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  An essential guide to achieving quality eggs

Poultry layers are big business with billions of eggs eased out of healthy hens into nests or rolling down from cages every year. Fresh eggs as animal food products are unique because what consumers see is not what they are going to eat. The edible part of the egg is immediately contained in a transparent living membrane and further by a thicker ‘dead’ and mostly pigmented shell made of calcium carbonate and completely obscuring any view of edible egg contents.


Consumers are therefore forced to judge purchases based on egg shape, dimensions and external surface characteristics. That said quality of the shell is more than a good indicator of egg content quality, because if an egg’s albumen is of poor quality a sound foundation on which to build a thick, high quality and strong shell is generally lacking. If consumers select eggs according to the industry standard governing external appearance, then fresh, good-quality nutritional content and composition should follow automatically.

Huge ongoing investment is made by the industry to achieve cost effective egg collection, handling, grading, packing and transportation. For an ideal situation where speed and efficiency is achieved without compromising egg integrity and safety, then detailed knowledge of egg shape and shell strength related to things like packing efficiency and egg damage level is of utmost importance.


Egg shape


Egg shape may be of interest to the consumer but is of crucial importance to the industry including producers and marketing companies if eggs are packed in uniform trays for transportation. Abnormally shaped eggs are easily broken during handling for the simple reason they do not fit exactly into custom-designed trays.

The ideal egg shape is defined by the relationship between two dimensions which are the length and breadth of the egg. An index of 75 (calculated as breadth/length x 100) is considered to be the best. Egg shape is of much less importance when eggs are not packed in container, as most of these will be going for processing.

Shape is important to purchasers/consumers of intact eggs. They want the classical egg shape and certainly not eggs which have an ‘equatorial’ bulge, typically occurring in eggs that have broken in the shell gland and partly repaired before being laid. Such eggs in which breakage occurs during shell formation display a bulge around the centre of the egg. Any single factor responsible for bird disturbance in the 10-14 hour period before the egg is actually laid will increase occurrence of this condition.


Shell quality


Shell quality can be divided into several components including surface characteristics, and shell thickness and composition which combine to determine shell strength. Egg shell comprises 3 per cent membranes and 97 per cent true shell largely composed of calcium carbonate (Ca CO3).


Shell colour and smoothness


Additional calcium can be deposited on the shell of some eggs causing a type of splashing or a pink/lilac coloured egg and referred to in the industry as ‘coated shell’. Such faults are usually the result of eggs staying in the shell gland for a longer than is normal and occur most frequently in young laying hens that have just come into production. Young hens in particular are very susceptible to stress, and any disturbance to flocks will encourage birds to retain eggs.

Two eggs can be in the shell gland at the same time to cause development of a specific form of rough shell referred to as a ‘target’ or ‘thumb-print’ condition. The same occurrence may also result in hens laying ‘slab-sided’ eggs. Other types of rough shell condition are generally caused by health and nutritional problems often related to age, and resulting in shells with ‘pimples’ or a very abrasive ‘sandpaper’ like surface.

Shell colour is determined by presence or absence of a surface layer of pigment called ooporphyrin, with darkness (brownness) of egg shells positively correlated with dark feathered breeds. In some western countries and especially the United Kingdom there is general assumption that brown eggs are more nutritious than white eggs, although there is not a speck of evidence to support this. That said the shell of some eggs laid by brown laying hens may be very pale coloured or even white. The effect is purely cosmetic but in these markets the eggs are inevitably rejected.

Incidence of pale shelled eggs laid by brown feathered birds tends to increase with older laying birds and some disease conditions. Ironically there is evidence to suggest that pale-shell eggs from brown feathered laying hens are more common when the flocks are raised free range. Brown-shell free-range eggs are considered ‘tops’ for nutritional quality in countries like the United Kingdom.


Shell thickness


Shell quality is directly and closely related to shell thickness and eggs with thin shells simply and inevitably break very easily during transit. This is a particular problem for eggs produced in hot climates where high temperatures in tandem with high humidity may affect feed intake resulting in hens laying thinner shelled eggs. Shell thickness can be measured destructively (by breaking the egg) and using a micrometer or be assessed non-destructively using specific gravity measurements.

Other factors including porosity of the shell (density of pores), thickness of the membranes, minerals content of the dead shell and thickness of the protein matrix all affect shells strength, but at the end of the day shell thickness is by far the most important.

