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A complete well-formed egg is indeed a work of art. Factors which combine to impart overall egg quality – size, shape, colour, smooth intact shell, internal integrity (albumen and yolk), quality and freshness – encourage purchasers to buy and consume eggs.
Be that as it may, as ‘living’ things, eggs are subject to ‘deterioration’ (rotting and chick embryo development if fertile).
Deterioration occurs when the egg is damaged internally― poor collection and storage conditions (e.g. temperature) are not conducive to continued freshness. And the dividing line between an attractive looking egg purchase offering good nutritious food, and a stale, rotten and repulsive item, which could cause serious illness, is very thin indeed. The following account considers the factors which affect egg quality. Those such as physical integrity are crucial while the importance of others including egg shape and colour will depend on sophistication of the market. The foundations for egg quality are laid early in the production cycle and relate to poultry breed and genetic selection, health of flock, nutrition and general management practice.
Importance of Shape
Eggs should possess the conventional ovoid shape. Eggs which are too rounded or too long are generally disliked by producers because they are difficult to pack in uniform egg trays, and are therefore more prone to breakage when moved. Ideal shape is defined by the ratio between the length and breadth of the egg multiplied by a factor of 100 with 75 as the optimum figure. Egg shape is clearly less important for small rural operations that do not pack eggs in custom-designed containers and in less sophisticated village markets misshapen and deformed eggs will usually find a ready market at a reduced price. Examples of deformity are ‘chalk head’, ‘sand head’ and ‘body check’.
Eggshell must be sufficiently robust to withstand the physical stresses of oviposition, collection and general handling. Once a shell is damaged, even by a hairline crack, the egg is subject to infection with diseases such as E. coli and Salmonella, especially from surface soil (faeces) and must not be sold at any price. There is a high positive correlation between shell strength and shell thickness. Trials show eggs with shell thickness of 25 micrometres (µm) or less suffer a high degree of breakage during handling, while those having a shell thickness of at least 35 µm generally withstand the rigours of commercial marketing. Shell thickness, which can be assessed destructively using a micrometer and non-destructively with specific gravity based tests, is governed by a variety of factors. Genetic heritability of shell thickness is about 30 per cent while physiology of the laying hen― which is determined by feed and nutrition, ambient temperature in the laying house and specific poultry diseases― has a major impact on shell development, thickness and quality.
While thickness is the most important factor governing shell strength other factors including shell porosity (density of pores), thickness of the membrane, chemical (mineral) content of the shell and thickness of protein matrix all play a part. Calcium and phosphorous minerals, together with Vitamin D (caciferol), are key components for shell strength. Infectious bronchitis is known to have a specific deleterious effect on shell development, and therefore quality and strength.Tropical producers face particular problems related to shell thickness and egg quality. High temperatures cause laying hens to produce thinner shelled eggs. These eggs are not only more liable to cracking and breakage but more prone to rapid deterioration because of high ambient temperature in the poultry house.
Shell colour, which is determined by a thin layer of ooporphryrin and governed by breed, has no effect on the nutritional quality and benefits of the egg. The myth of brown eggs being ‘better for you’ is widely believed in countries like the United Kingdom where consumers pay a premium for brown eggs because they are perceived to be more wholesome. This consumer preference can pose problems for producers, because poultry breeds producing brown-shelled eggs are generally less productive than those producing white-shelled ones. Examples of brown-egg and white-egg laying breeds are, respectively, Rhode Island Red/ New Hampshire and White Leghorn. Be that as it may, poultry breeders have striven to produce highly productive strains of brown egg producing birds. But this is made difficult because as a bird lays more eggs the shell becomes progressively lighter, irrespective of breed. The market has gone one stage further and now caters for the specific demand for ‘novelty’ eggs such as turquoise blue and olive-coloured eggs produced by the ancient and now resurrected ‘Old Cotswold Legbar’ breed.
Size and weight
Uniformity of eggs for size and weight, within the range of categories operating in their market, is a key consideration for producers. Egg size and weight are characteristics with a fairly high degree of heritability but are negatively correlated with increased egg production and generally affected by the age of the pullet at the point of lay. In breeding terms, this means that selection of layers for high production results in breeds and strains that produce small eggs, but this can be overcome by the use of selection indices. Mean egg weight in hens (chickens) increases by one gram every seven days for a period of some 42-50 days but then levels off. To obtain any meaningful comparison, selection must therefore be made between birds at equivalent stages of production.
