethylamine (TMA) in eggs as the result of an inability to oxidise the TMA found in feed ingredients such as canola meal and fish-meal. This is no longer a problem owing to selective breeding.

d) Induced moult
Induced moulting is practised in some countries to extend the laying life of the flock and to improve egg quality. However, the improvements in egg quality may be short-lived which means that moulting tends to be used as a management practice to ensure continuity of supply and at times which costs of replacement flocks are high.

e) Stress
A range of types of general stress can affect eggshell quality. High population densities were shown some time ago to increase the production of body-checked eggs. Body-checked eggs are thought to result from contraction of the shell gland while the eggshell is in the early stages of formation.

Stress can also induce delays in the timing of oviposition when hens retain their eggs and this can result in an increased incidence of white- banded and slab-sided eggs. The stressors of relocation, or exclusion from nest boxes of birds that normally had access to them, can cause an increase in the incidence of calcium dusted, white-banded, slab-sided and misshapen eggs. Even handling of birds, which are not used to handling can increase the incidence of cracked eggs. Many of the deleterious effects of general stress on egg quality can be induced by injections of adrenaline.

The high temperatures experienced in most parts of Australia and also in other countries during the summer can result in smaller eggs and reduced shell quality via a number of physiological processes occurring within the bird. Heat stress reduces feed intake and limits the availability of blood calcium for eggshell formation. It may also reduce the activity of carbonic anhydrase, an enzyme, which results in the formation of bicarbonate, which contributes the carbonate to the eggshell. Therefore, supplementation during heat stress may improve eggshell quality.

Feeding practices in hot weather should focus on ensuring that birds are receiving adequate levels of essential nutrients. Diets need to be formulated to match feed consumption and it should be recognized that birds will tend to eat most during the cooler times of the day. The addition of fat to the diet during hot weather has beneficial effects, apparently via a number of mechanisms. Provision of half the dietary calcium in a coarse particulate form can improve eggshell quality in heat stressed birds. However, there is no evidence to suggest that increasing the calcium level of the diet above that necessary to achieve an adequate intake of calcium has any beneficial effect. The phosphorus requirement of laying hens increases slightly at hot environmental temperatures. Other dietary remedies that have been tried to alleviate the negative effects of heat stress include addition of sodium bicarbonate to the diet and supplementation of dietary electrolyses and addition of aluminosilicates. However, the results of using these additives have been variable. The provision of cool drinking water can alleviate the effects of heat stress.

f) Disease

A number of diseases have been reported to affect egg and eggshell quality and any disease that compromises the health of the bird may result in defective eggs and eggshells by indirect means.

Infectious bronchitis virus causes loss of shell colour, elongation of eggshells (lower shape index) and reduced albumen quality. Other viruses that have been shown to affect production and quality around the world include egg drop syndrome, swollen head syndrome, avian encephalomyelitis, avian influenza, Newcastle disease, and laryngotracheitis. Bacterial diseases that have been shown to decrease production and egg quality around the world include Salmonella, Mycoplasma gallisepticum, Escherichia coli, Ornithobacterium rhinotracheale, Gallibacterium anatis, the spirochaetes Brachyspira pilosicoli, B. Intermedia. Syndromes that can affect egg production and quality include fatty liver syndrome, cage layer osteoporosis.

g) Production system
The type of production system may influence eggshell quality. Early problems with cracked eggs in furnished cages have been greatly improved by changes in design of the furnished cages to include egg saver wires and long nest curtains as well as increasing nest attractiveness and lowering perch height. Direct comparisons among the different types of production system have been made difficult by the shortage of experiments in which all other variables have been maintained constant. Some of the problems with eggshell quality reported from free range systems may result from an inability to ensure a balanced diet for the hens. Some studies have found effects of cage density on eggshell quality whereas others report no consistent effects. Some strains of birds appear to be more suitable for particular production systems.

Egg quality and product safety

The relationship between egg quality and the safety of the egg for human consumption has received considerable attention in recent years. Table eggs are regularly implicated in outbreaks of food-borne illness, which makes it essential that the commercial industry conducts regular monitoring to ensure a safe food product.

Salmonella can contaminate intact eggs by vertical transmission, where eggs are contaminated via the reproductive tract of the hen while the egg is being formed, and horizontal transmission. Egg contamination by horizontal transmission occurs when Salmonella penetrates the eggshell during or following oviposition and can lead to contamination of the internal contents. For salmonellae other than S. Enteritidis, horizontal transmission is the most common route for the contamination of egg internal contents.

Bacteria such as Salmonella can multiply within the yolk of the egg but in order to reach the yolk, they need to pass through the shell either via defects or pores, cross the two shell membranes, move through the albumen and then across the perivitelline membrane and into the yolk. The hen and the egg possess many defence mechanisms that protect against bacterial contamination. Lactobacillus flora in the cloaca and vagina, have an inhibitory effect on bacteria. Eggs with intact cuticle blocking the pores are less likely to be contaminated. The cuticle, eggshell organic matrix and shell membranes have antibacterial properties. The albumen contains antibacterial substances in the form of lysozyme, ovoinhibitor, cystatin and ovotransferrin. The perivitelline membrane has antibacterial properties in addition to acting as a mechanical barrier. Finally, the yolk itself contains antibodies, which may afford some protection.

In Australia, the serovar of greatest concern is S. Typhimurium, which appears to contaminate eggs via horizontal transmission, although there has been some speculation about types of S. Typhimurium being able to enter eggs by vertical transmission. Abnormalities in eggshells can potentiate the entry of food borne pathogens into the eggs.

In many countries, including Australia, eggs are commonly washed to remove external contamination although there is considerable discussion concerning the effectiveness of washing. Washing must be conducted correctly or it can damage the cuticle, which may, in turn, make eggs more susceptible to bacterial ingress. Commercial layer flocks are monitored for the presence of Salmonella and management and sanitation programs are used to control Salmonella in layer flocks. Approaches to control of Salmonella will vary according to the type of production system. Increasingly high consumer expectations and pressure from public health authorities mean that continued vigilance will be essential for the egg industry.

Presented at the 26th Annual Australian Poultry Science Symposium