The hen's egg: Density of egg shell and egg contents

One hundred eggs were used, from hens of seven widely differing strains: two commercial brown‐egg hybrids (one British, one American), two commercial white‐egg hybrids (one British, one Canadian), one broiler strain, one highly inbred strain of laboratory White Leghorns and one laboratory strain of Brown Leghorns. The volume occupied by the shell of each egg was estimated from its surface area—itself estimated by means of a three‐parameter model (Carter, 1968)— and its mean thickness, measured with an anvil micrometer. The volume occupied by the egg contents was estimated by subtracting the shell volume from the egg volume, which was also estimated by means of the three‐parameter model. Mean overall shell density (counting as “shell” all mineral matter and spaces between the outer surface of the mineral shell and a surface through the tips of the mammillae) was estimated, by regressing shell weight on shell volume, to be 2.241 ±0.004 g./cm.³; covariance analysis showed that the strains were homogeneous in this respect. Mean incremental shell density (i.e. the density of shell distal to the mammillary region) was estimated, by regressing shell weight per unit surface area on shell thickness, to be 2.386±0.004 g./cm.³; the strains were homogeneous in this respect too. The mean depth of the intermammillary spaces was estimated to be 19.9 μ. All the residual deviation from the common regression line can be attributed to measurement error. The estimated density of incremental shell is lower than that of calcite; the packing fraction of the crystals in the shell aggregate and/or the atoms in the crystals was estimated to be 92.8 per cent. The density of the egg contents (at the temperature of the bird) was estimated by regressing weight of contents on volume of contents; covariance analysis revealed significant differences between strains, one of the brown‐egg hybrids having the highest density of egg contents, 1.045 g/cm.³, and the laboratory Brown Leghorns the lowest, 1.033; both commercial white‐egg hybrid strains gave a value of 1.040 g./cm.³.     

The hen's egg: Relationship between shell thickness and the amount of organic matter in the shell

Percentage weight loss on ashing was measured in 110 shell segments: 4 segments per shell—broad polar cap, narrow polar cap and two halves of the “barrel” —from 28 shells from 4 strains of chicken. The membranes had been removed, by hand, from each shell segment and shell thickness measured with a biconvex anvil micrometer. For each segment the function γ = T (1 + Wodm/Wmdo) was calculated, where T is the mean thickness, Wo the weight loss upon ashing, Wm the ash weight, d0 the density of the organic matter in the shell (assumed to be 1.0075 g/cm3) and dm the density of the mineral matter (2.386 g/cm3). Govariance analysis of the regression of T on T for each shell segment within each strain showed that the regression was linear, that the lines did not differ in slope, but that there were significant differences between the adjusted means for strains and for segments. This is interpreted as demonstrating that (a) the amount of organic matter in incremental shell is constant, estimated at 0.68 per cent by weight, and (b) the amount of organic matter outside incremental shell is variable. The equivalent thickness of the latter was estimated for each segment of each shell with a standard deviation of 0.4 μm. Other evidence suggests that the variation in organic matter outside incremental shell is due mainly to variation in cuticle thickness. Implications for poultry breeding and for future research are discussed.     

The hen's egg: A method for estimating the principal curvatures at any point on the shell

1. Estimates of curvatures of the shell of a well‐formed egg can be obtained by the method described, which is based on six measurements of the shell.

2. Estimates of the radii of longitudinal curvature at the equators of 24 eggs selected for their extreme shapes or sizes ranged from 284 to 42–7 mm; the standard deviation of the difference between such an estimate and one obtained for the same egg by the 3‐point method (using a slot and dial micrometer) was 1.2 mm.

3. Other tests, which are described, indicate that the method is reliable for estimating the principal curvatures at any point on the shell and not merely at the equator.     

The hen's egg: Factors affecting shell splitting during boiling

Experiments are described from which it is concluded that the shell will split when an egg is boiled if thermal expansion of the egg's liquid or gaseous contents, which have a higher coefficient of expansion than the shell, is insufficiently relieved by escape through the membranes and pores. The probability of splitting was enhanced if the shell had low structural strength due to a pre‐existing crack or to large size, skew shape or low effective thickness; once these factors had been taken into account there was no significant effect of the shell's aspect ratio, colour, glossiness, roughness or degree of ridging, nor of the hen's age or a strain history of selection for egg production. The incidence of splitting depended also on storage time and temperature: it was very low in newly laid eggs; in eggs stored at room temperature it rose to a maximum at about 5 d, then fell and was again low after about 21 d; in eggs stored at — 2 °C the time‐scale was increased and the incidence of splitting was still high after 14 d storage. These effects are due to two changes occurring during storage: progressive reduction of permeability of the membranes and shell to the liquid contents of the egg, which impedes escape of the contents when they expand during cooking, and progressive increase in the size of the air space, which provides expansion space. Allowing refrigerated eggs to warm to room temperature before cooking and adding salt to the cooking water did not significantly affect the incidence of splitting; putting eggs into cold rather than boiling water did not reduce the incidence of splitting except when the air space was large, following prolonged storage at room temperature.