Shell strength does not show linear (proportional) reduction with decrease in shell thickness but actually falls much quicker than would be expected. Very few eggs having a shell thickness of 35 micrometres or more break during transportation while almost all eggs with a shell thickness of 25 micrometres or less will break during handling and if not later during transit. Shell thickness and strength can be boosted by good nutrition and management but that is not the whole story because shell thickness is 30 per cent controlled by heritable genetic factors.

Thin and weak shelled eggs are quite commonly laid by older hens and especially those which are approaching the end of their useful laying life, having often produced a very high output of eggs in the preceding months. When ‘soft’ shelled eggs appear in younger flocks they are often associated with ‘coated shell’ and ‘rough shell’ eggs that have remained in the shell gland for an inappropriately long time. If an egg stays in the shell gland for too long a period then subsequent ovulation will still take place at the usual time but before the previous egg has been laid. The second egg may therefore spend less time than is normal or beneficial in the shell gland leading to a soft or even shell-less egg. In such situations a hen may ‘miss a day’ then lay two eggs, one coated and one soft-shelled, on the following day.

De-synchronisation of the egg-laying process lasting several days or more is easily achieved by not paying sufficient attention to the well being of hens, even if threatened with just a single and seemingly mild disturbance subsequently leading to stress. The problem may well end up to be accumulative causing prolonged effects on egg quality and featuring a number of different egg shell abnormalities occurring over that period of time.


Dietary Calcium


Nutritional deficiencies are also responsible and because egg shells are predominantly of calcium carbonate then adequate supplies of calcium-rich feed are clearly required by laying hens. The calcium requirement of the laying hen is complicated by the progress of egg shell formation in the uterus, which relies on temporary supplementation from the skeletal system.

The egg shell is formed during the 24 hour period that the egg is inside the uterus. Almost 10 per cent of the shell is secreted around the egg during the first four hours with secretion accelerating and shell formation completed during the remaining 16 hours. Hens are only able to absorb 50 per cent of the calcium ingested in feed and the maximum amount that can be absorbed in 24 hour period is 2.0 g. Egg shells normally contain between 1.6 and 2.4 g of calcium and research shows that during the final hours of shell formation deposition of calcium exceeds the maximum rate at which calcium can be absorbed from the intestine.

The shortfall is made up by mobilising calcium from the bones. Hens coming into lay should be offered diets which are high in calcium (3-4 per cent) or diets with supplemental calcium provided by calcium grit, sourced for instance from oyster or mussel shells. This extra calcium ideally fed ‘ad libitum’ is deposited in the bones (especially the medullary bones), helped by hormonal changes in the hens at this time. Failure to build up this skeletal store of calcium and to maintain adequate calcium in the feed ration through the laying period will lead to permanent depletion of skeletal calcium.

This strategy is especially critical at higher temperatures (25ºC and above) when temporary shell thinning may occur due to reduced feed intake caused by heat- induced stress. But shells rapidly regain normal thickness when temperature falls and feed intake resumes at normal level. Serious shortfalls of calcium over a period of time will lead to rapid decline in egg production and eventually cessation of egg lay.


Damaged shells


Cracking and damage to eggs post laying is the consequence of poor shell quality, thin shells with low strength and the most common reason for downgrading. Damage may be due to poor quality egg shells or inappropriate handling during collection, grading, packing or transit, but if the two occur together then producers will be literally ‘treading on egg shells’.

A trio of egg shell crack types is generally recognised within the industry.

  1. The hardest to see and identify are ‘hairline cracks’ especially when they occur in newly laid eggs. Very skilled monitoring and inspection by egg candling is required to pin-point and identify such damage. This type of damage is commonly caused by an egg colliding with a surface that has no ‘give’ (is inflexible).

  2. Star cracks which emanate from a central point of impact are much more visible than hair-line cracks under normal light conditions and very easy to identify during inspection using egg candling procedures. This type of damage is the typical result of one egg impacting on another.

  3. So called ‘pin-hole’ and ‘toe-hole’ cracks may be caused by the hens themselves ‘stepping’ on or pecking at eggs, or by contact with any sharp point or protrusion. There is evidence to suggest that a similar type of damage can occur while the egg is still inside the bird.


Soiled shells


Once laid the egg is subject to contamination with all sorts of organic and inorganic matter including mucous, faeces and blood in general, and dust, mud and litter where birds are not kept in cages. Difficult soiling to clean from eggs is so called ‘glazing’ or ‘varnishing’ caused when the contents of broken eggs spill over intact eggs. Surface contamination of any nature not only ruins appearance of the eggs but may raise the issues of health and hygiene. Surface soiling is an important factor contributing to the downgrading eggs.


Dr Terry Mabbett