The eggs added to yeast breads serve a few distinct purposes. First, they're emulsifiers. The richest breads, such as Italian panettone, French brioche or German stollen, all contain significant amounts of butter. The eggs help the butter remain incorporated in the dough, instead of simply melting and running out. Second, baked goods get their structure and texture when proteins in their ingredients congeal in the heat, trapping air bubbles. Eggs add protein, and therefore structure, to the bread. Finally, eggs add tenderness and richness to the bread's dough.

Many sweet breads use whole eggs as a major ingredient. The yolks contribute their fattiness, emulsifiers and flavor, while the whites provide additional protein. The high level of protein in the egg whites helps sweet breads bake to a relatively light texture, compensating for the rich bread's weaker gluten development. Recipes heavy in whole eggs use relatively little water or milk, with the egg whites providing the dough with moisture.

Sweet bread recipes that use egg yolks instead of whole eggs tend to be denser and richer. The egg yolks themselves add fat, richness and flavor to the bread, and they emulsify large quantities of butter. That means sweet breads using egg yolks have an unusually rich and soft crumb, almost cake-like in texture. The yolks also give the breads a pleasantly golden color. Historically, this was important, because for centuries gold-colored foods were symbolic of prosperity and good fortune.

There are hundreds of variations on sweet breads, ranging from barely sweetened to heart-stoppingly rich. Although some recipes use only whole eggs and some use only yolks, it's more common to see a combination of both. A recipe that supplements whole eggs with a few additional yolks has the lighter, chewier texture of an egg-based bread, but with a richer flavor. A bread that primarily uses yolks but adds a whole egg or two is rich and soft, but shows more lift than a bread made with all yolks. Sweet doughs are relatively forgiving, so if you have a favorite recipe it's easy to experiment by varying the proportion of whole eggs to yolks.