Shell characteristics of different weight-grade eggs: E. Casiraghi et al.
Influence of weight grade on shell characteristics of
marketed hen eggs
E. CASIRAGHI1*, A. HIDALGO and M. ROSSI
Dipartimento di Scienze e Tecnologie Alimentari e Microbiologiche, Università degli Studi di
Milano, via Celoria, 2 - 20133 Milano, Italy
*[email protected]
Keywords: eggshell; egg weight; shell strength; shell thickness
Summary
In egg marketing, shell breaking during both packaging and delivery is the main cause of losses. As a
part of a wider study devoted to the description of commercial eggs quality, eggshell characteristics
were evaluated as a function of the weight grades established by the European regulation.
To this end, 27 samples of commercial grade A eggs, belonging to S, M, L, and XL weight grades,
laid by cage hens and purchased in different retail shops in Northern Italy, were analysed. Each
sample included at least 36 eggs from the same lot.
The following parameters were determined: cracks percentage; egg weight, surface area, equator
diameter, height and shape index; percentage of eggshell; eggshell index and thickness. Breaking
strength of eggshell was evaluated using an Instron dynamometer equipped with a plate and a probe
(8 mm diameter) as compression/penetration elements.
Differences among weight grades were significant (p£0.001) for egg weight, surface area, height
and equator diameter. Instead, shape index values of different weight grade eggs partially overlapped.
A significant (p£0.001) decrease of eggshell percentage was observed as egg weight increased, along
with a reduction of eggshell thickness in XL eggs compared with M and L eggs. Eggshell thickness
was lower at the broad pole than at the narrow pole and the equator.
Penetration breaking strength was significantly higher in grade S eggs (43.4 N), while XL eggs
leaned to the lowest average value (35.5 N). Furthermore, XL eggs showed the highest average
percentage of cracked eggs.
The use of different penetration/compression elements highlighted several significant correlations
between breaking strength and various egg geometrical and structural parameters.
Introduction
In intensive egg production systems egg losses, due to shell breakage during collection and
distribution to the consumer, amount to 6-8% (Hamilton, 1982; Coucke et al., 1999). Even though
several studies (Hamilton, 1982; Thapon, 1994; Coucke et al., 1999) analysed many factors (i.e.
laying house characteristics, feeding, strain and age of hens, composition and structure characteristics
of shell) possibly responsible of losses in egg production, the influence of egg weight on shell quality is
not yet well established.
Aim of this work was the evaluation of geometrical and physical eggshell characteristics as a
function of weight grade (S, M, L and XL) (EU, 2003).
Materials and methods
Twenty seven samples of grade-A shell eggs of different weight grades, laid by cage hens, and
purchased in different retail shops in Northern Italy, were analysed. Each sample was made up of at
least 36 eggs (with a maximum of 42 eggs). The eggs were candled in order to determine the
percentage of cracked eggs and to select intact eggs for the evaluation of eggshell characteristics.
Intact eggs were cleaned with blotting paper, to eliminate filth, and weighed to classify them in the
correct weight grade. Thus 6 samples of S grade, 8 of M grade, 7 of L grade and 6 of XL grade were
analysed within 24 hours from purchase. Fourteen eggs for each sample were submitted first to nondestructive and then to destructive analytical evaluations.
XI th European Symposium on the Quality of Eggs and Egg Products
Doorwerth, The Netherlands, 23-26 May 2005
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Shell characteristics of different weight-grade eggs: E. Casiraghi et al.
Surface area (SA, cm2) of each egg was evaluated using the equation reported by Thompson et al.
(1985): SA = 4.67 (egg weight) 2/3.
Equator diameter and height (cm) of each egg were measured using a manual callipers. Shape
index was calculated as percent ratio between egg diameter measured at equator and egg height,
using the equation reported by Khalafalla e Bessei (1995).
Shell percentage on total egg weight was determined by weighing clean and albumen-free shell,
including cuticle and membrane.
Shell thickness, including cuticle and membrane, was measured at equator, broad and narrow pole,
as suggested by Narushin (2001), using a 550-501 NSK digital micrometer (Japan). Results of
thickness measurements are the average of three determinations at each zone per egg.
Shell index (SI, g/cm2) was calculated using the equation proposed by Rodriguez-Navarro et al.
(2002): SI = shell weight/(equator diameter · egg height).
Shell strength (N) was measured at room temperature, using an Instron Universal Testing Machine
(model 4301, Instron Ltd, High Wycombe, England) supported by a serie IX Automated Material
Testing System software. Compression and penetration tests were carried out each on seven single
per sample, at the constant cross-head speed of 20 mm/min using a load cell of 100 N.
A 35 mm diameter plate was used as a compression device, while a 8 mm diameter probe was
used as penetration element. Strength (N), displacement (mm) and energy (N · mm) at breaking point
were determined by both compression and penetration tests. Instead, slope of force/deformation curve
(N/mm) and Young modulus (N/mm2), i.e. the slope of the stress/deformation curve, indicating shell
strength at small deformations, were only determined by the penetration assay.
