Introduction

Internal and external egg quality

Quality has been defined by Kramer (1951) as the properties of any given food that have an influence on the acceptance or rejection of this food by the consumer. Egg quality is a general term which refers to several standards which define both internal and external quality. External quality is focused on shell cleanliness, texture and shape, whereas internal quality refers to egg white (albumen) cleanliness and viscosity, size of the air cell, yolk shape and yolk strength.

Internal egg quality

Internal egg quality involves functional, aesthetic and microbiological properties of the egg yolk and albumen. The proportions of components for fresh egg are 32% yolk, 58% albumen and 10% shell (Leeson, 2006).

The egg white is formed by four structures. Firstly, the chalaziferous layer or chalazae, immediately surrounding the yolk, accounting for 3% of the white. Next is the inner thin layer, which surrounds the chalazae and accounts for 17% of the white. Third is the firm or thick layer, which provides an envelope or jacket that holds the inner thin white and the yolk. It adheres to the shell membrane at each end of the egg and accounts for 57% of the albumen. Finally, the outer thin layer lies just inside the shell membranes, except where the thick white is attached to the shell, and accounts for 23% of the egg white (USDA, 2000).

Egg yolk from a newly laid egg is round and firm. As the egg gets older, the yolk absorbs water from the egg white, increasing its size. This produces an enlargement and weakness of the vitelline membrane; the yolk looks fl at and shows spots.

As soon as the egg is laid, its internal quality starts to decrease: the longer the storage time, the more the internal quality deteriorates. However, the chemical composition of the egg (yolk and white) does not change much.

In a newly laid egg the albumen pH lies between 7.6 and 8.5. During storage, the albumen pH increases at a temperature dependent rate to a maximum value of about 9.7 (Heath, 1977). After 3 days of storage at 3 �C, Sharp and Powell (1931) found an albumen pH of 9.18. After 21 days of storage, the albumen had a pH close to 9.4, regardless of storage temperature between 3 and 35 �C (Li-Chan et al, 1995).

Heath (1977) observed that when carbon dioxide (CO₂) loss was prevented by the oiling of the shell, the albumen pH of 8.3 did not change over a 7-day period of storage at 22 �C. In oiled eggs stored at 7 �C, albumen pH dropped from 8.3 to 8.1 in seven days (Li-Chan et al, 1995).

Increases in albumen pH are due to CO₂ loss through the shell pores, and depend on dissolved CO₂, bicarbonate ions, carbonate ions and protein equilibrium. Bicarbonate and carbonate ion concentration is affected by the partial CO₂ pressure in the external environment.

In newly laid eggs, the yolk pH is in general close to 6.0; however, during storage it gradually increases to reach 6.4 to 6.9. Egg quality preservation through handling and distribution is dependent on constant care from all personnel involved in these activities. The quality of the egg once it is laid cannot be improved, so efforts to maintain its quality must start right at this moment.

The decrease in internal egg quality once the egg is laid is due to the loss of water and CO₂. In consequence, the egg pH is altered, resulting in watery albumen due to the loss of the thick albumen protein structure. The cloudy appearance of the albumen is also due to the CO₂; when the egg ages, the CO₂ loss causes the albumen to become transparent, compared with fresh eggs.

To minimize egg quality problems two things are important: frequent egg collection, mainly in the hot months, and rapid storage in the cool room. The best results are obtained at a temperature of 10 �C. There are six main factors affecting internal egg quality: disease, egg age, temperature, humidity, handling, and storage.

Disease: Newcastle disease and infectious bronchitis produce watery albumen, and this condition may persist for long periods after the disease outbreak has been controlled (Butcher, 2003). Egg age: eggs several days old show weak and watery albumen, and the CO₂ loss makes the content alkaline, affecting the egg flavour. Temperature: high temperatures cause a rapid decrease in internal quality. Storage above 15.5 �C increases humidity losses. Humidity: high relative humidity (RH) helps to decrease egg water losses. Storage at an RH above 70% helps to reduce egg weight losses and keeps the albumen fresh for longer periods of time. Egg handling: rough handling of the eggs not only increases the risk of breaking the eggs, but also may cause internal egg quality problems. Storage: eggs are very prone to take on the odours of other products stored with them; separate storage is therefore advised.

The variables mentioned above are particularly important to ensure that a 1-week-old egg, properly handled, can be as fresh as a day-old egg kept at room temperature.

If the egg is properly handled during shipment and distribution, it will reach the consumer�s table with adequate freshness.

External egg quality

Poor eggshell quality has been of major economic concern to commercial egg producers, with estimated annual losses in the USA of around 478 million US dollars (Roland 1988). In Australia in 1998, the impact was of the order of 10 million Australian dollars per year. Information obtained from egg grading facilities indicates that 10% of eggs are downgraded due to egg shell quality problems. Based on values for the UK, Germany and the USA, it has been estimated that the incidence of broken eggs ranges between 6 and 8% (Washburn, 1982). In Mexico in 2005 it was estimated that the egg industry lost between 30 and 35 million US dollars, based on average figures of 2.5% broken eggs and 4% weak shells. These losses occur only between laying and packing, not taking into account losses in transit to the end consumer (DSM Mexico, 2005, unpublished data).

To maintain consistently good shell quality throughout the life of the hen, it is necessary to implement a total quality management programme throughout the egg production cycle.

It has been always recognized that the hen has the most extraordinary method of obtaining and depositing calcium (Ca) in the entire animal kingdom. An egg has an average of 2.3 g of calcium in the shell, and almost 25 mg in the yolk (Etches, 1987). A modern hen laying 330 eggs per cycle will deposit 767 g of calcium; assuming a 50% calcium retention rate from the diet, the hen will consume 1.53 kg of calcium per cycle.

Exterior egg quality is judged on the basis of texture, colour, shape, soundness and cleanliness according to USDA (2000) standards. The shell of each egg should be smooth, clean and free of cracks. The eggs should be uniform in colour, size and shape.

There are five major types of shell problems in the egg industry: 1. cracks due to excess pressure; 2. cracks due to thin shells; 3. body-checks; 4. pimpled or toe holes, and 5. shell-less eggs.

When a producer complains about an increase in downgrade eggs, the first thing required is to determine which types of problems have increased. In a processing plant with 97% A-quality eggs, a typical distribution of the different types of shell problems (downgrade) might be 2.13% stains, 0.85% blood spots, 0.85% meat spots, 61% pressure cracks, 9.8% thin shell cracks, 6.8% body-checks, 13.6% pimpled and 5.1% toe holes. If the percentage of any type of shell problem is abnormally high, then that is the problem needing attention.