The belief, common among housewives, that shell splitting occurs more commonly now than in earlier decades may be well‐founded but, if so, the change is probably due to a combination of two practices that have been introduced to maintain interior egg quality but which coincidentally raise the probability of shell‐splitting during boiling: rapid distribution from hen to housewife and refrigerated storage. There is no evidence that shells are now thinner, that hens of modern, high‐production strains lay eggs with weaker shells or that white shells are weaker than brown ones.     

The cuticle: A barrier to liquid and particle penetration of the shell of the hen's egg

The cuticle of the egg shell prevents water‐soluble dyes and carbon black from entering the majority of pores. A small percentage of eggs obtained from a commercial flock had no cuticle and their shells were easily invaded by carbon black. Chemical or physical removal of cuticle resulted in the pores being flooded with water which carried in carbon black. Such eggs did not, however, absorb water at a rate equal to that of eggs from which a piece of shell had been removed. This indicated that the pores without a cap and plug of cuticular material resisted the movement of water. The role of the cuticle and shell in repelling water is discussed and a function akin to that of the plastron of insect eggs proposed.     

The hen's egg: Variation in shell deformation under static load with relative humidity and age of egg

Deformation of an intact egg shell under static load, which is used for the nondestructive estimation of shell thickness, is commonly assumed to be independent of the relative humidity of the atmosphere surrounding the egg and of the age of the egg. Two experiments are described in which these assumptions were tested. In the first it was found that deformation under a load applied at the poles depends on both factors. In the second it was found that the same was true for equatorial loading and that the change in deformation was independent of the number of times the load had been applied, the porosity of the shell and the strain of bird. Deformation increased linearly with relative humidity and decreased exponentially with the age of the egg, the half‐value period being about 3.5 d. It is suggested that the effect may be due to variation in the elastic properties of the organic moiety of the shell. The effect is small relative to that of shell thickness and curvature and may usually be neglected if relative humidity is in the normal range and the egg is more than 10 d old. With fresh eggs or extremes of relative humidity, however, it is desirable to apply corrections. A table of correction factors is given (Table 6).     

The hen's egg: Evidence on the mechanism relating shell strength to loading rate

1. The decrease in strength of an egg shell which is known to accompany an increase in the period of time over which a load is applied might come about by either of two mechanisms: a decrease in the strength of the material constituting the strong, outer layer of the mineral shell, or a decrease in the thickness of this layer brought about by deepening of the crevices that are normally present between adjacent crystal columns in the weak, inner layer of the shell.

2. Experiments designed to discriminate between these mechanisms are described: the results indicate the second mechanism.

3. This mechanism offers explanations for various other shell phenomena, including delayed fracture under a static, cyclical or recurrent load.

4. The main implication for the poultry industry is that any external insult to a shell is likely to weaken it by doing damage that is local, internal, irreparable and cumulative, even though it may be invisible from the outside.     

The fine structure of the pores in the shell of the hen's egg

1. No fibrous or crystalline material was present in the lumen of pore canals which were exposed by radial fracture and examined with a scanning electron microscope.

2. The walls of the canals were rough, but they did not have a characteristic ultrastructure.

3. The campanulate outer orifice of the pores was plugged with cuticle in which radial cracks formed channels through which the respiratory gases presumably diffuse.     

The behaviour of mixed bacterial infections in the shell membranes of the hen's egg

The selection of Gram‐negative bacteria from a heterogenous flora on the shell membranes of the hen's egg occurred during two phases of the infection process. When the infection was confined to the shell membranes, the Gram‐negative bacteria achieved dominance over the Gram‐positive organisms. This dominance was accentuated when the albumen was invaded and it was noted, moreover, that a particular strain of Gram‐negative bacteria predominated whereas several strains co‐existed in the shell membranes.     

The hen's egg: Estimation of shell superficial area and egg volume,using measurements of fresh egg weight and shell length and breadth alone or in combination

1. Shell superficial area (S, mm²) and volume (V, mm³) of an egg can be estimated from any one or a combination of the following measurements: fresh egg weight (W, g), shell length (L, mm) and maximum shell breadth (B, mm).

2. Estimation equations and their error standard deviations are given.

3. The random error of the estimate is smallest when estimation is based on L, B and W together: 0.4% for S and 0.6% for V.

