Munankuori

Munankuori, commonly referred to as eggshell, is the hard outer covering of a bird egg. It is primarily composed of calcium carbonate in the form of calcite, reinforced by an organic matrix of proteins and polysaccharides. The shell accounts for roughly 9–12 percent of an egg’s weight and varies in thickness from about 0.3 to 0.4 millimeters in many species, with variations among breeds and ages.

An egg’s shell consists of two shell membranes beneath the surface: an outer shell membrane and an inner shell membrane, plus the outer protective cuticle. The shell also contains thousands of microscopic pores that allow gas exchange while helping limit water loss. Color is determined by pigments deposited during shell formation in the oviduct, such as protoporphyrin (brown shades) or biliverdin (blue/green in some breeds).

Shell formation occurs in the shell gland, or uterus, of the oviduct as the egg travels. Calcium carbonate is deposited around a framework of organic matrix proteins; minerals and vitamins, notably calcium and vitamin D3, influence shell quality.

Function and significance: The eggshell provides mechanical protection and a barrier to pathogens, while pores permit respiration for the developing embryo and humidity control. Poor shell quality or thin shells can reduce hatchability and increase breakage; factors include hen age, nutrition, calcium availability, disease, temperature, and humidity. Cleaning eggs can remove the protective cuticle, increasing microbial risk.

Uses and byproducts: Eggshells are used as a calcium supplement in poultry diets and in agriculture as lime substitutes or soil amendments. In culinary contexts, shells are usually discarded or ground for compost.