## 3 Results and Discussion
It is worth noting that the percentage decrease is consistent with those noted by [PERSON] et al. [50], indicating a reduction of 45.4% of the initial albumen height for eggs stored for 28 days under 21 \({}^{\circ}\)C temperature. To sufficiently characterize the freshness of the stored eggs, the Haugh unit was calculated following the methodology proposed by [PERSON] in 1937 as one of the crucial markers next to the eggshell thickness and eggshell strength [51]. At the start of egg storage, the Haugh unit approximated 68.3%, and the observed value was in line with data indicated by [PERSON] [45] (Figure 8). Figure 7: Changes in albumen height (mm) 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) are significantly different at \(p<0.05\), while means within the same hen breed with different capital letters (A–D) are significantly different at \(p<0.05\). However, analogous to albumen height, the value of the Haugh units was greatly influenced by storage time, particularly during the first 14 days of storage. On the 11 th day of egg storage, the percentage reduction corresponded to 20.3%. A reasonably similar decrease in Haugh units was reported by [PERSON] et al. (2016), revealing a decrease of 24.9% after 11 days of untreated egg storage under 25 \({}^{\circ}\)C temperature. Up to the 15 th day of storage, the eggs corresponded to grade A as proposed by the United States Department of Agriculture (USDA) ([PERSON] et al., 2016). A further decrease in Haugh unit values was also observed; however, it was less apparent than at the initial storage stage. After 22 days of storage, the percentage decline reached 30.7% in comparison with the initial value. The most apparent decline in the Haugh unit was observed in eggs produced by enriched cage H/HN/CCE and cage-free H/D/BWE laying hens. Further storage for up to 36 days revealed the most noticeable Haugh unit loss after 28 days of storage, which corresponded to a 36.9% loss. The observed Haugh unit's reduction was less pronounced than that reported by [PERSON] et al. (2016), identifying almost a 50% loss from the initial Haugh unit value. As expected, the values of Haugh units were well correlated with albumen height; however, no direct relationship was confirmed between protein quality and thickness of the eggshell and its membrane. Based on the data presented in the work of [PERSON] et al. (2013), it is apparent that after laying eggs, a significant part of CO2 has already been lost, which leads to a substantial rise in the albumen pH. The rise of albumen pH is considered a natural process since it ensures the high antimicrobial activity of albumen against certain pathogenic bacteria ([PERSON] et al., 2013). However, with the progression in pH rise, the enzymes destabilizing the ovomucin-lysozyme complex and degradation of clusterin and ovalbumin become more active. An increase in their activity leads to solubilization and deterioration in the gelling properties of albumen (2013). As reported by [PERSON], the most intense liquefaction of albumen was observed at pH 9.0 ([PERSON] et al., 2016). It was observed that the pH of albumen on the 1 st day of egg storage varied in the range from 8.9 to 9.3, with the albumen derived from eggs of cage-free housed hens H/NH/BCE having the lowest value and albumen derived from eggs of cage-free hens  
Figure 8: Changes in Haugh units 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.