## 4 Conclusions  
A structural analysis of eggshells performed by scanning electron microscope coupled with energy dispersive X-ray spectroscopy revealed no substantial deviations in the morphology of eggshells. The thickness of the eggshell and membrane varied in the range from 356.2 to 366.4 \(\upmu\)m and from 20.0 to 59.9 \(\upmu\)m, respectively. No statistically significant difference was observed between the thickness of the eggshells tested. Furthermore, the eggs derived from cage-housed laying hens demonstrated substantially thinner layers of eggshell membranes. The lack of essential micronutrients such as Cu in the diet of hens under cage-housed conditions is a possible cause of poor egg membrane development. From the total number of eggs investigated (\(n=50\)), the eggs derived from cage-free hens of H/HN/BCE represented no apparent signs of eggshell surface cracking, which likely contributed to the preservation of water and CO2 loss during 36 days of storage. The most  
Figure 9: Changes in albumen pH during 36 days of egg storage. Note: H/LS/CCE—eggs from enriched cage-housed laying hens; H/HN/CCE—eggs from enriched cage-housed laying hens; H/HN/BCE—eggs from cage-free housed laying hens; H/D/BWE—eggs from cage-free housed laying hens. The lack of small and capital letters on the plot indicates no statistically significant variations in the pH values between storage days and hen breed. significant changes in the quality of eggs occurred after 28 days of storage, especially with such parameters as weight and breaking strength. Eggs obtained from cage-free housed hens H/D/BWE showed the most significant weight loss, and eggs from the cage and cage-free housed hens H/LS/CCE and H/HN/BCE the lowest, respectively. It was expected that egg shrinkage would harm eggshell integrity. However, data obtained revealed no direct relationship between weight loss and breaking strength values after 11 or 28 days of egg storage. It is hypothesized that the main factor contributing to the weakening of eggshell strength was the availability of long and thick spider-like microcracks on the eggshell surface of H/LS/CCE eggs. Protein quality indicators such as albumen height and Haugh units were well correlated with each other, and the intensity of their reduction during egg storage to a greater extent was found to be storage time-dependent. No substantial depletion of egg albumen was observed during the first 15 days of egg storage. The quality of eggs still complied with the regulations outlined by the United States Department of Agriculture quality standards and belonged to grade A (reasonably firm). However, after 18 days of storage, the albumen had undergone the most substantial depletion since Haugh unit values dropped below 60, making eggs enter the B grade (weak and watery). The most apparent reduction in the Haugh unit was revealed in eggs produced by enriched cage H/HN/CCE and cage-free H/D/BWE housed hens, and these values were well correlated with albumen pH values. The EDS mapping analyses of the entire eggshell from four egg samples revealed almost similar profiles and composition of elements (data not represented). In general, six base elements, i.e., Na, Mg, P, S, Ca, and O, were detected in the eggshell. Overall, the quality of eggs was found to be storage time-dependent, and the rate of their deterioration was primarily associated with the genetic background of the laying hens and housing conditions rather than with the structure of the eggshell. Conceptualization, V.K. and V.R. ; data curation, V.K. and V.R. ; formal analysis, V.R., V.K. and D.J. ; investigation, V.R., V.K. and D.J. ; methodology, V.R., V.K., D.J. and S.M.-B. ; resources, V.K. ; software, V.R. ; visualization, V.K., V.R. and D.J. ; writing--original draft, V.K. and V.R. ; writing--review and editing, V.K., V.R. and K.J.-R. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable.