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Studies on physical properties of egg shells Tung, Marvin Arthur

Abstract

Physical characteristics of egg shells and their relationships to shell strength were studied In 2,733 eggs collected over thirty-two weeks from a flock of sixty Single-Comb White Leghorn pullets. Shell strength under quasi-static loading was measured as maximum force at failure and as energy absorbed at failure when load was applied at the equator of the egg. Area under the force-deformation curve was taken as energy absorbed by the shell up to failure and the slope of the curve as shell stiffness. Egg size was measured as egg weight, width and length. Shell weight, thickness at the equator, percent egg as shell, and shell weight per unit surface area were studied as measures of shell quantity. Shape index, roundness, and three concepts of sphericity were used to describe egg shape. Hardness in radial sections of 25 shells was tested with a micro-indentation technique. Variation in hardness across the thickness of egg shells was examined in radial and tangential sections of nine shells. Force at failure as a measure of shell strength showed high multiple correlations with combinations of physical properties, whereas energy absorbed at failure had relatively small multiple correlations with physical characteristics. Shell stiffness was found to be the most important indirect measure of shell strength along with lesser effects of egg weight, shell width, shape index, and hardness. Shell quantity characteristics, along with egg size and shape, were shown by means of theoretical and statistical analyses to be largely responsible for shell stiffness. Shape index proved to be the most satisfactory measure of egg shape with respect to reducing residual variance of force at failure after stiffness was considered and was judged to be the most accurate of the shape measurements studied. Shell hardness was found to vary in a parabolic manner across the shell thickness, reaching minimum values near the midpoint of the shell. Comparable hardness gradients were observed in both radial and tangential shell sections. No appreciable change in hardness or its gradient resulted from removal of shell membranes with sodium hydroxide solution. The proportions of variation in force at failure explained by the non-destructive variables shell stiffness, egg size, and shape were 60.5, 77.7, and 86.3 percent in pooled-egg, bird average per period, and overall bird average analyses respectively.

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