Egg Quality Defects in Poultry Management and Food Safety
Poultry egg is a vehicle for reproduction; it also serves as a source of food for human consumption. The size and shape of avian eggs differs among the various species of birds, but all eggs have three main parts-yolk, albumen and shell. The quality of eggs depends on physical make up and chemical composition of its constituent parts. Due to the diversity in the potential uses of poultry eggs and the subsequent consumer demands that egg quality become extremely difficult to define. Egg quality is the more important price contributing factor in table and hatching eggs. It is obvious that quality of egg is important from producers point of view. One of the biggest challenges for the poultry industry is to provide consistent quality egg products to the consumer. Thus breeding companies are shifting selection emphasis to improved egg quality. Problems associated with egg quality include: egg shell defect and internal defects which can be broadly categorized into three groups namely: defects affecting yolk quality, defects affecting albumin quality and defects affecting overall quality. Egg quality defects are usually easily resolved, but can be costly if they are not dealt with quickly.
May 10, 2010; Accepted: June 28, 2010;
Published: October 11, 2010
Kramer (1951) defined food quality as the sum of characteristics
of a given food item which influence the acceptability or preference for that
food by the consumer. Based on this definition, it is clear that egg quality
will mean different things to different people and the consumers perception
of quality is likely to vary depending on their intended use of the egg and
their own preferences. This is clearly illustrated by a brief review of the
regulatory requirements for eggs sold around the world.
Under the European Union Egg Marketing Regulations, enforced in all European
Union Countries, eggs are classed as either class A or B eggs (Council
of the European Union, 2006; European Commission, 2003)
and only eggs graded class A can be sold for direct human consumption or retailed
(Council of the European Union, 2006). The characteristics
of grade A and B eggs are detailed in Table 1. Similarly,
the United States Department of Agriculture (USDA) has developed three grades
of eggs based on the interior quality of the egg, appearance and condition of
the egg shell (USDA Food Safety and Inspection Service, undated). US grade AA
and US grade A eggs are usually retailed while U.S. grade B eggs are usually
sent for further processing. In contrast, no defined grades have been developed
for shell eggs sold in Nigeria and little attention has been focused on internal
egg quality factors, such as yolk or albumin quality.
However, the Animal Products Specification for Products Intended for Human
Consumption Notice 2000 requires that shell eggs intended to be traded in the
||Be visibly clean
||Have no cracks that are visible on candling (or equivalent) unless they
have been treated by a process that destroys pathogenic organisms
||Have no evidence of embryo development or putrefaction, and no significant
||Not have been incubated
||Be handled and stored under conditions that minimise condensation on the
surface of the eggs
Eggs which do not meet these minimum requirements can only be sold for human
consumption if they have been pasteurized (or undergone an equivalent process)
and meet the microbiological criteria. Thus, although, grading systems for shell
eggs may vary from country to country or region to region, however, regardless
of the grading or classification system used egg shell quality and interior
quality are important factors in determining egg quality.
Egg shell quality: The vast majority of eggs sold in Nigeria are sold
in their shell and a consumers first impression of any egg purchased is
based on their perception of shell quality (Okoli and Udedibie,
2000). Shell quality is one of the most important factors that influence
hatchability (Roque and Soares, 1994). The productivity
and quality of breeding eggs have an overall significant for the continuity
of the flocks and for an economic breeding (Ojedapo et
al., 2009). Peebles and Brake (1987) reported
that reduction in eggs shell quality depressed hatchability and result in weakening
of embryos. Egg shell quality has significant impact on the reproductive fitness
of the parent (Bennett, 1992; Abanikannda
et al., 2007). The shell thickness and porosity help to regulate
the exchange of carbon dioxide and oxygen between the developing embryo and
the air during incubation (Roque and Soares, 1994).
Shell thickness also has a very significant effect on moisture loss during incubation
(Bennett, 1992). Thin-shelled eggs lose more moisture
than do thick-shelled eggs, causing the chick to have difficulty hatching (Roque
and Soares, 1994).
