Research On Mechanics Of Duck Eggshells And Its Permeability Of Porous Ultrastructure

Egg salting is an ancient method of egg preservation dating back hundreds of years. Processing eggs into salted ones by immersing into salt water not only makes the flavor more unique, but also alleviates the seasonal supply and demand of the eggs. In static state, eggshell quality and its ultrastructure will have direct impact on the maturity of the eggs, with salt water penetrating inside through eggshell and its membrane during the process of making salted eggs. On the basis of deep understanding of the researches both at home and abroad, the mechanical characters and fractal characteristics of pore structures were investigated using duck eggshells as an object of study in this paper. Firstly, the basic physical parameters of egg were measured, and then the mechanics of different loading parts of eggshell were tested by using utilizing texture analyzer, theory analysis and Finite Element Analysis (FEA) numerical simulation, respectively. Secondly, the ultrastructures of the sharp end, the blunt end and the equator of the eggshells were measured using Scanning Electron Microscope (SEM) and Image-Pro Plus (IPP) software, the porous penetration mass transfer characteristics of eggshell were predicted based on fractal theory. Lastly, the influences of temperature and pressure on the physical parameters of the seepage were explored. The main research results are as follows:(1) Compressive experiment and puncture experiment on duck egg by application of TMS-PRO texture analyzer. In the compressive experiment, probe has only downwards compressive force on eggshell and the greatest loading force of long axis of eggshell is significantly greater than that of the short axis. However, in the puncture experiment, eggshell bears the shearing force and extruding force from probe, and the greatest puncture-resistance force of short axis of eggshell is slightly greater than that of the long axis.(2) Through the test of the basic physical parameters of the duck egg and research on the mechanical characteristics of eggshell, it has shown that egg weight has a certain degree of positive correlation with egg length and egg breadth, but no correlation with shape index. Egg weight has a certain degree of positive correlation with the largest loading force of the sharp end, but no correlation with the maximum loading force of the equator and of the blunt end. Egg shape index has a positive correlation with the largest loading force of different parts of eggshell, that’s to say, the rounder the duck egg, the larger the loading force. Eggshell thickness has positive correlation with the largest loading force of different parts of eggshell, and it’s appropriate to adopt the polynomial models to fit the relation of eggshell thickness and the largest loading force.(3) It is shown by the computational analysis of the membrane theory of thin shells that the compressive strength of the long axis of duck egg is much greater than that of short axis. Through simulation of the stress distribution of different parts of eggshell under different loads with FEM, the internal stress produced by eggshell when the load is born on the short axis is larger than the internal stress when the load is born on the long axis, that’s to say, the loading force is stronger on the short axis of the duck egg. Theoretical calculations and FEA numerical simulation results are consistent with the experimental result, which also proves why to place the eggs vertically during transport-tation.(4) Application of SEM and image processing technology in the quantitative study of the pore size of the eggshell. The average area, perimeter and diameter on the blunt end are all larger than those on the sharp end and the equator of the same eggshell. According to the Box-counting Method (BCM) and Slit Island Method (SIM), this paper studies the fractal dimension of pore size distribution and pore section boundary of eggshell, showing that the pore size distribution on the blunt end is much wider than that on the point end and at the equator, thus making it more conducive to penetration; but the pore shape distribution is extremely irregular.(5) Fractal models of porosity and of permeability of eggshell are built based on the fractal theory. The calculation shows that pore tortuosity has little influence on porosity and permeability. The porosity is very small with the value close to0, that because the maximum pore diameter of eggshell is usually10μm.Theoretically, the permeability is an inherent property of the eggshell, only affected by the pore structure of the eggshell. It does not increase or decrease with the variation of porosity. It is predicted that eggshell permeability is very low with its magnitude about10-8μm2.(6) During the process of making salted eggs, the external pressure, to a certain extent, can make eggshell pore stress change, resulting in deformation of solid skeleton, thus changing the porosity and permeability. Temperature has little influence on the deformation of solid of eggshell, and its influence on eggshell porosity and permeability can be ignored.