The mechanical properties of animal and plant materials are necessary considerations in the design and effective utilization of the equipment used in the transportation, processing, packaging and storage of agricultural products. In a natural environment, eggshells must be strong enough to prevent cracking in order to preserve the embryo until hatching. In the context of a farm, shell strength is necessary to prevent damage from handling and to preserve eggs during transport from farm to market. There is natural variability in egg shape and this variability can be characterized using a shape index (SI). Eggs are characterized by the SI as sharp, normal (standard) and round if they have an SI value of <72, between 72 and 76, and >76, respectively (Sarica & Erensayin, 2004). Normal chicken eggs have an elliptical shape. If the chicken eggs are an unusual shape; such as being long and narrow, round, or flat-sided; they are not regarded as Grade-AA or -A (USDA, 2000). Round and unusually long eggs have poor appearance, and they do not fit properly in preformed packaging. Further, they are less resistant than normal shaped eggs to rupture during shipping (Jacob, Milles, & Mather, 2000).A chicken egg is an packaged food and an important quality aspect of the packaged egg material is the mechanical strength of the eggshell. A commonly used technique for the measurement of the shell strength is the quasi-static, non-destructive compression of an egg between two parallel steel plates (Coucke et al., 1998 and De Ketelaere et al., 2002).Egg size and the eggshell thickness are strongly related to each other (Harms, Rossi, Sloan, Milles, & Christmas, 1990). While egg weight increases during the production period, eggshell thickness and breaking strength usually decrease. Eggshell quality depends on egg size and weight. Egg properties such as SI and shell thickness affect the proportion of damaged eggs during handling and transport (Anderson, Tharrington, Curtis, & Jones, 2004). These physical properties of eggs, and their resistance to damage through mechanical shock, can be characterized by measures such as rupture force, specific deformation, rupture energy and firmness (Abdallah et al., 1993, Altuntaş and Yıldız, 2007, Olaniyan and Oje, 2002, Voisey and Hunt, 1969 and Vursavuş and Özgüven, 2004).Eggshells must be strong enough to prevent cracking, weak enough to allow the chick to break through when hatching and thin enough to allow gas exchange. Eggshell strength has been described using various variables such as thickness of eggshell, shell stiffness and rupture force (De Ketelaere et al., 2002). The rupture force of hen eggs depended on various egg properties such as egg specific gravity, egg mass, egg volume, egg surface area, egg thickness, shell weight, and shell percentage. The strongest correlation was found between shell rupture force and shell percentage (Narushin, van Kepmen, Wineland, & Christensen, 2004). Breaking strength is correlated with shape index (Carter, 1976).
Several researchers have investigated the physical and mechanical properties of chicken and Japanese quail eggs by testing them against various compression loads (De Ketelaere et al., 2002, Lin et al., 2004, Narushin et al., 2004 and Polat et al., 2007). Breaking strength as a direct variable to measure of eggshell strength, is a difficult variable to measure, because only one measurement can be taken from each egg, and is highly dependent on compression speed (Voisey & Hunt, 1969). However, there is a paucity of technical information and data in the scientific literature with regards to the mechanical behaviour of chicken eggs under different compression orientations.
The objective of this study was to investigate the effect of SI value on the mechanical properties of chicken eggs. The mechanical properties examined were rupture force, specific deformation, rupture energy and firmness.
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