Candling is a non-destructive technique that involves shining a bright, concentrated light through an eggshell to illuminate its internal structure. This method allows observers to inspect the contents without breaking the shell. Poultry keepers use this practice to monitor the growth and viability of developing embryos during incubation. Candling is also a fundamental procedure in the commercial egg industry for assessing the quality of eggs intended for human consumption.
Monitoring the Hatching Process
The primary purpose of candling during incubation is to confirm that a fertile egg is progressing successfully into a live embryo. By the fifth to seventh day, a healthy egg displays a dense network of fine, red blood vessels radiating outward beneath the shell. This vascular system, often described as a spider-like pattern, confirms the presence of a live, developing embryo.
The embryo appears as a small, dark spot situated within this web of blood vessels. As development continues, particularly by day 14, the embryo grows significantly, becoming a larger, dark mass that fills more of the egg when viewed against the light. Monitoring the air cell at the blunt end of the egg is also an indicator of proper moisture loss, as it should gradually increase in size throughout the incubation period. Tracking these visual changes ensures that only eggs with thriving embryos remain in the incubator, optimizing the chances for a successful hatch.
Detecting Non-Viable Eggs and Internal Defects
Candling allows for the identification and removal of eggs that are infertile or have ceased development. Eggs that remain entirely translucent, showing no signs of an embryo or blood vessels after several days, are termed “clear” and indicate infertility or very early death. Another sign of failure is the “blood ring,” a dark, distinct circle formed by the remnants of a circulatory system when an embryo dies early in development.
Removing these non-viable eggs prevents contamination risks within the incubator, as a rotten egg can sometimes explode and spread bacteria to healthy eggs. Candling also reveals physical defects, such as hairline cracks in the shell that compromise the egg’s integrity. Eggs showing signs of contamination, such as a cloudy appearance, dark spots, or unusual contents, must be promptly eliminated to safeguard the entire clutch.
Grading Eggs for Freshness and Consumption
The application of candling extends beyond hatching, serving as the standard method for determining the interior quality and freshness of market eggs. This process shifts focus to evaluating the egg’s internal components for commercial grading standards. The size of the air cell is the most reliable measure of freshness, as moisture loss causes the air cell to expand over time.
For example, a high-quality United States Department of Agriculture (USDA) Grade AA egg must have an air cell depth of no more than 1/8 inch. Candling also assesses the condition of the yolk and albumen; a fresh egg has thick albumen that holds the yolk firmly in the center. When a fresh egg is quickly rotated, the yolk’s shadow moves only slightly. In contrast, an older egg with thinner albumen allows the yolk to move more freely and appear closer to the shell. This visual assessment of air cell size and component mobility is used to assign consumer grades like AA, A, and B.