Yolk

Yolk is the nutrient-rich part of an egg. It gives food to the growing embryo. In animals like birds, reptiles, and insects, the yolk is made in the mother's body. In smaller animals, such as some fish and invertebrates, the yolk is inside the egg cell.

Yolks have vitamins, minerals, fats, and proteins. These help the embryo grow. The amount of yolk can change how the embryo grows after fertilization.

Yolks are not living things like protoplasm. They are mostly made of deutoplasm. The food and control parts are put in when the egg is formed. Some plants, like algae, also store food in their cells.

Avian egg yolk

In bird eggs, the yolk is usually yellow and round. It floats in the egg white, also called albumen, thanks to special spiral bands of tissue named chalazae.

The yolk, along with the egg’s center where the embryo forms, is protected by a special layer called the vitelline membrane. The yolk is a big part of the egg’s inner material, called cytoplasm, and is made of fats, proteins, and enzymes. After an egg is fertilized, the embryo starts to form in a spot called the germinal disc.

Chicken egg yolks are a great source of vitamins, minerals, and nutrients. They contain all the egg’s fat and cholesterol, plus almost half of its protein. When you fry an egg sunny-side up, the yolk stays in the middle, surrounded by egg white. If you mix the yolk and white together before cooking, like in omelets or scrambled eggs, the whole mixture turns yellow.

The yolk gives food to the growing embryo inside the egg. People sometimes separate yolks from egg whites for cooking. Yolks are used to make foods like mayonnaise, custard, and hollandaise sauce. They are also used in paintings called tempera and in making special paints for old books. Yolks have a special ingredient that helps protect the embryo from harmful germs. They are also used to make sweet drinks like advocaat and eggnog, and to get egg oil for skincare and health products.

The yolk makes up about one-third of an egg’s weight and has more energy than the egg white because of its fat. It contains important vitamins like A, D, E, and K, and many minerals such as calcium, iron, and zinc. The yellow color of yolks comes from special pigments in the chicken’s food, like carrots or green plants, but this color does not tell us how nutritious the egg is.

Eggs with two yolks, called double-yolk eggs, happen when a hen’s body makes two yolks at once. These eggs are rare and usually cannot hatch on their own.

Eggs without yolks, called yolkless or wind eggs, are usually laid by young hens who are learning to lay eggs. These eggs cannot hatch because they lack a yolk.

The color of an egg yolk depends on what the hen eats. Feeds with yellow pigments, like green plants, can make yolks more yellow. Even red or green yolks can happen depending on the hen’s diet, but this does not change how healthy the egg is.

In fish

All bony fish, some sharks, and rays have yolk sacs when they are growing. Fish that lay eggs keep their yolk sacs after they hatch. Some special sharks called Lamniform sharks grow inside their mother’s body. Their eggs hatch inside her, and the baby sharks eat unfertilized eggs for food.

In crustaceans

The yolk in crustacean eggs helps baby animals grow. It gives them the food they need. In animals called decapod crustaceans, a special protein called apolipocrustacein (apoCr) helps make this yolk. This protein is different from ones in other egg-laying animals.

ApoCr is a big protein made of about 2,600 tiny parts called amino acids. It has special areas that help it work. Scientists found that apoCr is more similar to proteins in insects and vertebrates than to yolk proteins in most animals. In decapods, apoCr is made in the ovary and another organ called the hepatopancreas, helping with fat and yolk creation. In some species, the gene for apoCr has made copies of itself, creating different versions that work in specific tissues.