The Egg Producers Federation of New Zealand (Inc) (EPF) Code of Practice (EPF
and NZFSA, 2002) lists 14 possible eggs shell defects, although, these can
be grouped into five main categories, defects associated with egg shell integrity,
texture, shape, colour and cleanliness. Some of the causes of shell quality
problems are detailed in Table 2.
Egg shell integrity: Defects considered under the category of egg shell
integrity include gross cracks, hairline cracks, star cracks and thin shelled
or shell-less eggs. As cracked eggs cannot be made available for retail sale
(Food Standards Australia New Zealand, 2006), high number
of cracked eggs will have a negative impact on the profitability of any egg
producer. One of the most obvious reasons for egg shell cracks (including gross
cracks, hairline cracks and star cracks) is mechanical damage (Awosanya
et al., 1998) caused either by the birds themselves or as a result
of poor management practices, such as infrequent collection of eggs, rough handling
and poor design and/or maintenance of the cage floor. Egg shell strength ultimately
affects the soundness of the shell, with weaker shelled eggs more prone to cracks
and breakages and subsequently microbial contamination (Yoruk
et al., 2004). Shell strength can be affected by a wide range of
Egg size: Smaller eggs have stronger shells than larger ones, as hens
have a finite capacity to deposit calcium in the shell and as a result, the
same amount of calcium is spread over a larger area (Butcher
and Miles, 2003; Rajkumar et al., 2009).
Age of bird: Older birds tend to lay bigger eggs and have a higher egg
output, which impacts on shell strength as described above (Butcher
and Miles, 2003). Very young birds with immature shell glands may produce
shell-less eggs or eggs with very thin shells (Rajkumar
et al., 2009). Delaying the onset of sexual maturity by one to two
weeks will prevent this (Coutts and Wilson, 1990).
Stress: A single stress or disturbance to a flock of laying hens can
be enough to de-synchronise the process of egg formation for several days, during
which time, a number of different egg quality faults may be seen (Clunies
et al., 1992). For example:
||Any factor which results in oviposition prior to completion
of shell deposition will result in soft or thin-shelled eggs. Activities
which create disturbances in and around the layer shed should be minimised
(Coutts and Wilson, 1990)
||If an egg is retained in the shell gland, any subsequent egg laid may
spend less time than normal in the shell gland, resulting in insufficient
shell deposition and a soft shelled or shell-less egg (Clunies
et al., 1992). Solomon (1991) noted that
once an imperfect mammillary layer has formed (as a result of stress experienced
before the egg reaches the shell gland), subsequent layers are disorganized
and thin or soft shelled eggs are a common phenomenon after stress
Elevated environmental temperature: High (above 25°C) environmental
or shed temperatures may affect the feed (and therefore calcium) intake of the
bird, thus, resulting in a decreased availability of calcium for shell deposition
(Okoli et al., 2006). As well as decreasing feed
intake, laying hens will try to overcome heat stress by panting (Koelkebeck,
1999). However, this causes a decrease in the amount of carbon dioxide (CO2)
in the hens blood, a condition known as respiratory alkalosis (Noddegaard,
1992; Koelkebeck, 1999). As egg shells are made up
of 95% calcium carbonate (CaCO3), this decrease in blood CO2
levels, combined with an increase in blood pH and a subsequent decrease in Ca2+
ions for shell formation leads to an increase in the number of thin or soft
shelled eggs produced. Arima et al. (1976) found
that the egg quality of older hens was more severely affected by increased temperature
than younger hens.
Nutrition and water quality: The provision of adequate dietary minerals
and vitamins is essential for good eggshell quality (Yoruk
et al., 2004). Similarly, as water quality varies from country to
country and region, the role of drinking water in mineral and trace element
supply should not be overlooked (Anyanwu et al.,
2008). Calcium and phosphorous are essential macro minerals with calcium
forming a significant component of the shell and phosphorous playing an important
role in skeletal calcium deposition (Frost and Roland, 1991)
and subsequent availability of calcium for egg shell formation during the dark
period (Boorman et al., 1989). Coetzee
(2002) investigated the effect of calcium levels in drinking water on shell
integrity in South African laying hens and demonstrated that birds supplied
an additional 200 mg of calcium per litre of drinking water laid eggs with mean
shell strength of 42.6±9.0. This was in comparison to those receiving
un-supplemented water, whose eggs had mean shell strength of 38.9±7.0.
