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The Impact of Eggshell Thickness on the Qualitative Characteristics of Stored Eggs Produced by Three Breeds of Laying Hens of the Cage and Cage-Free Housed Systems. #### Article  
The Impact of Eggshell Thickness on the Qualitative Characteristics of Stored Eggs Produced by Three Breeds of Laying Hens of the Cage and Cage-Free Housed Systems. 65% of the total sale, the remaining 5% represents a marriage sorted out at the initial processing stage due to non-compliance with the quality standards. Scientific evidence reveals that eggs are an excellent source of essential amino acids and provide such groups of bioactive compounds as micro and macroelements and vitamins that represent significant importance for human health and functionality [4; 5; 6; 7]. Moreover, readily digested and assimilated nutrients make eggs essential in human nutrition [7]. On the other hand, the availability of fatty acids, especially representatives of unsaturated ones, along with high moisture content and water activity, make eggs highly perishable [8; 9]. According to Commission Regulation (EC) No 589/2008, the shelf-life of eggs at an ambient temperature and relative air humidity of 50% is defined as 28 days after the laying date [10]. During this time, albumen as a freshness indicator undergoes substantial depletion, making grade A eggs with reasonably firm albumen move to grade B with watery albumen. In addition, irreversible changes in protein quality contribute to the increase in amino acid and fatty acid susceptibility to oxidation processes and loss of antioxidant and antimicrobial activities [8; 11]. Therefore, eggs of such quality are no longer suitable for fresh consumption. However, they can further be utilized as a raw material for producing liquid whole egg products, taking into account the issue of microbiological safety [12]. The loss of moisture and CO\({}_{2}\) through the eggshell pores and microcracks is the main factor leading to pH rise and, eventually, egg quality deterioration [13]. A change in pH triggers a cascade of reactions, such as degradation of the ovomucin-lysozyme complex, liquefaction of albumen, and weakening of the vitelline membrane [14]. Meanwhile, the changes in eggs' physical and chemical features are more intensive in the case of the thinner eggshell layer [15; 16]. This statement was further reinforced by [PERSON] et al. [17] and [PERSON] et al. [3], indicating the importance of eggshell thickness and structural integrity as a physical barrier to preserve bird eggs from moisture loss and protect them from contamination by pathogenic microorganisms. The nutritional value of eggs and structural integrity of the eggshell are determined by bird age and genotype [18]. The importance of a balanced diet for laying hens has been highlighted by [PERSON] et al. [19] and [PERSON] et al. [20], indicating the relevance of mineral sources used in the diet of laying hens derived from inorganic compounds such as carbonates, phosphates, oxides, and sulfates in the formation of eggshells [19; 20]. The presence of an adequate amount of trace minerals in the diet makes the eggshell structure more rigid and robust enough to prevent failure during packing and transportation. Along with nutrition, the matter of poultry housing conditions is highlighted by [PERSON] et al. [21], revealing that laying hens face high-stress circumstances during conventional housing, resulting in substantially higher cholesterol levels and weakening the eggshell. The storage of eggs in air-conditioned rooms at relatively low temperatures and appropriate humidity delays the ageing processes of eggs and maintains their quality [22]. This statement is consistent with the observation made by [PERSON] [13], revealing that the process of moisture evaporation can be delayed by refrigerated storage. However, ensuring such conditions in practice seems not economically feasible, particularly when transporting eggs over long distances to countries with warm climates.