RESUMO
We have studied in detail the changes that occur in eggshell structure and composition during a production cycle in order to better understand the deterioration of eggshell quality with hen age (at 33, 45, and 67 wk). To study changes in eggshell ultrastructure and microstructure characteristics (mammillary density, palisade layer thickness, size, and orientation of calcite crystals) and the cuticle composition, we used complementary analytical techniques such optical and electron microscopy, X-ray diffraction, and infrared spectrometry. The marked decrease in eggshell breaking strength from 5.8 Kg at 33 wk to 4.4 Kg at 67 wk (25% reduction) could not be solely explained by the modest reduction in eggshell thickness (6-10% reduction) and seems to be associate to abrupt changes in eggshell ultra- and microstructure characteristics (i.e., decreased mammillary density; increased size of crystal units), occurring in older hens. Particularly, the decrease in mammillary density reduces the attachment points of the eggshell mineral to the membranes and therefore should negatively impact eggshell mechanical properties. Also, the observed increase in the calcite crystal size making the shell could also reduce the cohesion of crystals and eggshell resistance against impacts. Additionally, there was a decrease in the amount of cuticle and internal egg quality parameters (egg albumen height) with hen age that could have a negative impact in egg safety and quality.
