What Is a Candled Egg?

A candled egg is one which has been held up to a bright, concentrated light source in order to view the interior contents without breaking the shell. When placing an egg against a light with an aperture of about an inch in an otherwise darkened room, an experienced candler can quickly determine an egg's quality and fertility, among other things.

  1. Checking Quality

    • Poultry breeders who incubate their own eggs use candling to check the quality of eggs before attempting to hatch them. Because eggs with hairline shell cracks or internal irregularities of the yolk, white or air cell have a poor chance of hatching, they are best identified early so that they can be replaced by better candidates. Internal conditions that may be detected include bloody whites, blood or meat spots, bacterial contamination and excessive age.

    Checking Fertility

    • By the third, fourth or fifth day after being placed in the incubator, a breeder can use candling to see which eggs are fertile and which are not. Infertile eggs should be removed, since they will often become contaminated and rotten in the warm, moist conditions of the incubator, possibly exploding and contaminating fertile eggs next to them. Fertile eggs, when candled, show a spidery network of veins with the small mass of the embryo visible in the center.

    Chick Development

    • Developing chick embryos with genetic or developmental abnormalities tend to die off in two waves; the first, after about a week of incubation, and the second, a few days before hatching. By candling after these two critical development periods, the breeder can remove unviable eggs from the incubator and get a better idea of expected hatch rates. Dead embryos in the early stage exhibit a blood ring or halo; in the later stage, they simply stop moving inside the shell.

    Grading Table Eggs

    • The United States Department of Agriculture has detailed guidelines for grading table eggs based on quality. Several of these guidelines deal with the interior contents of the egg, and can only be checked by candling or breaking the egg to visually inspect the yolk and white. While most eggs are checked by personnel using mass scanning devices, eggs are still spot-checked by individuals using hand candling devices to check the accuracy of the automated system.