However, the feeding of calcium levels above the requirement of the bird for
production has not been shown to improve shell quality (Kershavarz
and Nakajima, 1993). Indeed, feeding hens high levels of calcium may interfere
with the availability of other minerals (NRC, 1994) and
can have a negative impact on the ability of the bird to utilise calcium, particularly
if calcium levels in the diet are subsequently decreased. This is particularly
true in young pullets. Wideman and Lent (1991) also
reported that high levels of calcium can cause latent kidney damage, which has
a long term impact on bird survivability.
Similarly, feeding high levels of dietary phosphorous have also been shown
to have a negative effect on eggshell quality (Keshavarz
and Austic, 1990; Harms, 1982a, b;
Boorman et al., 1989; Taylor,
1965; Arscott et al., 1962). Although, Miles
and Harms (1982) showed a clear linear negative correlation between specific
gravity and plasma phosphate, it remains unclear whether excess plasma phosphorous
interferes with the mobilization of skeletal phosphorous reserves or the shelling
process itself (Boorman et al., 1989; Ahmad
and Balander, 2004) or if elevated levels of phosphorous increases calcium
excretion (Keshavarz and Austic, 1990).
Keshavarz and Austic (1990) also examined the interaction
of phosphorus and chloride and the role of chloride in egg shell integrity.
As with phosphorous, elevated dietary levels of chloride resulted in decreased
eggshell quality and lower levels of blood acid-base indicators. This is supported
by the work of Balnave et al. (1989), who observed
an increase in shell defects with no changes in egg production, egg weight,
feed or water intake, blood-acid base levels and electrolyte levels for birds
provided with 2000 mg of sodium chloride (NaCl) per litre drinking water. This
was in comparison to those hens provided water with 600 mg NaCl per litre. Birds
receiving 2000 mg of NaCl per litre drinking water also had an increased incidence
of shell-less eggs. Egg shell defects persisted after the sodium chloride was
removed from the drinking water. In contrast, Hess and Britton
(1989) fed hens diets low in chloride and found virtually no effect on shell
Trace mineral nutrition is a complex area of animal nutrition and a wide range
of interactions and antagonisms can result in poor absorption or utilization
of minerals essential for shell formation (Frost and Roland,
1991). It should also be noted that not all trace mineral sources are equally
available and consideration should be given to this aspect of premix formulation.
Burley and Vadehra (1989), Miles (2001)
and Mas and Arola (1985) reported varying amounts of
zinc, copper, iron and manganese in the shell and its associated membranes.
It is clear, therefore, that provision of adequate levels of these minerals,
which are key components of the shell matrix and play an essential role as co-enzymes,
is essential if shell integrity is to be maintained. Vitamin D also plays an
important role in the proper utilization of calcium and phosphorous and sufficient
amounts of this vitamin should be included in the feed (Amiri
Andi et al., 2006).
Finally, care should be paid to the mixing of the diet. Thorough mixing of
the feed is essential if each bird in the flock is to receive a similar amount
of any given nutrient (Natalie, 2009). This is particularly
true for layer hen diets which frequently contain raw materials with a wide
range of different densities.
Mycotoxicosis: Jewers (1990) reported that thin
rubbery shells which break more readily than normal have been observed during
field outbreaks of chratoxicosis. Similarly, in an outbreak of T-2 toxicosis,
egg breakages increased from a normal 3 to 15% with a further 18% of eggs broken
in transit to customers (Jewers, 1990). Zaghini
et al. (2005) reported that birds fed diets containing 2.5 ppm of
aflatoxin B1 had lower egg shell weights than those fed the control diet or
diets supplemented with mannanoligosaccharides.
Genetics: Clunies et al. (1992) found
that hens laying thick-shelled eggs retained more dietary calcium than those
laying thin-shelled eggs. Although, there was no difference in egg production
between thick and thin shell layers, both egg and shell weight were greater
for the thick shelled eggs.
