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In uence of the microstructure on the shell strength of eggs laid by hens of different ages A. RODRIGUEZ-NAVARRO, O. KALIN 1 , Y. NYS 2
AND
J.M. GARCIA-RUIZ
Instituto Andaluz de Ciencias de la Tierra-CSIC, Campus de Fuentenueva, 18002 Granada, 1 Departamento de Paleontología, Facultad de Ciencias, Universidad Complutense, 28008 Madrid, Spain and 2INRA, Rech Avicoles Stn, F-37380 Nouzilly, France Abstract 1. The eggshell is a bioceramic material constructed of columnar calcite crystals preferentially oriented with their c-axis perpendicular to the shell surface. 2. The in uence of microstructure (crystal size, shape and crystallographic orientation of crystal grains) on the mechanical properties of eggshells (shell strength) was investigated using eggs from hens of different ages. 3. There was a strong correlation between crystallographic texture and the strength of the eggshell in the case of eggs laid by young hens. The strength of eggshells increased as the preferential orientation of the crystals constituting the eggshell decreased. 4. By comparing two age populations, the effect of hen age on eggshell properties was evaluated. In general, eggshells from aged hens had a lower breaking strength (less than half that of those laid by young hens) and showed a greater variability in their structural properties such as thickness, grain morphology and crystallographic texture. 5. Texture analysis revealed that shells from eggs laid by aged hens have two preferred crystal orientations, after (001) and (104), compared with mainly one, after (001), in eggs laid by young hens. 6. These observed changes in eggshell properties could be due to changes in the organic matrix of the eggshell associated with ageing of the hens.
INTRODUCTION Eggshell is a bioceramic composite consisting of a mineral part (calcite aggregates, 95%) pervaded with an organic matrix (1 to 3·5%), resulting in a structure which has excellent mechanical properties (Solomon, 1991; Arias et al., 1993; Hunton, 1995). The mineral part consists of columnar calcite crystal units (palisade layer, about 200 µm thick) grown from separate nuclei (mamillary knobs) and covered by the cuticle. The disposition and size of the calcite crystals changes across its thickness, from being small and randomly oriented at the inner surface to being preferentially oriented and larger near the outer surface (Hamilton, 1982; Sharp and Silyn-Roberts, 1984). The function of the eggshell is to protect the contents of the egg from mechanical impacts and microbacterial invasions (for example, Salmonella) and to control the exchange of water and gases through the pores during the extrauterine development of the chick embryo (Fink et al., 1992; Nys et al., 1999). In the food market, the eggshell functions as a packaging material and its good quality is
crucial to consumer selection and safety. So, great care is needed to preserve it intact. That is why the mechanical properties of the eggshell, and, in particular, the strength of the eggshell have been the subject of extensive research; its determination is helpful in predicting and preventing breakage of eggshells in the eld (Romanoff and Romanoff, 1949; Tyler, 1961; Voisey and Hunt, 1974; Hamilton, 1982; Tullet, 1987). From the data in the literature it is clear that, in the majority of cases, a linear relationship exists between the strength of the shell and its thickness and/or density (Romanoff and Romanoff, 1949; Tyler, 1961). Although the latter are considered the most important ones in uencing the strength of the eggshell, they only account at best for 56% of the observed differences in strength (Richards and Swanson, 1965). This fact is explained by other authors (Voisey and Hunt, 1967; Carter, 1970; Bain, 1990) as a result of the in uence of other structural variables such as egg size and shell curvature. Nevertheless, the eggshell is essentially a ceramic material and the physical properties, the elastic and mechanical behaviour of a ceramic in
Correspondence to: Dr A. Rodríguez-Navarro, Instituto Andaluz de Ciencias de la Tierra, Universidad de Granada, 18002 Granada, SPAIN. Tel: +34-958-243360. Fax: +34-958-243384. E-mail:
[email protected] Accepted for publication 7th January 2002.
ISSN 0007–1668(print)/ISSN 1466–1799(online)/02/030395–09 © 2002 British Poultry Science Ltd DOI: 10.1080/00071660120103675
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particular, depend strongly on its microstructure (Green, 1998). The microstructure is de ned by the size of grains, their shape and how they are arranged and orientated in the structure (crystallographic texture). There are few studies analysing the in uence of structure on shell strength (Robinson and King, 1970; Meyer et al., 1973; Bunk and Balloun, 1978; van Toledo et al., 1982). In these studies, the structure of strong and weak eggshells is compared by scanning electron microscopy (SEM), which suggests that shell strength is dependent on both the thickness of the palisade layer and the organisation of calcite crystals in this layer (Meyer et al., 1973). In low strength eggshells, the basal region has a higher density of mamillary knobs (van Toledo et al., 1982). Also, the basal part sometimes is disorganised and displays a number of structural alterations not commonly observed in high strength shells (Bunk and Balloun, 1978; Solomon, 1991). Such evidence supports the conjecture that not only the density and thickness but also the ultrastructure has an important in uence on shell strength. However, none of these studies has examined the in uence that microstructure may have on shell strength. Therefore, our study was conducted to evaluate this relationship quantitatively. As hens approach the end of the laying year there is a remarkable decrease in shell strength (Hamilton et al., 1979; Potts and Washburn, 1983). Some authors have reported accompanying changes in the organic matrix component of the eggshells from older hens (Fraser et al., 1998; Panheleux et al., 2000). To determine whether changes in the microstructure associated with hens’ ageing are responsible for the observed weakness of the eggshells, the microstructure, crystallographic texture and other properties of shells from hens of different age were compared. METHODS Sample selection and measurement of eggshell mechanical properties Brown egg-laying hens (ISA Brown) were caged individually in a windowless poultry house and subjected to a cycle of 14 h light and 10 h darkness. They were fed a layer mash as recommended by the Institut National de la Recherche Agronomique. Shell properties were measured in hens from two age groups, one at the beginning and the other at the end of the laying year (30 and 58 weeks old, respectively). The eggs were weighed and selected (29 eggs from the old hens (EOH) and 25 from the young hens (EYH)) to eliminate the effect of differences in egg size when measuring shell
strength. Breaking strength of uncracked eggs was measured with an Instron testing machine (model 1140, Instron Ltd, Bucks, UK). A constantly increasing load was applied to an egg lying lengthways until it broke. The applied load at the time of breakage is the measured strength. After testing, the egg content was discarded and the shells washed, dried at room temperature and then weighed. Shell index was calculated as the weight of the shell per unit surface area (length ´ width). Thickness of each shell sample was measured with a digital micrometer with a precision of 1 µm. Microstructure of the eggshells Optical microscopy Eight samples (the two strongest and two weakest shells from each of the two age groups) were selected for further examination under an optical microscope. Thin slices (
