Student notes
EGG HYGIENE
The widespread use of eggs as ingredients of many foods makes them prime suspects in
food borne disease outbreaks. They possess an excellent nutritional environment that
supports bacterial growth. Therefore they require careful microbiological control in
production, storage and usage.
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Egg Composition
1. Egg shell
This is the outer covering of the egg. It constitutes 9-12% of the total weight of the egg.
It consists of an organic framework of fibres and an interstitial substance of inorganic
material, largely of calcium carbonate (94%) with small amounts of magnesium
carbonate, calcium phosphate and other organic matter including protein. It has two
layers: the outer or spongy layer and the inner or mammilary layer.
Shell strength is greatly influenced by mineral content (calcium, phosphorus and
manganese) and vitamin content (vitamin D) in the hens diet. If the diet is deficient in
calcium, for instance, the hen will produce a thin or soft shelled egg or possibly an egg
with no shell at all. Occasionally an egg maybe prematurely expelled from the uterus due
to injury or excitement. In this case, the shell has not had time to be completely formed.
Shell thickness is also related to the egg size which in turn is related to the hen’s age. As
the hen ages, egg size increases. The same amount of shell material which covers a
smaller egg must be “stretched” to cover a larger one, hence the shell is thinner.
6,000-8,000 tiny pores are distributed over the shell surface, a greater number at the large
end. Average pore size is between 20-45µm. The size will allow the passage of many
micro-organisms. Even yeast cells can be forced through cells and mold mycelia may
grow through the pores. As the egg ages, these tiny pores permit moisture and
carbondioxide to move out and air to move in to form the air cell.
The shell is covered with a protective coating called the cuticle or bloom. By blocking
the pores the cuticle helps to preserve freshness and prevent microbial contamination of
the contents. The cuticle therefore serves as the first line of defense against microbial
invasion of the egg. Second barrier is the egg shell.
2. Shell membranes
Inside the shell are two membranes the inner and outer membranes. The inner membrane
is more significant of the 4 barriers.
3. Air Sac
When the egg is laid it is warm. As it cools, the contents contract and the inner shell
membrane separates from the outer shell membrane to form the air cell. As the egg ages,
moisture and carbon dioxide leave through the pores of the shell, air enters to replace
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them and the air cell becomes bigger. It is usually located at the larger end of the egg but
may move freely toward the upper most point of the egg.
4. Albumen
Also known as the egg white. It accounts for most of an egg’s liquid weight about 6367%.
It contains more than half the eggs protein, niacin, riboflavin, chlorine,
magnesium, potassium, sodium and sulfur. The albumen consists of 4 alternating layers
of thick and thin consistencies. From the yolk outward, they are designated as the inner
thick or chalaziferous white, the inner thin white, the outer white and outer thin white.
The egg white tends to thin out as an egg ages because its protein changes in character.
That’s why fresh eggs “sit up” tall and firm in the pan while older ones tend to spread
out.
Albumen is more opalescent than truly white. The cloudy appearance comes from
carbon-dioxide. As the egg ages, carbon dioxide escapes, so the albumen of older eggs is
more transparent than that of fresher eggs.
Several egg white proteins have biological activity that tends to retard bacterial growth.
 Lysozyme: (3.5% of egg white solids)
This catalyses the hydrolysis of glycosidic linkages in peptidoglycan ; the structural
polymer that is responsible for the strength and rigidity of the bacterial cell wall. It
therefore leads to bacterial lysis of Gram+ve bacteria.
 Avidin and Ovoflavoprotein (0.05% of egg white solids):
These bind biotin and riboflavin, restricting the growth of those bacteria for which they
thick are essential.
 Ovotransferrin/conalbumin (12% of egg white solids):
This chellates ferric and ferous ions forming a stable complex with them rendering iron
unavailable to bacteria for growth.

The high pH (8.0-9.3) of egg white creates an unfavourable environment for bacterial
growth.
 Ovomucoid & ovoinhibitor (11% of egg white solids):
This inhibits enzyme activity of invading micro-organisms e.g. fungal and bacterial
proteinase of Aspergillus and bacillus respectively.
5. Chalazae
These are ropey strands of egg white which anchor the yolk in place in the center of the
thick egg white. The more prominent the chalazae is the fresher the egg.
6. Germinal disc
This is the channel leading to the center of the yolk. The germinal disc is barely
noticeable as a slight depression on the surface of the yolk. During fertilization, sperm
enter by way of the germinal disc, travel to the yolk center.
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7. Vitelline membrane
This covers the yolk protecting the yolk. It is weakest at the germinal disc and tends to
become more fragile as the egg ages.
8. Yolk
This makes up about 33% of the liquid weight of the egg. It is the site of embryo
formation in a fertilized egg. It contains all of the fat in the egg and a little less than half
of the protein. With the exception of niacin and riboflavin, the yolk contains a higher
proportion of the egg’s vitamins than the egg white. All the egg’s vitamins A,D, E and K
are in the yolk. Eggs are one of the few foods naturally containing vitamin D. It also
contains more phosphorus, manganese, iron, iodine, copper, and calcium than the white
and it contains all of the zinc. The yolk is responsible for the eggs emulsifying properties.