Disease: Infectious Bronchitis (IB), a viral disease caused by a coronavirus
which attacks the mucus membranes of the respiratory and reproductive tracts
(Butcher and Miles, 2003; Cavanagh
and Naqi, 2003), may result in egg defects. These include pale shelled eggs
and eggs with poor shell structure and integrity (Butcher
and Miles, 2003; Khan et al., 2004). Similarly,
birds affected by Egg Drop Syndrome (EDS), caused by an adenovirus, initially
produce pale eggs, quickly followed by thin-, soft-shelled or shell-less eggs
(McFerran and Adair, 2003).
Texture: Rough or sandpaper shells, pimples, pinholes and mottled or
glassy-shells are all egg shell defects associated with egg shell texture. These
defects are frequently a result of bird age, but may also be caused by other
factors (Coutts and Wilson, 1990; MAFF,
Disease: Certain diseases such as IB, Infectious Laryngotracheitis (ILT)
(Spadbrow, 1993) and Avian Encephalomyelitis (AE) (Coutts
and Wilson, 1990) have been implicated in the production of rough or sandpaper
Mycotoxicosis: As discussed above, ochratoxicosis may result in rubbery-shelledeggs
Genetics: The production of eggs with calcium deposits on the shell
(or pimples) is thought to be hereditary (Khan et al.,
Management: Overcrowding of birds, changes in the lighting programme,
poor shed ventilation and inadequate water supply can contribute to increased
incidence of shell defects associated with egg texture (Etuk
et al., 2004).
Shape: Misshapen eggs have a shape which differs from the smooth normal shape (for example, flat sided eggs and body checked eggs). This can be caused by a number of factors:
Age of bird: As with shell soundness, young birds with immature shell
glands may produce misshapen eggs (Hess and Britton, 1989).
Stress: Body checked eggs, marked by grooves and ridges, occur when
the shell of the egg breaks in the shell gland, during the formation process
(i.e., 10-14 h before the egg is laid) (Abanikannda et
al., 2007). Although, the damage can be partly repaired, a bulge forms
around the egg (Solomon, 1991). Flat sided eggs occur
where two eggs are in the shell gland at the same time (Solomon,
1991; Abanikannda et al., 2007). Both defects
may be caused by overcrowding, frights or other disturbances and poor lighting
patterns (Koelkebeck, 1999; Coutts
and Wilson, 1990). Jones (2006) stated that proper
handling can reduce the incidence of body checks.
Disease: As the albumen of the egg and surrounding membranes provides
the structure on which the egg shell is deposited, if the albumen quality is
very poor, there it not sound foundation on which to build the shell (Abanikannda
et al., 2007). As a result, those diseases which result in poor albumen
quality often cause an increase in the number of misshapen eggs. Examples of
these are IB (Butcher and Miles, 2003; Cavanagh
and Naqi, 2003), EDS (McFerran and Adair, 2003) and
certain strains of Newcastle Disease (NCD) or Avian Influenza (AI) (Spadbrow,
Colour: The colour of an egg shell is determined primarily by the genetics
of the hen, with white feathered hens laying white eggs and brown feathered
hens laying brown eggs (Fairfull and Gowe, 1990). During
the process of egg shell formation in brown egg layers, the epithelial cells
lining the surface of the shell gland synthesise and accumulate pigments, such
as biliverdin-IX, its zinc chelate and protoporphyrin-IX (Butcher
and Miles, 2003). In the final three to four hours of shell formation these
pigments are transferred to the viscous, protein rich cuticle. The quantity
of pigment in the cuticle which determines the colour of the egg (Fairfull
and Gowe, 1990). As the cuticle is deposited onto the eggshell at the same
time that shell deposition reaches a plateau (approximately 90 min prior to
oviposition), pigment distribution is not uniform throughout the shell, with
very little pigment contained in the shell itself (Khan et
al., 2004). Thus, any factor which causes a disruption, either in the
ability of the epithelial cells to synthesise pigment or in the deposition of
the cuticle, will affect the colour of the egg shell. These factors include:
Stress: Epinephrine, a stress hormone, will cause a delay in oviposition
and cessation of shell gland cuticle formation, which can cause pale shelled
eggs to be produced. Stressors may include, amongst others, high cage density,
loud noise and handling (Natalie, 2009).