One-way analysis of variance (ANOVA) and Least Significant Difference (LSD), considering weight
grade as a factor, were computed by STATGRAPHIC Version 4 software. The LSD test was
performed only for parameters that showed statistical differences by ANOVA. Correlations between
average values of all variables were evaluated according to Pearson, using SYSTAT Version 5.
Results and discussion
Figure 1 shows cracks percentage within each weight grade. Even though the differences are not
significant because of the great variability existing within each grade, XL grade eggs present a greater
mean percentage of cracked eggs in comparison to S, M and L grades.
ANOVA highlighted significant differences for all the variables, as a function of weight grade, with
the exception of displacement at break (for both compression and penetration tests) and slope of the
force-deformation curves. All the differences observed were highly significant (p£0.001) with the
exception of breaking strength and energy, respectively in compression and penetration tests (p £0.01)
and eggshell index (p£0.05 ).
Results of the LSD are shown in Tables 1-3. Table 1 confirms that weight and the other parameters
bound to egg dimension increase with egg-weight grade and only shape index values gradually
decrease from smaller to larger eggs. This is due to the fact that the increase, from S to XL grade, in
height is larger than in diameter; however, a partial overlapping between grades was observed.
As shown in Table 2, shell percentage of weight grades are all significant different: lower values are
found in larger eggs, probably as a consequence of thickness reduction. In fact, excluding S grade
eggs, thickness tends to decrease from M to XL grade eggs. Moreover a significant indirect correlation
between egg weight and shell percentage was observed (Figure 2). Independently from weight,
thickness at broad pole is the smallest. Moog (2001) reports similar results for L and M grade eggs,
while for XL grade eggs the Author found the thinnest shell at equator.
Strength and energy at breaking resulted significantly higher in S eggs than in the other weight
grades, indicating the greater resistance to fracture of S eggs and their lower brittleness (Table 3).
Resistance to breakage tends to decrease from S to XL weight-grades, in accordance with the larger
percentage of broken eggs shown in Figure 1 for XL eggs. Actually, a highly significant indirect
correlation exists between egg weight and breaking strength, evaluated by penetration test, as shown
in Figure 3. An opposite trend is observed when small deformations are applied: Young modulus of L
and XL eggs is significantly higher than S and M eggs. The differences in Young modulus
demonstrate that, at small deformation, larger eggs are stiffer and more brittle, while smaller eggs
appear to be more elastic. The above results are confirmed by the direct correlation found between
egg weight and Young modulus (Table 4).
While breaking strength measured by penetration test is highly correlated with all parameters
related to egg dimension, breaking strength measured by compression test resulted better correlated
to eggshell thickness and index (Table 4). Eggshell percentage is highly correlated with breaking
strength evaluated by both compression and penetration tests.
XI th European Symposium on the Quality of Eggs and Egg Products
Doorwerth, The Netherlands, 23-26 May 2005
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Shell characteristics of different weight-grade eggs: E. Casiraghi et al.
In conclusion eggshell resistance to impacts is indirectly correlated with egg weight and
consequently this affects the resistance to breakage of the different commercial weight-grade eggs.
Since egg size increases along with hen age, the differences observed could be ascribed to
ultrastructural changes of eggshell. In fact as reported by Roberts et al. (1995) hen ageing has an
adverse effect on eggshell structural characteristics.
References
EU (European Union) (2003) Commission Regulation (EC) No 2295/2003, 23 December, 2003,
Council. Official Journal, L 340, 16-34.
ROBERTS J. R., BRACKPOOL C. E., SOLOMON S. E. (1995) The ultrastructure of good and poor
quality eggshells from Australian layer strains. In Proceeding of the VI European Symposium of the
Quality of Egg and Egg Products. (25-29 September) (107-115) Zaragoza (Spain).
COUCKE, P., DEWIL, E., DECUYPERE, E., DE BAERDEMAEKER, J. (1999) Measuring the
mechanical stiffness of an eggshell using resonant frequency analysis. British Poultry Science, 40:
227-232.
HAMILTON, R. M. G. (1982) Methods and factors that affect the measurement of egg shell quality.
Poultry Science, 61: 2022-2039.
KHALAFALLA, M. K., BESSEI, W. (1995) Reliability of quasi–static compression as an indicator of
eggshell quality. In Proceeding of the VI European Symposium of the Quality of Egg and Egg
Products. (25-29 September) (67-71) Zaragoza (Spain).
NARUSHIN, V. G. (2001) What egg parameters predict best its shell strength? In Proceeding of the IX
European Symposium of the Quality of Egg and Egg Products. (9-12 September) (349-355)
Kusadasi (Turkey).
RODRIGUEZ-NAVARRO, A., KALIN, O., NYS, Y., GARCIA-RUIZ, J. M. (2002) Influence of the
microstructure on the shell strength of eggs laid by hens of different ages. British Poultry Science
43: 395-403.