Yolk colour depends on the diet of the bird. If the bird is fed on xanthophylls the yolk
will be yellowish orange. If the diet is mainly white cornmeal the yolk gains a grayish
white colour. Due to the high nutrient content and absence of antibacterial properties,
many bacteria are likely to be found in the egg yolk as compared to the egg white.
Nutritional Composition of an Egg
Composition
Protein
Sugar
Ash
Lipids
Egg white %
86.0
9.0
5.0
Trace
Egg Yolk %
31.0
2.0
3.0
64.0-Triglycerides - 41.9
Phospholipids 18.8
Cholesterol
3.3
Functional Properties of the egg
The egg yolk and egg white have properties that render them useful as food ingredients:
 Coagulation
 Emulsifying power (forming an emulsion i.e. a creamy mixture of liquids which
donot completely mix).
 Ability to form heat stable foams.
Characteristics of Quality (Fresh) Eggs
Quality eggs should have the following characteristics.
1. Should have a cuticle and shell without lesions, aberrations or indications of soiling.
The ideal thickness is 35µ. ≤ 25µ thick eggs will invariably break.
2. Should be symmetrical or oval shaped and with typical egg shape.
3. Should have visible even surface.
4. The air sac should be less than 4mm deep.
5. Egg white should be viscous, clear, translucent and free from any inclusions. The egg
white should be dome shaped because of the higher proportion of viscous egg white
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surrounding it. This is not so in older eggs, where the egg white is flat because the
egg white is less viscous. Upon storage in a low humidity environment the egg white
loses water to the yolk resulting in a thinning of yolk and shrinking of the thick white.
6. On candling, the egg yolk should be visible only as a mere shadow and located in the
central position.
7. Should not bear offensive smell. Offensive smells often result from microbial
spoilage, improper storage of bird feeds e.g. rancid fat in the fish meal component can
change the fatty acid pattern of the yolk and generate fish like odour.
The following are the different types of eggs:
Organic eggs
These are eggs from hens fed on rations having ingredients that were grown without
Pesticides, fungicides, herbicides or commercial fertilizers; such birds are often free
range birds. No commercial laying hen rations containing hormones are fed to these hens.
Due to higher production costs and lower volume per farm, organic eggs are more
expensive than eggs from hens fed conventional feed.
Fertile eggs
These are eggs which can be incubated and developed into chicks. Fertile eggs are not
more nutritious than non fertile eggs. They donot keep as well as non fertile eggs and are
more expensive to produce due to the extra cost of a cock. They may contain a small
amount of male hormone. They are perfectly alright, but there is a feeling that eggs for
eating should be infertile.
Abnormal eggs
Soft shelled eggs
These are a frequent problem, and the trouble is often a calcium-phosphorus deficiency or
imbalance for which the remedy is oystershell grit, or baked, ground egg shells. Other
causes include lack of vitamin D3, or manganese, inadequate feed intake, high
temperatures, high sulphonamides and old age. Sudden shocks may also cause the shell
secreting-secreting glands to malfunction. For example birds kept in places where there
is sudden loud sounds e.g gun firing training may have soft shelled eggs.
Double yolked eggs
They are fairly common and tend to occur more in second season hens. It is believed that
there may also be a hereditary tendency towards the simultaneous release of two ova
which combine to form one shelled egg.
Rough shelled eggs
This is caused by infections like Infectious bronchitis, New castle disease; excessive
antibiotic treatment, excess calcium in diet and old age due to reduced oviduct tone.
Eggs with blood spots
This is caused by bleeding into the yolk when follicles rupture from the ovary. It is also
seen in old birds, birds in cold environments and those fed on rations high in vitamin K.
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Blood spots have shown a hereditary tendency. Some commercial egg producers candle
eggs and discard spotted ones, but these are quite harmless.
Wind eggs
These are small eggs which contain no yolk. They are quite common in the young pullet
whose egg laying system is just beginning to function. They may also be the result of
foreign matter passing down the oviduct and stimulating the shell-secreting glands into
action. They also commonly occur as a hen is ceasing production. It is also sometimes
observed in birds with New Castle disease.
Cheezy Yolk
This is a result of prolonged chilling eggs and presence of stercuric acid in cotton seed
cake (oil).
Taint (stale eggs)
Repulsive egg odours are often caused by rotting caused by bacteria or moulds; feeding
on fish meal; seed dressings, absorbed odours during storage, detergents used for
cleaning and treatment with coccidiostat robenidine.
Egg Contamination
The risk of contamination stems from presence of microbes on egg shells, few germs are
derived from egg laying apparatus. More are originationg from the environment resulting
in high species variety. Egg shells that are macroscopically clean are on average
contaminated by 102 – 108 microbes.
