The basis for quality determination of eggs by nondestructive means in the United States is the Egg Grading Manual (U. S. Department of Agriculture 1977). The same characteristics laid out in the manual are used in all countries with various degrees of emphasis on the several quality factors. Quality determinations are divided into external and internal factors. External quality factors are soundness of the shell, cleanliness, and egg shape. The internal factors are air cell size, albumen viscosity, and yolk shadow. The internal factors are judged by passing the egg in front of a bright light source. Other internal factors are freedom from blood spots, bloody albumen, meat spots, or other inclusions. Equipment is currently available that allows for quality evaluation by candling at rates in excess of 72,000 eggs per hour.
In 1981, the quality standards for grades of eggs in the United States were modified (U. S. Department of Agriculture 1981). The grades of eggs that can be offered for sale at retail are AA, A, B, and B*, referred to as B star. The requirements for each grade are detailed for each external and internal quality factor. The application of these standards are discussed by J. Stadel-man and Cotterill (1995).
Numerous laboratory methods for quality evaluation of eggs have been prepared. Most of these methods are destructive in that the shell is broken and measurements are made on the liquid contents. The most widely accepted method is the Haugh Unit for expressing the albumen condition. This measurement was presented by R. R. Haugh (1937) and modified by A. W. Brant, A. W. Otte, and K. H. Norris (1951). A lesser used system is the United States Department of Agriculture (USDA) score as suggested by Brant and colleagues (1951), which attempts to correlate visual appearance of the broken-out egg, Haugh Units, and candled grade.
Evaluation of shell quality is on the basis of shell strength. The breaking strength has been found to be closely related to shell thickness. On broken-out eggs, measurement of shell thickness is a common method. As the specific gravity of freshly laid eggs is determined primarily by shell thickness, a nondestructive estimation of shell thickness can be made by determining specific gravity of the intact egg.
In terms of quality preservation, the most frequently considered item is albumen condition, which is often expressed as Haugh Units. For a high-quality or grade AA egg, the Haugh Units should be above 78. This value lowers over time as the thick albumen thins. The rate of albumen thinning is a function of temperature. The breakdown or thinning of albumen is relatively rapid at high temperatures, 40° Celsius (C), and slows to almost no change at 1° C. Other than temperature, the carbon dioxide content of the atmosphere surrounding the egg affects the rate of carbon dioxide loss from the egg. With the loss of carbon dioxide, the pH of the albumen rises from about 7.6 in a fresh egg to 9.7 in a stale egg.
Humidity of the atmosphere influences rate of water loss from the egg, which results in increased air cell size. Ideal conditions for long-term storage of eggs are a temperature between 1° C and 3° C and a relative humidity of about 80 percent. For long-term storage, egg shells are usually coated with a colorless, odorless mineral oil that seals the pores of the shell.
A frequently neglected consideration in quality preservation is maintaining cleanliness and soundness of the shells. In summary, egg quality preservation is a function of time, temperature, humidity, and handling.
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