EGG STORAGE

The hen does not begin sitting on her eggs as soon as the first egg is laid. Instead, she waits until she has accumulated a clutch. Storing eggs for a short period before incubation therefore mimics the bird's natural behaviour.

The research has been in for many years: fresher eggs hatch better. However, perhaps surprisingly for some, the research also shows that freshest is not best. If we look closely at the graph, hatchability actually rises during the first few days after laying before beginning its long decline.

Nature has adapted the egg to remain at body temperature for only the minimum time required for it to be laid. Once laid, the egg cools and development is suspended as it waits in the nest for the following eggs.

The fall in temperature, together with a dramatic change in the chemistry of the albumen, slows the chemical reactions and then halts them altogether. The embryo enters a state of suspended development while the egg is preserved and protected from bacterial attack.

For this reason, it is practical to hatch once a week or so. Hatchability diminishes with prolonged storage, but short-term storage simply imitates what happens naturally.

Ideal storage conditions

Temperature

There has been much research into the maximum temperature at which embryonic development is arrested—the so-called biological zero.

For practical purposes, our target is 21°C or lower.

The ideal temperature depends on how long the eggs are to be stored:

Cooler temperatures are beneficial for longer storage because they slow embryonic development even further.

It is not advisable to store eggs above 21°C. Higher temperatures increase the risk of false starts and may also encourage bacterial growth on the eggshell surface.

The minimum practical temperature is 4°C. Temperatures below this risk tissue damage due to freezing and thawing.

Humidity

The less water lost during storage, the better.

Our minimum target is 75% relative humidity, rising to 80% for longer storage periods.

Where the target humidity cannot be achieved, wrapping eggs in cling-film has been shown to arrest moisture loss.

There is also evidence that eggs stored for extended periods benefit from being stored inverted, with the air cell lowermost. In this orientation, the weight of the egg restrains the air cell rather than allowing it to move freely.

Turning

Turning stored eggs has nothing to do with the embryo.

The albumen exists in layers of different viscosity. Turning twice each day causes these layers to slide over one another and remain separated.

A gentle turn in the morning and another in the evening is sufficient.

Ventilation

There is no embryonic metabolism during storage and therefore no out-gassing requiring ventilation.

Unlike eggs in the incubator, stored eggs are not consuming oxygen or producing carbon dioxide. For this reason, preventing moisture loss is more important than providing ventilation.

Egg hygiene during storage

Some care about how and where eggs are stored removes unnecessary risks.

Cardboard cartons are a popular choice, but they are also candidates for harbouring pathogens. Used cartons may be reused many times, but there is little effort involved in placing them in a pre-warmed oven at 70°C for ten minutes to sterilise them.

If your oven does not go down to 70°C, simply use its lowest setting. Cardboard egg cartons are themselves manufactured using heat and will tolerate these temperatures.

Plastic egg cartons should be washed with a mild anti-bacterial dishwashing liquid, rinsed and allowed to dry before use.

Clean containers prevent contamination.

The chemistry of storage

When the egg is laid, both the yolk and the albumen have a pH of approximately 6.6–6.7, almost identical to the pH of the hen's blood.

The carbon dioxide concentration within the egg is also identical to that of the hen's bloodstream.

Once the egg is laid, carbon dioxide escapes from the albumen. As this happens, the pH of the albumen rises over the following days to approximately 9.3–9.5.

This high pH is hostile to bacteria and appears to function as an antibacterial defence system.

The yolk, however, remains at a pH of about 6.5. The difference between the yolk and the albumen is therefore about three pH units—a thousand-fold difference in hydrogen-ion concentration.

The embryo lies between these two environments and, at the time of laying, is only one or a few cell layers thick. Yet it survives this extraordinary chemical gradient.

Once incubation begins and the embryo starts to develop, it produces carbon dioxide of its own, and the pH of the albumen begins to fall again.

A note about unusual eggs

In eggs that are not truly oval—for example, elliptical or almost spherical eggs—the position of the air sac may not be obvious immediately after laying.

Allow such eggs to rest on a flat surface for a few days until the air-cell end can be identified and marked. If rolled gently on a bench, they will tend to favour the smaller end.

Remember

The hen stores eggs before she incubates them, and so can we.

Good storage simply means preserving the egg in the condition in which it left the hen: cool, clean, slightly humid and protected while nature waits for the clutch to be completed.