Influence of weight grade on shell characteristics of marketed hen eggs

In egg marketing, shell breaking during both packaging and delivery is the main cause of losses. As a part of a wider study devoted to the description of commercial eggs quality, eggshell characteristics were evaluated as a function of the weight grades established by the European regulation. To this end, 27 samples of commercial grade A eggs, belonging to S, M, L, and XL weight grades, laid by cage hens and purchased in different retail shops in Northern Italy, were analysed. Each sample included at least 36 eggs from the same lot. The following parameters were determined: cracks percentage; egg weight, surface area, equator diameter, height and shape index; percentage of eggshell; eggshell index and thickness. Breaking strength of eggshell was evaluated using an Instron dynamometer equipped with a plate and a probe (8 mm diameter) as compression/penetration elements. Differences among weight grades were significant (p≤0.001) for egg weight, surface area, height and equator diameter. Instead, shape index values of different weight grade eggs partially overlapped. A significant (p≤0.001) decrease of eggshell percentage was observed as egg weight increased, along with a reduction of eggshell thickness in XL eggs compared with M and L eggs. Eggshell thickness was lower at the broad pole than at the narrow pole and the equator. Penetration breaking strength was significantly higher in grade S eggs (43.4 N), while XL eggs leaned to the lowest average value (35.5 N). Furthermore, XL eggs showed the highest average percentage of cracked eggs. The use of different penetration/compression elements highlighted several significant correlations between breaking strength and various egg geometrical and structural parameters.