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Microcracks in Eggs

M. M. Bain^*,1, N. MacLeod, R. Thomson and J. W. Hancock

^* Division of Cell Sciences, Institute of Comparative Medicine, University of Glasgow Veterinary School, G61 1QH, UK; and Department of Mechanical Engineering, Faculty of Engineering, University of Glasgow, G12 8QQ, UK

¹ Corresponding author: m.bain{at}vet.gla.ac.uk

An experimental program of mechanical testing has shown that microcracks initiate in hens’ eggs at loads less than that necessary to cause total structural failure. In contact loading, computational modeling and numerical analysis show that very high stress levels develop on the inner surface of the eggshell as it conforms to a hard contact surface. This causes a series of microcracks to initiate at the inner surface of the shell and radiate out from the load site. A series of concentric circumferential microcracks also develop beneath the cuticle from the edge of the contact zone. Calculations relating to the eggshells’ dynamic response indicate that microcracks have a little effect on the structural stiffness and resonant frequencies of the egg. As a result, microcracks are unlikely to be detected by online crack detection systems, which rely on mechanical excitation. Eggs in retail outlets are likely to contain microcracks as a result of insults experienced during the collection, grading, and packing processes. Because the eggshell forms the first line of defense against potentially pathogenic microorganisms entering the egg contents, microcracks could potentially compromise egg safety.

Key Words: egg • eggshell • structural integrity • microcrack • damage