Egg storage is an essential aspect of
egg handling. The main goal is to preserve important egg quality
characteristic, since there is now year round egg production, long term
storage, to keep egg quality at a level acceptable for table eggs, is no longer
essential as it was. Barbosa et al.,
(2004), evaluating the effect of temperature and storage time (0,7,14,28 and
35) on egg quality, Verified that the increase of storage time caused reduction
in Haugh unit and the egg weight. They also reported that storage time affects
the quality of eggs, exerting a negative influence on the Haugh unit.
Egg lose weight due to evaporation of
water form the albumen. As long as relative humidity (RH) in the atmosphere
around the eggs is less than 99.6% water evaporates through the shell pores. Leandro et al., (2005) and Menezes et al., (2009). The rate of evaporation
increases linearly with the difference in relative humidity inside and outside
of the egg. It can be described in the equation: L =p(1-R) where
L = Loss
in weight
P = permeability
of the shell
R = atmosphere
relative humidity. This formular hold for single
egg fully surrounded by an atmosphere with a certain
relative humidity.
In a pile of packed eggs, other condition occurs
depending on the rate of ventilation through the pile. Pile eggs usually are
not well ventilated in the centre. This is the reasons why in the centre, the
relative humidity (RH) reaches equilibrium with the interior of egg. At that
point, an atmosphere has risen with a relative humidity of about 99% and weight
loss has been reduce to zero. Under this condition, mould growth often occur
and care should be taken that no where in the pile does the relative humidity
exceed 85%. This can be achieved by frequent collection of eggs, rapid storage
in the cold room separated from other products. (miles and Henry, 2004). A rule
of thumb is that under correct conditions, egg lose about 1% of their weight
each week, proper storage means that some loss in weight must be allowed.
Weight loss is restricted by oiling of the shell which decreases the
permeability of the shell by closing the pores.
In the first stage of storage, egg
lose carbon dioxides causes cloudy white which is a characteristics of newly
laid eggs. Due to the low content of carbon dioxide in the atmosphere, eggs
lose it in a few days. This phenomenon gives rise to an increase in PH of the
white.
Ageing of eggs causes a reduction in
the amount of thick albumen with it’s conversion into thin albumen. Eggs of several
days old show weak and watery albumen (Awosanya et al.,1998). Since this is a chemical process, progress depends on
temperature, the lower the temperature ,the slower the conversion of thick
albumen proceeds.
High temperature above 16.50c
cause rapid decrease in internal quality via increasing humidity loss (Samli et al., 2005). storage temperature
should be adjusted carefully to the ambient temperature and relative humidity.
immediately the surface temperature is low, the dew point of the air around it
water vapour will condense on the shell. This phenomenon of water condensation
on the shell is called “sweating” and should be prevented as it is very harmful
since an egg is never sterile, micro organisms will start to develop in the moist
environment. They will again access through the pores and continue to grow on
the membranes. When microbial growth reaches the yolk, the egg will spoil.
The yolk absorbs water due to osmotic
activity during storage. At the same time the strength of the vitelline
membrane decreases. These factors are responsible for the difference in
appearance between yolk from the stored eggs and those form fresh eggs. The colour
of the yolk also varies with the type of feed given to the chicken but does not
affect the nutritional content (Choprakem et
al., 1998). When eggs that has been stored for long period are boiled, the
surface of the yolk will have a greenish colour. This is caused by chemical
interaction between iron present in the yolk and sulphur coming from
decomposition in the albumen.
Other factors that may cause loss of
quality include disease. New castle
disease and infectious bronchitis produce. Watery albumen which may persist for
long after the diseases has been controlled (Aldene, 1996). Relative humidity
of 70-780h reduces the egg weight losses by decrease in egg water losses to keep
albumen fresh for long period of time (Keener et al., 2004).
Internal and external egg quality
Egg quality is a general term that relate to various
standards that are imposed on the eggs (USDA, 2000). Egg quality is a general
term which refers to several standards which define both internal and external
egg quality. External quality is focused on the shell cleanliness, texture and
shape. Whereas internal quality refers to egg white (albumen) cleanliness and
viscosity size of he air cell, yolk shape and strength. Internal egg quality
also involves functional aesthetic and microbiological properties of the egg
yolk and albumen. (Akbas, 1996).
The white is formed by four
structures. Firstly the chalalziterous layer or chalazae, immediately surrounding
the yolk accounting for 3% of the white .Next is the inner thin layer, which
surround the chalazae and account for 18% of the white. Third is the firm or
thick layer which provide an envelope or jacket that holds the inner thin white
and the yolk. It adheres to the shell membrane at each end of the egg and
accounts for 57% of the albumen. Finally, the outer thin layers lies just
inside the shell membranes except where the thick white is attached to the
shell, and accounts for 23% of the egg white (USDA, 2000).
Egg yolk from a newly laid egg is
round and firm. As the egg gets older, the yolk absorbs water from the egg
white, increasing the size. This provides an enlargement and weakness of the
vitalize membrane, the yolk looks flat and show spots.
As
soon as egg is laid , the internal quality starts to decrease, the longer the
storage time, the more the internal egg quality deteriorates. However, the
chemical composition changes much.
In a newly laid egg the albumen ph
lies between 7.6 and 8.5.
During storage, the albumen pH increases at a
temperature dependent rate to a maximum value of about 9.7 (Lichan et al., 1995), after three days of
storage at 30C. In oiling of the shell the albumen pH of 8.3 did not
change over a 7 days period of storage at 220c. In oiled egg stored
at 70C, albumen pH dropped form 8.3 t 8.1 in 7 days (Li-chan et al 1995).
Increase in albumen pH are due to the
carbondioxide(CO2) loss through the shell spores and depend on dissolved Co2
bicarbonate ions.
In a newly land eggs, the yolk pH is
in general close to 6.0 however, during storage it gradually increases to reach
6.4 to 6.9. Egg quality preservations through handling and distribution is
dependent on constant care from all personnel involved in this activities. The
quality of an egg once it is laid cannot be improved so effort to maintain it’s
quality must start right at this moment. (Cotterill et al.,1995).
To minimize egg quality problems, the
following are important;
* keep the birds confined
* gather eggs frequently at least three
times a day
* if egg must be cleaned, clean them
immediately after gathering
* dry clean slightly dirty eggs
* cool eggs as quickly as possible
* maintain
an egg storage room temperature of 10-130C and relative humidity of
75%
* the packing material should be clean and
odorless
* the
storage room should be cleaned regularly with odourless sanitizer and fee form
staining material.