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Cooled Egg Storage
Why
When laid the embryo in a fertile hens egg will have spent some 20h at the body
temperature of the hen (410C). As this temperature is above so-called physiological zero
(25-27oC) - in essence the temperature below which the embryo will not develop – cell
division takes place and when the egg is laid there are some 120,000 cells present on the
surface of the yolk. Normally the egg then cools to below physiological zero and the
embryo enters a dormant state. Providing the temperature remains stable and below
physiological zero fertile hens’ eggs can be held when the embryo is in this very early
stage of development for several days without the embryo being compromised. This
ability has evolved over time so as to allow the hen to lay a clutch of eggs which, when
completed, it can incubate with all embryos developing at the same rate and hence
hatching at the same time.
If the egg is stored under temperature conditions that are not stable and fluctuate around
or above physiological zero then the embryo will continue to develop. However once the
embryo has moved significantly beyond the 120,000 cell stage it cannot return to its
dormant state and if the temperature falls below physiological zero the embryo will die.
Therefore storing eggs in climates where the temperature is around or above physiological
zero is, in the absence of conventional refrigeration or similar, likely to result in relatively
high embryo moralities. As this happens in the very early stages of development (when
the embryo is just a mass of undifferentiated cells) the egg is often incorrectly thought to
be infertile.
Keeping eggs cool during the storage period is therefore critical to maximising hatchability.
How
In the absence of refrigeration the environmental temperature can be reduced by the use
of evaporative cooling. A technique that has been trialed and found to provide positive
results uses the following approach.
A container typically a shallow basket (but could be a clay pot or similar) is filled with sand,
soil (or a mixture of the two) to a depth of 10-15 cm. The soil/sand is then moistened with
water and a jute sack placed over the surface. The sacking helps protect the eggs from
spoilage bacteria that will be in the soil while allowing the water to evaporate through the
material and so cool the eggs that are placed on the sacking. Eggs can be stored on their
sides or pointed end down (ie not broad end down). Because evaporative cooling will only
reduce the ambient temperature by a few degrees it is important that the basket containing
the eggs should be placed in a cool area, out of the sun, to achieve the lowest storage
temperature.
The soil mixture needs to be checked every few days and more water added as required
to keep the mix moist. It is important that when adding water care is taken not to wet the
eggs as water on the shell will allow spoilage bacteria to penetrate the shell and will cause
the egg to spoil.
Storage times should be minimised as much as possible as even under ideal temperature
conditions (approx 15-18 oC) eggs cannot be stored for much more than 7 days without
hatchability decreasing.
N Sparks
21/12/05