THAPON, J. L. (1994) Structure de l’œuf de poule. In L’œuf et les ovoproduits; Thapon, J. L., C. M.,
Ed.; Technique et Documentation (Lavoisier), Paris, France, pp 2-5.
THOMPSON, B. K., HAMILTON, R. M. G., GRUNDER, A. A. (1985) The relationship between
laboratory measures of egg shell quality and breakage in commercial egg washing and candling
equipment. Poultry Science 64: 901-909.
XI th European Symposium on the Quality of Eggs and Egg Products
Doorwerth, The Netherlands, 23-26 May 2005
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Shell characteristics of different weight-grade eggs: E. Casiraghi et al.
Table 1 Mean values and LSDs of the different-grade eggs for weight, surface area, height,
equator diameter and shape index.
Egg weight
grade
S
M
L
XL
Egg weight
Egg surface
area
(cm2)
(g)
49.1 X
59.8 X
67.4 X
76.0
X
62.5 X
71.4 X
77.4 X
83.8
X
Egg height
Egg equator
diameter
(cm)
(cm)
5.3 X
5.6 X
5.9 X
6.2
X
Egg shape index
(%)
4.1 X
4.3 X
4.5 X
4.7
X
77.4 X
77.1 XX
76.3 XX
75.7
X
Table 2 Mean values and LSDs of the different-grade eggs for eggshell percentage, eggshell
thickness at equator, narrow and broad pole, and eggshell index.
Egg weight
grade
Eggshell
percentage
(%)
S
M
L
XL
11.8 X
11.3 X
10.7 X
10.4
X
Eggshell
thickness
equator
(mm)
0.40 X
0.42 XX
0.42 X
0.41 X
Eggshell
thickness
narrow pole
(mm)
0.41 X
0.42 X
0.42 X
0.41 X
Eggshell
thickness
broad pole
(mm)
0.40 XX
0.41 XX
0.41 X
0.39 X
Eggshell index
(g/cm2)
0.27 X
0.28 X
0.27 X
0.27 X
Table 3 Mean values and LSDs of the different-grade eggs for breaking strength and breaking
energy, evaluated by plate compression and probe penetration, and Young modulus evaluated
using a probe.
Egg weight
grade
S
M
L
XL
Breaking
strength
compression
(N)
43.5 X
41.0 XX
40.3 XX
38.0
X
Breaking
energy
compression
(N mm)
8.0 X
6.8 X
6.8 X
6.1 X
Breaking
strength
penetration
(N)
43.4 X
37.6 X
37.5 X
35.5 X
Breaking
energy
penetration
(N mm)
9.0 X
6.6 X
6.0 X
6.3 X
Young
modulus
penetration
(N/mm2)
110.3 X
118.0 X
131.6 X
132.3 X
Table 4 Coefficients and significance of correlation between parameters, obtained in
compression and penetration tests, and geometrical and structural variables.
Breaking
Breaking
strength
energy
compression
compression
Egg weight
-0.5*
-0.6***
Surface area
-0.5*
-0.6***
Egg height
-0.5**
-0.6***
Equator diameter
-0.4*
-0.6**
Eggshell percentage
0.6***
0.6***
Eggshell thickness
0.5**
ns
Eggshell index
0.5**
ns
*= p£0.05; **= p£0.01;***= p£0.001; ns= not significant
Breaking
strength
penetration
-0.7***
-0.7***
-0.7***
-0.7***
0.7***
ns
ns
XI th European Symposium on the Quality of Eggs and Egg Products
Doorwerth, The Netherlands, 23-26 May 2005
Young
modulus
penetration
0.6***
0.6***
0.6**
0.7***
ns
ns
ns
186
Shell characteristics of different weight-grade eggs: E. Casiraghi et al.
35,0
% cracked eggs
30,0
25,0
20,0
15,0
10,0
5,0
0,0
S
M
L
XL
weight grades
Figure 1 Mean percentage of cracked eggs for different weight grades. Bars represent ± one
standard deviation.
13,0
Shell (%)
12,5
12,0
11,5
11,0
10,5
10,0
y = -0,0571x + 14,658
9,5
9,0
40
50
60
70
Egg weight (g)
80
Figure 2 Correlation between egg weight and shell percentage (p £0.001, r= -0.8)
XI th European Symposium on the Quality of Eggs and Egg Products
Doorwerth, The Netherlands, 23-26 May 2005
187
Shell characteristics of different weight-grade eggs: E. Casiraghi et al.
Breaking strength (N)
48
46
44
42
40
38
36
34
y = -0,2713x + 55,511
32
30
40
50
60
Egg weight (g)
70
80
Figure 3 Correlation between egg weight and breaking strength (penetration);
(p£0.001, r = -0.7)
XI th European Symposium on the Quality of Eggs and Egg Products
Doorwerth, The Netherlands, 23-26 May 2005
188