Age of bird: As birds age increases, the intensity of pigment decreases.
This may be due to decreasing production of pigment or increased surface area
over which available pigment is distributed (Abdullah et
Chemotherapeutic agents: Certain drugs have been shown to affect egg
shell colour. For example, nicarbazin (an anticoccidial drug) fed at a level
of 5 mg day-1 can result in the production of pale eggs within 24
h, while higher doses can lead to complete depigmentation (Cavanagh
and Naqi, 2003). Chlortetracycline (600-800 ppm) may also result in yellow
egg shells (Cavanagh and Naqi, 2003).
Disease: Viruses, which affect the mucus membranes of the respiratory
and reproductive tract, such as NCD and IB, not only cause a decrease in egg
production, but also cause the shell to become abnormally thin and pale (Spadbrow,
Cleanliness: Cleanliness is probably the easiest aspect of egg shell
quality control and good management plays an important role. Most eggs are clean
when laid and subsequently become contaminated with faecal material or other
|| Summary of standards for interior quality of chicken eggs
Defects which fall into this category, include cage marks, stained eggs and
fly marks (EPF and NZFSA, 2002). Although, fungus or
mildew on shells is listed as a defect, it is only likely to occur in poor conditions.
Good management practices will help reduce the number of dirty eggs (Etuk
et al., 2004). These practices include frequent collection of eggs,
as well as regular replacement of litter material in nest boxes or regular maintenance
and cleaning of cage floors and rollout trays. Whilst less common, fly stains,
water stains and grease or oil stains may occur and can be prevented by good
shed and equipment maintenance or management (Etuk et
al., 2005). Morover, factor which causes diarrhoea in the birds, (for
example high dietary salt levels), will also result in an increase in the number
of dirty eggs collected. Blood smears on eggs can be minimised by good pullet
management, including lighting and beak trimming if necessary.
Internal egg quality: Unlike external (shell) quality, internal quality
of the egg begins to decline as soon as the egg is laid (Okoli
and Udedibie, 2001). Table 3 shows detail summary of standards
for interior quality of chicken eggs by candling. Thus, although factors associated
with the management and nutrition of the hen do play a role in internal egg
quality, egg handling and storage practices do have a significant impact on
the quality of the egg reaching the consumer (Okeudo et
al., 2003). Similarly, although the shell provides a unique package
for the distribution of the egg contents to the consumer, it is in fact the
internal quality of the egg that is most important to the consumer (Okeudo
et al., 2003). These aspects of internal quality are significantly
more difficult to observe or evaluate in the intact egg, even with the use of
candling. In addition to the obvious, nutritional quality of the egg, internal
egg quality is extremely important because of its many functional and aesthetic
properties. For example, eggs are used as thickening agents in custards and
puddings, egg whites are used as smoothing agents to give icings a desirable
texture and egg yolks add colour and richness to food (Okeudo
et al., 2003).
In recent years, much attention has been focused on increasing the omega 3:6
ratio and vitamin content of eggs, principally through manipulation of the diet.
However, although these enhancements further complicate the issue of egg quality
(Anyanwu et al., 2008). The EPF Code of Practice
(EPF and NZFSA, 2002) lists a total of nine internal
defects and these can be broadly categorised into three groups; namely: defects
affecting yolk quality, defects affecting albumin quality and defects affecting
Yolk quality: Yolk quality is determined by the colour, texture, firmness and smell of the yolk.
