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Eggs: fragile. The words follow almost automatically. The features which make the eggshell a suitable container for a developing chick embryo are not necessarily those that make the shell suitable for the eggs in your supermarket basket. Every year, egg producers count the cost of the millions of eggs with shells damaged or broken before they reach the consumer – between 6 and 8 per cent of all eggs laid. Worldwide, this damage costs the producers more than $600 million.

Engineers and biologists have devoted many years to the fundamental problem that you do not have to hit an egg very hard before it breaks. They have tried to reduce the impact on eggs from the moment they are laid until they reach the super-market shelves. They redesigned cage floors to reduce the impact on eggs as they are laid, and chicken farms collect eggs more frequently to minimise collisions on rollaways. Despite these efforts, cracked and broken eggs remain a serious problem for the poultry industry. We need stronger eggshells.

Immediately, this raises two important questions. First, do we know what determines the quality and strength of an eggshell? And if not, how can we find out? During the past 20 years, biologists have successfully exploited the scanning electron microscope to study in detail the structure of the eggshell. They now know that it is made up of several layers. From the inner to the outer surface these are: inner and outer shell membranes, the mammillary layer, the cone layer, the palisade layer, the vertical crystal layer and the cuticle. The shell is formed mainly in the hen’s shell gland pouch, where the egg spends 20 hours. The membranes consist mainly of protein and carbohydrate; the other layers are built up of calcium carbonate in a crystal form called calcite.

In 1971, Toby Carter, head of what was then the Agricultural Research Council’s Poultry Research Centre in Edinburgh, suggested that only the outer two-thirds of the shell-everything except the mammillary layer and the membranes-make any real contribution to the strength of an egg. But the method by which he arrived at this result is now open to debate. Since then, researchers in the poultry research group at the University of Glasgow have linked weak and poor quality eggshells with changes in the mammillary layer. The strength of the eggshell seems to depend rather on a fine balance between its different components.

Engineers have devised many methods for measuring eggshell strength. In one test, eggs are compressed between two parallel plates by a steadily increasing load. The strength of the eggshell is described in terms of the force needed to break the egg. Another popular type of test is the impact test in which a steel ball is dropped onto eggs from various heights. In this method, the strength of the shell is expressed in terms of the height required to fracture the shell. But these tests have their limitations. They measure precisely the maximum force or height required to cause breakage, but the equipment is expensive and only one region of each egg can be tested. Peter Hunton of the Ontario Egg Producers’ Marketing Board, amongst others, also doubts whether they reflect what really happens to eggs during handling and transport.

For these reasons, most egg producers have traditionally assessed the quality of eggshells, either by measuring the specific gravity of the whole egg or by measuring its nondestructive deformation. Both tests measure a physical property which, they assume, has an indirect influence on the strength of the eggshell. Specific gravity is taken to be related to the amount of shell covering the egg, and the nondestructive deformation test measures its stiffness.

There are two ways of measuring specific gravity: by flotation in salt solutions, or by Archimedes’ principle. Flotation involves immersing the egg in a series of salt solutions of increasing specific gravity. The specific gravity of any egg is equal to that of the solution in which it first floats. In the second method the egg is weighed in air, then in water. Specific gravity is then calculated by dividing its weight in air by the difference between its weight in air and in water. The assumption is that the higher the specific gravity, the thicker and stronger the shell, but no one has yet worked out exactly how these are related.

The nondestructive deformation test measures how much an egg deflects when a standard load is applied at a point on its equator. When the load is removed, the egg springs back to its normal shape, so this test measures the stiffness or elasticity of the eggshell. A small deformation should indicate a stronger shell, but this is not always the case. It seems that either the test is inaccurate or something else apart from stiffness is affecting the egg’s strength.

Engineers often find that the stiffness and strength of a material are unrelated. Reinforced concrete, for example, is stiff and strong. Glass is stiff but relatively weak. To understand the eggshell’s strength, we need to distinguish between these two properties.

When a load is applied to an object, the object experiences counteracting internal forces in the form of stresses and strains. How good an object is at absorbing these internal stresses and strains-its stiffness-depends on two things. One is a property of the material from which it is made, called the elastic modulus. The other is the object’s shape and thickness.

According to classical theories of engineering, an object under load will only break when one or more of these counteracting stresses builds up to a critical level. In the 1970s Peter Voisey and his colleagues at the Engineering and Statistical Research Service and Animal Research Centre of Agriculture in Ottawa applied this theory to an eggshell under load. They concluded that the stresses were at a maximum at the inner surface of the eggshell, directly beneath the load-that is, at the point where adjacent columns of calcite in the palisade layer fuse, because stress cannot cross empty space (see Figure 1). But their theoretical predictions for the force needed to break eggs were at least 20 times lower than that needed in practice. There were several reasons why. First, they had concluded that the all the layers in the eggshell make an equal contribution to how the egg performs under load. Secondly, they had ignored the shape of the egg. Thirdly, they had treated the eggshell as a classical ‘thin shell’ structure, ignoring components of shear acting through the shell wall. Fourthly, they had concluded that the eggshell breaks as soon as the stresses at its innermost surface build up to a critical level-in other words, that crack initiation and catastrophic failure happen simultaneously.

