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Physical, chemical and rheological studies of the hen's egg Tung, Marvin Arthur

Abstract

A three-part investigation is described in which hardness at regular intervals across the thickness of egg shells is measured and related to chemical composition at similar positions in the same shells; egg shell membranes are viewed by electron microscopy to compare the structure of inner and outer layers; and rheology of egg albumen is studied using a rotational viscometer. Microindentation tests of 27 egg shells reveal a maximum hardness at the outer surface, intermediate hardness at the inner surface and a minimum hardness midway across the thickness of the shell. Electron probe microanalyses indicate a gradual linear increase in calcium toward the inner shell surface. Magnesium and phosphorus are a maximum at the outer surface and follow quadratic and exponential gradients respectively across the egg shell thickness. Hardness gradients, composition gradients and the hardness-composition relationships change from shell to shell. For individual egg shells, an average of 91 percent of the variation in hardness is accounted for by variations in chemical composition. In pooled data of 27 shells, 48 percent of hardness variance is associated with composition changes; whereas 81 percent may be attributed to variations in composition and position in the shell. Electron microscopical study and measurement of four egg shell membranes show that the structures consist of an open network of fibers built up in layers parallel to the membrane surfaces. Two regions are evident in transverse section. The outer membrane is three times the thickness of the inner membrane and the combined dimension is about 100 microns. Each fiber has a central core encased by a granular mantle. For the outer membrane, fiber core diameters are significantly greater and fibers usually occupy a larger percentage of membrane volume than is the case for inner membranes. The inside surface of the inner membrane is lined by a 0.1 micron layer of material similar to the fiber mantle. Egg shell membrane structure is discussed in relation to microbial penetration. Viscous behavior of egg albumen is described at 10, 20, 30 and 40°C between shear rates of 220 and 3140 sec⁻¹. Egg albumen is a time-dependent pseudoplastic fluid. Apparent-viscosity decay at constant shear rate is partially recoverable between 32 hour tests and is affected by the temperature and shear rate used. Flow behavior is accurately described by the power-law and Ellis models. Shear history strongly influences flow behavior with higher shear rates resulting in greater pseudoplasticity and sensitivity to temperature effects.

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