Eggshell microstructure, shell quality indices, mineralogy, and UV–Vis absorbance of domestic eggs of Iran

The structure and quality of eggshells (ES) vary around a mean value depending on different items, and these variations are important in their many industrial and technological applications as will be mentioned (vide infra). Five commercial egg brands of different laying hens are examined and their quality indices, morphological microstructure, elemental composition, and light absorbance at 200–700 nm UV/Vis spectra were compared. The ES layers include a limiting membrane, inner and outer shell membranes, a mammillary layer, a palisade layer, a surface vertical crystal layer, and a bilayer cuticle. The elemental composition of each layer reflects the proteinous or calcified nature of the layers, and various elements, C, O, N, Ca, Fe, Mg, Mn, Pb, Sr, Sc, Hf, Co, and La, were found. The weight difference between egg brands was significant, as well as the difference in ES percentage between white and colored eggs. ES weight possessed a positive relationship with the thickness of the calcified layer. So, the ES is heavier and the egg is larger and heavier when the calcified layer is thicker. The calculated average ES index was 7.99% ± 0.16 SE. In all ES samples, the absorbance in UV wavelength spectra (300–350 nm) was slightly higher than in Vis spectra (400 - 700 nm) and their difference was significant. The difference in absorbance of various treatments was significant and mean absorbance was the highest in the after-furnace (ash) samples and was the lowest for acid-treated ones. It seems that turning the ES into ash can improve the absorbance capability, especially in white ESs.