Even though most researchers accept that the thickness of an eggshell is related to its stiffness, only now do we know exactly how. From the diversity of structure in the layers that make up an eggshell you might expect each one to contribute differently to its strength. Last year we chemically removed individual layers from an eggshell to find out what contribution each one makes to its performance under load. Removing the cuticle, vertical crystal layer and compact palisade layers in this way decreased the stiffness of the remaining shell, but removing the mammillary layer had no effect. So when we try to relate the thickness of an eggshell to its stiffness, we are really only talking about the stiffness of the layers above the mammillary layer.

To analyse levels of stress in complex structures made up of many layers-to predict the behaviour of oil rigs, planes and buildings, for example-engineers use a method called finite element analysis. First they create a three-dimensional model of the structure using computer graphics. Then they subdivide the model into a number of elements or blocks. Each one can be subjected to a load, or its properties changed. Engineers then translate this information, together with the thickness and properties of each layer, into a series of codes which the computer uses to calculate the theoretical levels of stresses and strains throughout the model.

At Glasgow in 1989 we applied this method to the case of the eggshell under load. For many years researchers disagreed about how much the shape of an egg influences the strength of its shell. It is almost impossible to devise an experiment that separates the effect of shape from the effects of differences in thickness and structure. But finite element analysis does allow us to separate these effects. Using various egg-shaped models we found that the rounder the egg, the less it should deform.

We also discovered that the elastic modulus of eggshells resembles that of bone. Engineers usually find out the elastic modulus of a material of a particular shape and size by loading it steadily until it breaks. Eggs prove a little more tricky than most materials because they are curved and all have slightly different structures. From finite element models we calculated the elastic modulus of an eggshell, taking into account both the egg’s shape and the fact that the mammillary layer does not contribute to its stiffness.

Shells are not so simple

It also seems that although the hypotheses of the early 1970s were right to the extent that the stress is at a maximum directly beneath the load, shear is also important in making the shell resistant to load. Eggs are not the simple ‘thin-shelled’ structures researchers once thought them to be.

We now also think the strength of the eggshell depends on the shell’s resistance to the growth of cracks as well as on the rate at which tensional stresses build up. We loaded an egg with a weight of less than 1 kilogram. To the naked eye it appeared perfect, but candling-the technique of placing a light behind the egg, first used in the 1920s-revealed a star-shaped translucent patch we had not been able to see before. When we looked even closer with a scanning electron microscope we found a series of cracks radiating across both inner and outer surfaces of this patch.

A 3-kilogram weight produced an all-too-familiar cracked egg with a definite star-crack pattern, as well as one or more major crack lines. We suspected that there might be a time interval between the start of cracking, as seen by the series of small radiating cracks, and the growth of one or more of these cracks which results inevitably in the shell breaking. Why?

The answer seems to lie in the mammillary layer. This is organised in such a way as to influence both crack initiation and crack growth. The scanning electron microscope revealed that cracks tend to follow the path of least resistance. So shells in which the mammillary bodies-calcite deposits-are aligned offer little resistance to crack growth. Whatever their thickness, shells with a random arrangement of mammillary bodies hold up the movement of cracks, making them more resistant to loads. Materials scientists now know that the resistance of a material to the growth of cracks depends on the size and number of defects it contains. Eggs, with all their structural diversity, are no exception.

Two things must happen before a crack will grow. First, enough stress must be available at the tip of the crack to make it grow. Secondly, sufficient energy must flow to the crack tip to supply the energy required to create new surfaces. The quantity that links these two parameters is the fracture toughness. This quantity indicates the severity of defects in otherwise identical materials. We are now using finite element analysis to calculate the fracture toughness of eggshells. This work should help us solve problems such as how a hen’s diet and surroundings affect the strength and quality of its eggshells, with the aim of producing stronger ones.

So next time you reject a cracked egg in its egg box, take comfort in the knowledge that the same techniques used to ensure planes fly safely are also being applied to improve the humble egg.

Maureen M. Bain is a research assistant with the poultry research group at the University of Glasgow. Sally E. Solomon is a senior lecturer in veterinary anatomy at the University of Glasgow.

This article is based on chapter 9 of Egg & Eggshell Quality by S. E. Solomon, Wolfe Publishing, Aylesbury, 1991.