Quality of eggs

 

There are several aspects to the quality of eggs that will be discussed. However, quality as it relates to hatching and hatchability will not be included here as the application of such knowledge is specific to the poultry breeding and reproduction industries.   Internal quality of the eggs. In general, the internal quality of the egg does not affect its nutritional value, but it affects the appearance and other aspects of the egg before and after cooking and consumption.

            

Quality of the egg white (albumen) The appearance of the egg white of broken-out eggs is the most important aspect of quality. When broken onto a flat surface (such as a plate, a  fry-pan or griddle) the yolk should be centred, surrounded by a compact circle of thick albumen, which in turn is surrounded by a small area of thin albumen. [insert illustrations here] For quality control purposes in an egg production/processing context, objective measures of egg quality become necessary.

The best known of these is the Haugh Unit. This is a computed value, in which the height of the thick albumen mid-way between the yolk and the thin albumen is corrected for the weight of the egg. It is intended that the Haugh Unit will have a uniform value for eggs of the same appearance, regardless of their size.  Newer research has shown that for most purposes, the correction for egg weight is unnecessary, and the use of the albumen height measurement alone is adequate.

The quality of the egg white is profoundly influenced by two factors.  These are the age of the hen that laid the egg, and the length and conditions (primarily temperature) in which it is/has been stored.  There is a marginal genetic effect, and one specific genetic defect that can cause “watery whites”.  In addition, hens infected with Infectious Bronchitis will lay eggs with very poor albumen quality. The freshly laid egg has the best quality possible for the specific hen that laid it. Older hens lay eggs with lower albumen quality. Freshly laid eggs may have a slightly opaque (cloudy) appearance, but this soon dissipates and the white becomes transparent.

 

Egg white quality begins to decline as soon as the egg is laid. The rate of decline depends primarily on the temperature in which it is kept. As a rule of thumb, temperatures higher than 15ºC result in rapid loss of albumen height. Eggs held for 1 day at 20ºC lose as much quality as they do when held for 7 days at 12ºC. Egg storage in the production/processing chain should aim at temperatures no higher than 12ºC. Consumers should be advised to keep eggs in a refrigerator (~ 4ºC) for best results. Refrigeration also limits bacteriological multiplication in the unlikely event that a consumer receives eggs containing SE. The decline in albumen quality is a result of chemical changes in the egg white. When laid, the albumen is slightly alkaline, with a pH of about 7.5. This increases to 9.5 or more as storage is prolonged.   Quality of the egg yolk The most obvious quality of the egg yolk is its colour. Depending on nutrition of the laying hen, yolk colour can vary from a very pale yellow to deep orange. Pigments are transferred directly from the diet to the forming egg yolks. Hens fed diets in which the cereal components are primarily wheat, barley, sorghum or other non-pigmented grains will produce eggs with pale yolks. Eggs from birds fed a corn-based diet will have yellow yolks, while those from hens fed natural or synthetic pigments, or such feed ingredients as alfalfa meal, will be various shades of orange. There are strong consumer preferences for egg yolk colour. These are geographically based. North Americans generally prefer the yellow yolks associated with corn-based feeding, although in Western Canada, where wheat is the primary grain, much paler yolks are readily accepted. Europeans prefer darker yolks, and hens must be fed diets containing sources of orange or red pigment to achieve the desired colour. Natural sources of pigment include alfalfa, marigold petals and corn gluten meal. Synthetic pigments are available as feed supplements where permitted; these include canthaxanthin, astaxanthin and β-apo-8-carotenoic acid.

Yolk colour is most frequently measured using the Roche Scale on which 12 graded colours are available for comparison with egg yolk samples. In addition to its colour, the quality of the egg yolk (vitelline) membrane is important in some contexts, specifically if the egg yolk is to be separated from the white. The vitelline membrane deteriorates with age of the egg. Where egg components are industrially separated, it is important that no yolk material contaminates the albumen, as this can seriously compromise the functional properties of the resulting liquid egg white.

 

External Quality of the Egg

From the consumer's perspective, there are two external quality factors of importance. These are 1)  the colour of the shell and  2)  the shell should not be cracked, dirty or damaged. The colour of the shell is strictly a genetic affair. ISA, along with all breeders, sells brown-egg and white-egg varieties.

The colour of the shell is strictly a genetic affair. ISA, along with all breeders, sells brown-egg and white-egg varieties. Intermediate between these is the tinted egg, but demand for these is not great. As hens age, the colour of the brown shells tends to deteriorate, but not usually to the point that they would be unacceptable in the market. A few diseases may reduce the intensity of the brown colouration but this is rare. Accidental feeding of drugs (especially nicarbazin) may also eliminate the colour from the shells. White eggs have to be pure white; a slight tint shows up in cartons and such eggs may be rejected by consumers.

Cracked eggs should be detected and eliminated during the primary processing and grading procedure. Eggs may be cracked prior to processing either because they have weak shells, or because they were exposed to an environmental “insult” that even a strong shell could not withstand.  Eggs may be cracked after processing but before reaching the consumer, usually due to mechanical damage during processing or transportation.

Weak shells are generally the result of nutritional faults; inadequate or incorrect form of dietary calcium, inadequate Vitamin D, and insufficient feed intake are the most common factors.  In addition, older hens produce eggs with weaker shells, and an environmental insult that eggs from a young hen would tolerate, may crack them. The best way to monitor egg shell quality in layer flocks is to measure the specific gravity of freshly laid eggs.

This is a quick method of estimating shell thickness which, while not the only factor affecting shell strength, is a reasonable guide.  The egg is exposed to environmental insults from the moment it leaves the hen's cloacae. It may fall too great a distance before hitting the cage or nest-box floor. Hens that squat to lay have a better chance of their eggs surviving than those that stand up when they lay. The material the egg first encounters may not be appropriate. If a cage floor has too steep an angle, the egg may roll out too fast and crack when it hits another egg, or the front of the cage. Mechanical egg gathering systems have improved enormously since first introduced, but still have to be maintained and monitored. Every time the egg changes direction, there are opportunities for collisions that may cause cracks. Artificial eggs with sensing devices have been designed to identify places in gathering systems where eggs may be damaged.

  

Thermal cracks may occur if egg washing water temperature is too high or eggs, too cold, when they encounter it. Egg processing equipment must be continuously monitored and maintained so as to limit damage to eggs during processing. Dirty eggs are the result of environmental faults in the production system or the egg collection system.

In countries where egg washing is routine, most dirty eggs are cleaned before packing and dirty eggs in retail cartons are minimal. Where egg washing is not practiced, dirty eggs should be identified and rejected prior to packing. A consumer who finds a dirty egg in a retail carton may be lost forever, so it behoves everyone in the egg production and processing industries to eliminate all dirty eggs from the system.