Egg weight and size, and factors that control them, are of key importance in countries where marketing is based on weight. But where eggs are sold per item the number of eggs obtained from a single bird is more important than egg size or weight. Productivity by weight is calculated by expressing egg collections as kg of eggs per started number of layers. In sophisticated markets very small (pullet) eggs are discarded, while ‘double yolk’ and ‘Jumbo’ eggs are normally sold outside of the mainstream market to overcome packing and transportation problems.
Sub-standard internal quality with associated taint, bad smell and even serious illness can put off consumers. The highest standards of internal quality must be encouraged, achieved and maintained. Internal quality is traditionally measured by candling (viewing the egg against the light) and assessing the following parameters.
• Egg shell must be completely intact – reject eggs with even a hair line crack
• The air space should be fixed and 1.0 - 1.5 cm in width – reject eggs if air space is loose or more than 1.5 cm
• Yolk should be centrally located and firm – reject eggs in which yolk is loose and/or to the side
• Acceptable eggs must possess clear egg white (albumen) – reject all eggs having blood and meat spots
Blood and meat spots
Blood and meat spots both appear in the border areas between the yolk and albumen. They are caused by different events, but have the same negative impact on discerning consumers. Blood spots are caused by the release of blood into the yolk when the follicle bursts out from the ovary. Meat spots originate from lesions in the mucosa of the oviduct and are most commonly associated with albumen. Blood spots and meat spots are not harmful to consumers, but by the same token are unlikely to encourage further purchases from the same source. Occurrence of either is inherited to a high degree (up to 50 per cent heritability) and therefore (in theory) can be selected out. High levels of Vitamin K in poultry rations increase the frequency of blood spots in eggs.
Deep coloured egg yolks are generally regarded as more healthy and nutritious but science explodes yet another myth surrounding egg characteristics and healthy eating. Yolk colour is governed by concentration of xanthophylls and carotenes that are natural yellow and orange pigments found in plants. Some are precursors of Vitamin A but yolk colour is no guide to nutritional quality since Vitamin A is colourless. In fact yolk colour is largely determined by elements in the diet governed in turn by how they are reared and produced. Rich yellow yolks are obtained by feeding hens with grass meal, alfalfa meal or other appropriate green plant material. Free-range and backyard hens will generally acquire such material for themselves. Producers can control yolk colour to a high degree by inclusion of artificial pigments such as canthaxanthine and beta apocarotenoid in the ration.
A large proportion of thick viscous egg white (albumen) is a hallmark of quality. It is ‘selectable’ in a breeding programme, but other factors including length of storage also affect the proportion of thick albumen. At the beginning of storage the proportion of thin white starts to increase at the expense of thick white. Thus genetic trials aside, a large proportion of thick viscous albumen is also the mark of a freshly laid egg.
Egg collection and cleanness
Basic egg quality is determined long before laying occurs, but conditions furnished for laying hens, collection technique and storage conditions will ensure established egg quality is maintained for as long as possible.
Production must take place under clean conditions to ensure that eggs arrive at the point of sale in a fresh, palatable, nutritious and healthy condition. This is achieved by keeping all equipment as clean as possible and by collecting eggs at least three times a day, especially during hot weather. This avoids the accumulation of soiling, frequency of breakage and contamination of intact eggs with the contents of those that are broken.
Provide nest boxes that are stuffed with clean, dry nesting materials to prevent egg breakage and soiling. Reject floor laid eggs on the grounds of poor sanitation. If necessary, wash dirty eggs in an egg washing machine with warm water and recommended disinfectant and sanitising solution. Do not wipe with a damp cloth as this just transfers disease from one egg to another.
Store collected eggs at an optimum temperature of 12 to 15°C at a relative humidity of 75 to 80 per cent and away from other products including chemicals because eggs readily absorb smells from the atmosphere which in turn taint the internal contents.
By Dr Terry Mabbett