Yolk colour: Although, yolk colour is a key factor in any consumer survey
relating to egg quality (Okeudo et al., 2003),
consumer preferences for yolk colour are highly subjective and vary widely from
country to country. The primary determinant of yolk colour is the xanthophyll
(plant pigment) content of the diet consumed (Silversides
et al., 2006). It is possible to manipulate the yolk colour of eggs
by the addition of natural or synthetic xanthophylls to layer hen feeds. This
ability to readily manipulate egg yolk colour can be an advantage in meeting
market demands. However, the ease with which yolk colour can be manipulated
can lead to unwanted colour changes. For example, the inclusion of higher than
recommended levels or incorrect ratios of pigments can lead to orange-red yolks
(Silversides et al., 2006). Similarly, diphenyl-para-phenylenediamine
(DPPD), an antioxidant, has been reported to cause excessive deposition of pigments
in the egg yolk.
The inclusion of more than 5% cottonseed meal in a layer diet will result in
olive or salmon coloured yolks (Esonu, 2006), while the
inclusion of certain weeds or weed seeds may results in green yolks. Similarly,
inadvertent omission of xanthophylls from the diet will lead to pale yolks (Esonu,
2006). Both inadequate mixing of the diet as well as excessive mixing of
the diet will also result in a heterogeneous feed and subsequent variation in
the amount of xanthophylls consumed by each hen in the flock, This will result
in egg yolk colour not being uniform throughout the flock.
Pale yolks can result from any factor which alters or prevents the absorption
of pigments from the diet or the deposition of these pigments in the yolk. These
factors could include:
||Worms (Coutts and Wilson, 1990)
||Any factor which inhibits liver function, subsequent lipids metabolism
and deposition of pigment in the yolk. For example, mycotoxicosis caused
by aflatoxin B1 (Zaghini et al., 2005)
||Coccidiosis, although this is rare in adult hens
Mottled yolks (with many pale spots and blotches which vary in colour, size
and shape), occur when the contents of the albumen and yolk mix as a result
of degeneration and increase permeability of the vitelline membrane (Amiri
Andi et al., 2006). Factors affecting mottling were reviewed in detail
by Cunningham and Sanford (1974). Dietary factors which
may cause mottled yolks include:
||The presence of nicarbazin (an anticoccidal agent) in the
feed has shown by numerous authors to cause mottling (Jones
et al., 1990; Cunningham and Sanford, 1974)
||Worming drugs such as phenothiazine, dibutyltindialaurate (Berry
et al., 1968) and Piperazine (Amiri Andi et
al., 2006). However, Berry et al. (1968)
did not observe yolk defects when Piperazine was fed at the manufacturers
recommendations. Similarly, they only observed defects when dibutyltindialaurate
was fed at the recommended level but over a much longer period.
||Gossypol from cotton seed meal (Berry et al.,
1968; Esonu, 2006)
||Certain antioxidants such as gallic acid (from grapes, tea and oak bark)
and tannic acid, or tannins from grains such as sorghum (Esonu,
||Calcium deficient diets (McCready et al., 1972;
Cunningham and Sanford, 1974)
Storage time and temperature has also been shown to affect the degree of egg
yolk mottling (Okoli and Udedibie, 2001). Okeudo
et al. (2003) stated that as the internal temperature of the egg
increases above 7°C, the protein structures of the thick albumen and vitelline
membrane breakdown faster. As the membrane degenerates during storage, water
enters the yolk causing mottling and after prolonged storage, albumen proteins
also enter the yolk increasing the severity of mottling. In order to reduce
the rate of breakdown of the vitelline membrane, eggs should be collected regularly,
reducing the time they are exposed to higher environmental temperatures and
contaminants and stored at temperatures of 7-13°C and humidity of 50-60%.
Cunningham and Sanford (1974) also identified hen age,
oil coating of eggs and movement of eggs as possible factors affecting mottling
Yolk firmness: The yolk of a freshly laid egg is round and firm (Okoli
and Udedibie, 2000). However, as the egg ages and the vitelline membrane
degenerates, water from the albumen moves into the yolk and gives the yolk a
Yolk texture: Rubbery yolks may be caused by severe chilling or freezing
of intact eggs, the consumption of crude cottonseed oil or the seeds of some
weeds (Jacob et al., 2000).
Albumin quality: Albumin quality is related to the consistency, appearance and the functional properties.
