## 3 Results and Discussion
However, after 28 days of egg storage, the size of air cells was exponentially enlarged by an average of 81.5% (for all tested eggs), and the observed mean values corresponded to 5.8 mm, respectively. Meanwhile, an additional 17 days of egg storage did not lead to exceeding a critical value of 6.0 mm. However, further storage of eggs negatively affected the enlargement of air cells, reaching an average value of 8.1 mm on day 36. Considering this observation solely, the maximum shelf-life of the selected eggs could be defined as 28 days, which is also in line with EC regulations No 589/2008 [10]. No apparent relationship between the weight loss and air cells can be seen after 11 or 28 days of egg storage. It is believed that the increase in air cell space and moisture and CO\({}_{2}\) loss promote the weakening of the eggshell structure and disrupt its integrity. Indeed, an increase in the space between the outer and inner membranes leads to the loss of eggshell strength. However, the results of this experiment revealed ambiguous data regarding this criterion, indicating a relative fluctuation in eggshell-breaking strength as a function of storage time (Figure 6). For instance, on the 11 th day of storage of eggs only, a 2.5% reduction in the eggshell-breaking strength was observed in eggs derived from cage-free housed laying hens H/HN/BCE, while an up to 20% increase was revealed in eggs produced by cage-free housed hens H/D/BWE. The most apparent reduction in the eggshell strength was observed in eggs produced under an enriched cage system H/LS/CCE after 28 days of storage, corresponding to a 13.5% decrease (42.4 N). The observed value, however, does not differ significantly (\(p\geq 0.05\)) from that indicated by [PERSON] et al. [47]. After 36 days of storage, minor changes took place in the structure of eggs obtained from cage-free housed laying hens H/D/BWE, as only a 5.6% increase in their strength was observed compared with the initial value. It was expected that egg shrinkage would harm eggshell integrity, though data obtained confirmed no direct relationship between weight loss and breaking strength values after 11 or 28 days of egg storage. Figure 6: Changes in eggshell-breaking strength (N) 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. Means within the same day of storage with different small letters (a, b, c) are significantly different at \(p<0.05\), while means within the same hen breed with different capital letters (A–D) are significantly different at \(p<0.05\). The albumen height was the main indicator of egg quality to be considered critically during the evaluation, since a direct relationship between albumen height and egg freshness was repeatedly documented [48; 49]. Furthermore, the pH shift from acidic to alkaline caused by water and CO\({}_{2}\) loss led to albumen liquefaction and viscosity decline [8]. The results of this study revealed a gradual decline in albumen height over 36 storage days (Figure 7). As shown, the reduction in albumin height after 11 days of egg storage varied from 18.6 to 35.5%, with eggs from cage-free housed laying hens H/HN/BCE having the lowest reduction and eggs from cage-free eggs housed hens H/D/BWE the highest. However, the values found still complied with the criteria framed in the EC regulation. The observed initial albumen height values are slightly lower than those reported by [9; 14; 48], perhaps due to the difference in breeds of laying hens. The most significant reduction in albumen height was found in eggs from enriched cage-housed hens H/HN/CCE, while no significant differences were revealed for the other three laying hens. The most notable reduction in albumen height was uncovered during the following storage of eggs for an extra 17 days, reaching a reduction of 48.6% of the initial height.