4. Estimation of S based on L and B together has the advantage over that based on W alone that it is unnecessary to measure the egg soon after lay; the random error is the same (0.7%).     

The hen's egg: Incidence and inheritance of some egg abnormalities

1. More than 9000 eggs, each laid by a known hen, were examined and malformations of six types recorded: double‐yolked, A, B, rough, shell‐less and thin‐shelled.

2. A and B malformations are those seen in, respectively, an egg that is held in the shell gland for an abnormally long period and one that reaches the shell gland while an A‐egg is still there; many of the eggs recorded as rough‐shelled were probably A‐eggs for which the corresponding B‐eggs had been lost.

3. The birds were of three strains: a medium‐body‐weight brown‐egg‐laying strain and a White Leghorn strain, both of commercial origin, and a laboratory strain of Brown Leghorns.

4. The incidences of egg malformation in the three strains were 8.3, 9.2 and 0.5%.

5. A‐, B‐ and rough‐shelled eggs constituted more than two‐thirds of all malformed eggs.

6. Evidence from two generations suggests that these malformations have a high heritability.     

The hen's egg: Relationship of mean strain energy at shell fracture to shell compression speed,the nature of the compressing body and the location on the shell of the point of contact

1. The mean energy, E_f, required for fracture of an egg shell at its equator by a flat plate is known to depend on shell compression speed, v_m, for speeds in the range 20 μm/s to 2.lb5 mm/s; analysis of published data shows that E_f bears a linear relationship to log (v_m ) throughout the industrially important speed range from 20 μm/s to 1.lb1 m/s, increasing from 1.lb3 to 5.lb0 mJ.

2. At lower speeds down to 2 μm/s E_f is constant.

3. E_f is reduced if the flat plate is replaced by a sphere or cylinder; the greater its curvature, the smaller E_f.

4. E_f for the narrow pole is greater and for the broad pole smaller than that for the equator but these differences may be due in part to . systematic differences in shell thickness as well as curvature.     

The hen's egg: Test of an industrial plerophory

1. Experiments with eggs from fowls of three strains failed to support a breeder's belief that the sex of a chick could be predicted from the position of the air space relative to the axis of the egg.

2. Relative to sex found at dissection, the accuracy of sexing newly hatched chicks by endoscope was 98% and by head markings (in Brown Leghorns) 88%.     

The hen's egg: Effect of a hole in the shell on the incidence of splitting during cooking

Two experiments, involving nearly 1000 eggs, were designed to show whether or not piercing the shell at the broad pole prevents it splitting when an egg is soft‐boiled. With eggs stored in the laboratory the incidence of splitting was low after storage for 1 d and there was no indication that the presence of a hole affected it; after 5 d splitting was reduced; after 28 d it was prevented. This may be because at 1 d the shell is still permeable, so even if there is no hole the build‐up of internal pressure as the egg contents expand is limited; at 5 d, when shell permeability has fallen, pressure build‐up is prevented initially by the presence of a hole, though it can occur in the later stages of cooking because the hole becomes sealed by congealed albumen; at 28 d the hole remains patent and the air space can accommodate all the expansion of the egg contents. The nature of the hole, whether made by piercing with a needle or by drilling to 1·5 mm diameter with a dental drill, did not significantly affect the response of eggs of any age.

Housewives should pierce eggs before boiling them, since if they are fresh it will do no harm and if they are stale it will prevent splitting.     

The hen's egg: Fracture of shells loaded very slowly

1. Eggs were loaded at the equator, the load on each egg increasing at a constant, low rate dW/dt in the range from 8.7 to 0.39 g/s.

2. When loading continued until shell failure the mean value of the force at failure, Pf, decreased as the loading rate decreased.

3. Among eggs from a given population the mean value of the product P_fR_a was related to log ê (dW/dt) in a linear manner, where R_a is the reciprocal of the average curvature of the shell at the point of application of the load.

4. When loading had been halted shortly before the expected time of failure, delayed failure was observed in more than a quarter of the eggs.

5. The more closely the load approximated to the expected load at failure, the shorter the delay in eggs showing delayed failure.

6. There is a risk of loss of eggs through delayed shell fracture if they are stored in stacks of trays that permit some of them to bear loads that are not large enough to cause immediate fracture but are nevertheless fairly large.     

The use of specific gravity of the egg to estimate shell thickness

1. Errors of the order of 10 μm could be introduced if specific gravity (SG) was used to estimate shell thickness when the comparison was being made between eggs from hens of different breeds or of different ages.

2. The precision of estimates made on the basis of SG measurements on eggs from within a flock of hens of uniform breed and age could be significantly improved by the use of egg weight information.