Consistency: Albumin quality is measured in terms of Haugh Units (HU)
calculated from the height of the albumin and the weight of the egg (Haugh,
1993). A minimum measurement in HU for eggs reaching the consumer is 60.
However most eggs leaving the farm should be between 75 and 85 HU (Zaman
et al., 2005). Albumin consistency is influenced by:
Age of the hen: HU will decrease with increasing bird age value, with
HU decreasing by around 1.5 to 2 units (Awosanya et al.,
1998) for each month in lay. Doyon et al. (1986)
stated that HU decreases at a fairly constant rate of 0.0458 units day-1
of lay as the hen ages. Doyon et al. (1986) also
noted that in an ideal situation, HU should be on average 102 at 20 weeks of
age, falling to an average of 74 HU by 78 weeks of age.
Genetics: Strain of bird has also been shown to play a role in albumin
consistency, with some strains consistently producing eggs with thin albumin.
Rajkumar et al. (2009) reported that brown egg
layers produced eggs with higher HU, while other authors (Sell
et al., 1982; Williams, 1992) reported that
HU values were more variable within the brown egg layers compared with those
that lay white eggs. High producing birds tend to lay eggs with relatively lower
amounts of thick albumin and, although this can be influenced by selective breeding,
egg numbers are usually considered more important.
Age and storage of the egg: As the egg ages and carbon dioxide (CO2)
is lost through the shell, the contents of the egg become more alkaline, causing
the albumin to become transparent and increasingly watery (Okeudo
et al., 2003). At higher temperatures, loss of CO2 is
faster and the albumin quality deteriorates faster. Decreasing shed temperatures
in the hotter months, combined with regular collection of eggs will help to
reduce deterioration of the albumin before collection.
Eggs stored at ambient temperatures and humidity lower than 70% will lose 10-15
HU in a few days from point of lay. By 35 days, these eggs will lose up to 30
HU (Natalie, 2009). Storage of eggs at temperatures of
7-13°C and a humidity of 50-60% (as discussed under mottling), will reduce
the rate of degeneration of thick albumen proteins and, consequently, egg albumin
quality will be maintained for longer (Jones, 2006).
Oiling of eggs can also help to reduce CO2 losses and thus help maintain
internal egg quality (Okoli and Udedibie, 2000, 2001;
Okeudo et al., 2003) but is not a substitute
for cool storage.
Vanadium: Henry and Miles (2001) reviewed the
effects of vanadium on poultry performance. They noted that poorer albumin quality
has been reported from laying hens consuming as little as 6 ppm. Sell
et al. (1986) showed that the interior quality, of eggs decreased
in two strains of laying hens fed 3 or 6 ppm added vanadium. Duyck
et al. (1990) fed laying hens 10 ppm of vanadium for 30 days. HU
from these hens averaged 71 HU after one day of storage (62°F (16.6°C)
and 60% Relative Humidity) and 64 after seven days of storage. This was in contrast
to the average of 82 and 74 HU after one and seven days storage, respectively,
observed for hens fed the control diet. Henry and Miles
(2001) reported that the negative effects of vanadium may be overcome by
feeding cottonseed meal, ascorbic acid, vitamin E or carotene, although this
is dose dependant.
Diseases: Diseases such as certain strains of EDS, IB, NCD and ILT (Jacob
et al., 2003) can all cause a decrease in albumin consistency.
Appearance: Normal albumin is transparent, with a slightly yellow green
colour. Discolouration of the albumin may occur if the eggs are stored for an
extended time in poor conditions, with the albumin becoming much yellower (Cavanagh
and Naqi, 2003). Cyclopropene fatty acids from cottonseed meal and the certain
weed seeds (Sell et al., 1986) can cause albumin
to turn pink after storage. Green whites are caused by excesses of riboflavin
(vitamin B2) in the diet. Cloudy whites may be caused by the oiling of eggs
within 6 h of lay (Silversides et al., 2006).
