How an incubating egg changes from the inside
A balut begins as a fertilized duck egg, not an ordinary unfertilized egg. Under warmth, the embryo develops using nutrients already stored in the yolk and egg white. The yolk supplies much of its energy; the white, or albumen, provides water and protein. The shell is porous, allowing oxygen to pass in and carbon dioxide to pass out while limiting moisture loss.
As development proceeds, the embryo’s body takes shape and its organs, bones, and other structures mature. Meanwhile, the contents of the egg change: the embryo uses some of the stored food, and its body becomes more distinct from the surrounding yolk and albumen. Incubation conditions and the point at which an egg is removed affect how far these changes have gone.
Cooking freezes a moving process
Boiling heats the egg enough to stop development and cook its contents. It does not make every balut look alike, because cooking preserves the different stages at which eggs were taken from incubation. A less-developed egg may contain mostly liquid and soft tissue; a more-developed one can have a recognizable body and firmer structures.
That variation is part of what makes balut unlike a uniform packaged ingredient: the egg records a particular moment in an embryo’s growth. Even eggs prepared together can offer different textures and proportions of yolk, liquid, and embryo.
