Chicken farmers need to know whether the eggs they are sending to market have been fertilized and are growing a chicken, or unfertilized and can be used for scrambled eggs and other recipes. Before the invention of the electric light, farmers held individual eggs in front of a candle in a dark room. This allowed the farmer to see inside the egg, but it was a long process. The farmer also uses candling to determine how well-developed the embryo is inside of the egg during incubation.
Next came the electric light, which enabled farmers to candle many eggs at once using a light source under many eggs at once. Today’s commercial operations are computer-operated and can distinguish instantaneously between fertilized and unfertilized eggs.
When the computer inspects the eggs, it looks for a yolk shadow, signaling that the egg can be sent to stores. If it sees a dark spot surrounded by a web of blood vessels, the egg has begun to grow into a chicken (see the duck egg on the right). If the computer identifies an egg with red spots, it means that the embryo has stopped growing and the egg is useless for either incubation into a chicken or for making French toast.
You can candle your own eggs like this: Take a halogen flashlight and a fresh egg into a dark room. Hold the light behind the egg so you can see through it. What do you see? Hopefully you’ll see the yolk shadow and can use that egg in your next recipe!
Look for more interesting STEM lessons in each recipe included in Everybody Cooks! STEM Facts and Recipes for Family Cooperation and Healthier Eating - Holiday Favorites Edition.