Blood spots: Blood spots may vary from indistinguishable spots on the surface
of the yolk to heavy contamination throughout the yolk (Cavanagh
and Naqi, 2003). Although, blood spots are normally closely associated with
the yolk, occasionally blood may be diffused through the albumin. Blood spots
occur when small blood vessels in the ovary rupture when the yolk is released.
Vitamin K plays an important role in blood clotting. Vitamin K deficiency can
result in an increased occurrence of blood spots (Bains, 1999).
Some strains of birds appear to be predisposed to blood spots although the incidence
is low (Rajkumar et al., 2009). Avian encephalomyelitis
has been reported as a cause of blood spots (McFerran and
Adair, 2003). Jewers (1990) reported an increase
in blood spots from essentially 0 to 3% in birds affected with T-2 toxicosis.
Bains (1999) suggested that mycotoxicosis may reduce vitamin
K absorption and this may explain the elevated incidence of blood spots in hens
affected by T-2 toxicosis.
Meat spots: These are usually associated with the albumin rather than
the yolk and often consist of small pieces of body tissue (Curtis
et al., 1985). However, some may consist of partially broken down
blood spots or pigments. The occurrence of blood spots varies with strain of
bird, increases with age of bird and is reported to be higher in brown egg layers
(Abdullah et al., 2003).
Bacterial or fungal contamination: Solomon (1991)
suggested that while pores on the surface of the egg do represent possible ports
of entry for bacteria, particularly as the cuticle hardens just after oviposition,
these are of secondary importance to the structural defects that may occur.
Structural defects, because of their magnitude, offer a much more likely route
for bacteria to enter the egg contents. Bacterial and fungal contamination of
eggs usually results in black, red or green rot. The egg looks and smells putrid
when broken out of the shell. Bacterial and fungal contamination of eggs, resulting
from faecal contamination of the egg, can be prevented by good management practices,
including regular replacement of nesting materials or good cage maintenance
as appropriate (Etuk et al., 2004). Bacterial
contamination of the egg contents may also occur as a result of an infection
in the oviduct of the hen and any affected hens should be culled (Etuk
et al., 2005).
Proper handling and storage of eggs following collection will minimise the
opportunity for bacterial or fungal contamination. However, improper washing
procedures, high storage temperatures and humidity will increase the incidence
of bacterial of fungal contamination (Etuk et al.,
2005). Careful attention should be paid to feed source, as Salmonella
sp. can be transmitted through the feed.
Roundworms in eggs: Burley and Vadehra (1989)
reported that where roundworm infestation of the intestinal tract occurs, worms
may migrate from the cloaca into the oviduct and become enclosed in the egg.
This can be prevented by good flock management.
Off odours/flavours: Although, off odours and flavours are rare if eggs
are stored correctly (Okoli and Udedibie, 2000), eggs
readily absorb strong odours or flavours. Storage of eggs in close proximity
to fish oils and meals, sour milk, strongly scented or decaying fruit and vegetables,
mould, disinfectants and kerosene is likely to result in the development of
off odours or flavours (Okeudo et al., 2003).
However, eggs that have been oiled are less likely to absorb foreign odours.
Old eggs and eggs stored at high temperatures are more likely to exhibit off
odours or flavours (Okoli and Udedibie, 2000). Other
causes of off odours or flavours include strongly flavoured feed ingredients
such as fish meal or fish oil, some vegetables (including onions, turnips and
excessive amounts of cabbage) and rapeseed or canola.
The consumer seldom sees many of the egg defects detailed above, as eggs are graded and most defects are normally removed prior to retail. Assuming adequate nutrition, the use of suitable raw materials, proper feed hygiene and a lack of contamination of the birds diet with foreign matter and/or objects, there is little effect of nutrition on egg quality defects. Flock shed and feed management, egg storage and egg handling remain the three most important factors in determining the incidence of egg quality defects. However, the breeding companies should be aware of the market requirements for egg quality and should include these traits in their selection programmes, where possible. Finally, I will like to call on the Product Standard Organisation of Nigeria in collaboration with the Animal Product Researchers and Ministry of Agriculture in Nigeria to develop a standard and have a defined grade for shell eggs sold in Nigeria to meet the international standard and consumers demand and aspiration.
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