Primary Contamination(Transovarial)
This takes place in the hen’s egg laying apparatus before eggs are laid (Transovarial
infection) e.g. Salmonella enteritidis, S. pullorum , Mycoplasmas , Avian infectious
bronchitis, Avian infectious encephalitis, Staphylococcus aureus, E. coli, Proteus,
Enterococci and Mycoplasma synoviae.
Is less important due to:
 Few numbers of organisms involved
 Public health significance of the pathogens involved is less important.
Secondary Contamination (penetration)
This is the contamination that occurs after the egg has been laid. It is the most important
mode of contamination. It is influenced by various factors which are directly related to:
a. Poultry house hygiene. This is in turn is determined by:
 Type of nest and floor litter,
 feeder and drinker designs,
 ventilation,
 occurrence of diarrhea causing diseases,
 ratio of laying birds to nest and
 frequency of egg collection
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b. Storage conditions
Freshly laid eggs are generally sterile or possess very few micro-organisms. However,
short period thereafter numerous micro-organisms may be found on the outside. Under
favourable conditions these may enter the eggs.
The rate of entry depends on:
 Temperature of storage
Rapid cooling of eggs soon after oviposition to temperatures of 4-7 oC results in fewer
bacteria penetrating. A study of the migration of artificially contaminated S. enteritidis
from the albumen into the egg yolk using 800 eggs, revealed that the bacterium could be
detected within a day in the yolk at temperatures of 30 oC and after 14 days when
temperatures were 7 oC.
 Relative humidity
The ideal relative humidity for egg storage is 70-80% . Upon storage in a low humidity
environment the thick egg white loses water to the yolk resulting in a waterly flat yolk
and shrinking of the thick white. This phenomenon makes it possible for the yolk to come
into contact with the inner membrane hence easily infected by micro-organisms.
 Duration of storage.
Under the above study, it was observed that day old eggs were more resistant than 4
weeks old eggs to contamination of the yolk.
c. Shell quality (Structure and hygiene):
Faecal contamination of eggs is a common occurrence. It occurs in the cloaca or after the
egg is laid in the nest. Cracks in the shell present an avenue for contamination of egg
contents, this is referred to as egg leakage.
d. Egg cleaning: Bacterial contamination can also occur through improper cleaning.
Dry cleaning can be done using abrasives. These are effective on lightly soiled eggs.
They include, Sand sponges, loofers, sand paper and steel wool. Dry clean cloths can
also be used to dry clean eggs.
Wet cleaning: Wet egg shells provide easy access to bacteria to penetrate into the eggs if
washing is done improperly. When eggs are washed, the temperature of the wash water
should be11- 20 oC higher than the temperature of the eggs. If the wash water is at lower
temperature, egg contents contract causing bacteria on the egg shells to be drawn into the
egg. Sanitizing compounds that can be used include: Sodium carbonate Na2CO3 , Sodium
hypochlorite and potassium hydroxide KOH. The detergent used should be non scented.
Tertiary Contamination
This takes place at the following levels
a. Processing level.
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This is the stage where the shells and membranes are removed. The liquid egg is prone to
contamination with organisms on the shell, equipment, man’s hands, environment, added
ingredients and the final container.
b. Consumer level.
Contamination often occurs after preparation of egg containing meals. Hence eggs
become fomites.
c. Distribution level.
Contamination occurs during the handling of the eggs or egg products. Organism
commonly associated with this is vibrio parahaemolyticus.
Egg Spoilage
The common causes of spoilage are bacteria and moulds. Bacterial spoilage of eggs is a
condition often known as rotting. Once the bacteria have gained entry into the yolk, their
growth is marked by the presence of protein and amino-acid metabolic products e.g.
Hydrogen sulphide. This results in foul smells; the yolk becomes “runny” and
discoloured. Rotting is commonly caused by gram –ve bacteria e.g. Pseudomonas,
Serratia, Proteus, Alcaligenes and Citrobacter. The various bacteria will form different
forms of egg spoilage.
In case of moulds, these first multiply in the airsac with very high humidity moulds will
be seen forming over the egg surface.
Microbial defects of Egg and Egg Products
Product
Shell eggs
Liquid whole
egg
Defect
Black rot
White rot
Green white
Sour, Musty
Castard
Yellowish-green rot
Fishy rot
Off odour, sour
Liquid albumen
Off odour
Micro-organisms
Aeromonas, Proteus
Citrobacter, Alcaligenes
Pseudomonas fluoresens
Pseudomonas
Citrobacter, Proteus, Enterobacter
Alcaligenes and flavobacterium
Pseudomonas, flavobacterium,
chromobacterium
Proteus, Pseudomonas, Alcaligenes,
flavobacterium, E.coli, Bacillus
Pseudomonas, Acinetobacter, Enterobacter.
Prepared by Dr. Joanne Kisaka.
Presented by Dr. Luke Nyakarahuka.
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