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The Relationship Between Laboratory Measures of Egg Shell Quality and Breakage in Commercial Egg Washing and Candling Equipment1,2
T h e R e l a t i o n s h i p B e t w e e n L a b o r a t o r y M e a s u r e s of Egg Shell Q u a l i t y and B r e a k a g e in C o m m e r c i a l Egg Washing a n d C a n d l i n g E q u i p m e n t 1 ' 2 B. K. THOMPSON, R.M.G. HAMILTON 3 , and A. A. GRUNDER 3 Engineering and Statistical Research Institute and Animal Research Centre, Agriculture Canada, Ottawa, Ontario, Canada K1A OC6 (Received for publication March 12, 1984)
1985 Poultry Science64:901-909 INTRODUCTION
A variety of methods have been proposed for measuring egg shell quality (Hamilton, 1982). However, as Bowman and Challender (1964) noted, these methods are of value to industry only if they relate closely to shell breakage under commercial conditions. Information concerning such relationships seems to be generally lacking. Bowman and Challender (1964) and Shrimpton and Hann (1967) considered shell thickness and shell deformation, respectively, as they relate to egg breakage from laying house to the packing station. In proposing their method for measuring shell deformation, Schoorl and Boersma (1962) conducted a test for breakage during transportation. Holder and Bradford (1979) looked at the possibility of using specific gravity deter-
1 Contribution numbers: 1-589 Engineering and Statistical Research Institute and 1234 Animal Research Centre. 2 Mention of a trade name, proprietary product, or specific equipment does not imply its official endorsement by Agriculture Canada to the exclusion of other products that may be suitable. 3 Animal Research Centre.
mined by flotation as a method of predicting shell breakage in an egg-sizing machine. Other researchers have restricted their attention to breakage in the laying house. Wells (1967a,b) examined egg specific gravity, shell weight per unit area, and shell deformation as indicators of breakage. More recently, Siegel et al. (1978) and Belyavin and Boorman (1981) considered shell thickness and its relationship to breakage, with the latter also including egg specific gravity and shell weight in their study. However, of all these studies, only Wells (1967a) and Siegel et al. (1978) relate breakage and laboratory measurements of shell quality to individual bird data, the area usually of primary concern in breeding programs. One problem in examining laboratory measurements of shell quality, as they relate to breakage, is that methods such as quasi-static compression fracture strength, and impact fracture destroy the eggs (Hamilton, 1982). Even nondestructive methods may weaken the egg (Voisey and Hamilton, 1975; Carter, 1979; Hamilton and Thompson, 1981), thereby influencing subsequent records of breakage. Moreover, some methods, both destructive and nondestructive, cannot be applied to cracked or broken eggs. Hence, the same set of eggs
901
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ABSTRACT Eggs from 494 hens of two White Leghorn strains were passed through egg-washing and candling equipment three times. At each cycle, the cracked and broken eggs were removed and a sample of the intact eggs obtained. Records were kept of the hen that laid each egg. Egg and shell quality measurements were obtained from additional eggs of the same hens; they were used to rank the birds as to the quality of their eggs by each measure. Breakage of eggs was high for the three cycles of washing and candling (7.8, 13, and 13.3%, respectively). The shell weight, percentage shell, shell thickness, specific gravity (by Archimedes' method), and shell weight per unit surface area were significantly (P<.05) higher for the intact than for the cracked and broken eggs. However, in each case, the range of values almost completely overlapped, indicating that factors other than shell quality influence shell breakage. For each of these variables, as well as quasi-static compression fracture strength and nondestructive deformation, the patterns of breakage in the four quartiles, obtained by ranking the birds, were remarkably similar. The incidence of shell damage over the three cycles was about 45% in the lowest quartile and 15% in the highest, whatever the variable used to define the quartiles. (Key words: Specific gravity, deformation, compression strength, shell weight, shell thickness, eggs, breakage)
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THOMPSON ET AL.
MATERIALS AND METHODS Eggs were collected from 494 Single Comb White Leghorn hens for three 3-day periods, the first beginning when the birds were 535 days of age, the second when they were 592 days of age, and the third when they were 596 days of age. The eggs from the first and third periods were used to obtain laboratory measurements of shell quality, and those in the second were passed through the egg-handling equipment. The flock included almost equal numbers of two strains, one commercial (Shaver Starcross 288) and the other a two-way cross (1 X 8) between Ottawa Strains 1 and 8, as described by Fairfull et al. (1983). The birds were between 60 and 65% egg production during the experiment. Older birds were used to ensure less than optimal shell quality with the hope that breakage would be fairly high in the egg-handling equipment. After the eggs had been properly identified by bird, they were placed in fiber Keyes trays and stored overnight in 15 doz cardboard egg boxes at about 10 C. The eggs of the first and
4
"Egomatic" Model SCF stainless steel spring cushion feed table, Otto Niederer Sons, Incorp. Pennington, NJ. 5 ST Manufacturing Corporation, P. O. Box 97, Benton Harbour, MI. 6 "Egomatic" Model SR-2 with two row spring cushion feed and mercury vapor lamps.
third periods were then moved to the laboratory for egg and shell quality measurements, and those of the second period were moved to the room containing the egg washing and grading equipment. Before any measurements were taken in the laboratory, the eggs were warmed to room temperature. Each egg was weighed to the nearest .1 g, then submerged in tap water at room temperature and reweighed to enable the determination of the egg's specific gravity (SG) by Archimedes' principle. After all eggs had been weighed, they were carefully dried with a paper towel and two measures of shell strength were obtained using the eggshell tester described by Voisey and MacDonald (1978). Each egg was compressed at 20 mm/min, and the nondestructive deformation (DFM) was measured as the applied force increased from .98 to 10.79 Newtons (N). The quasi-static compression fracture strength (CFS) was determined at the time of fracture. After the eggs were broken, the contents of the eggs were discarded and the shells, including membranes, were washed with warm tap water, dried, and weighed, according to the procedures described by Hamilton (1978). Shell thickness was measured on three pieces of shell taken near the point of fracture with a dial gauge comparator (see Voisey and Hunt, 1974). Subsequently, three additional measures of shell quality were derived using these data: percentage shell, shell weight per unit surface area (SW/SA), and specific strength. Shell weight per unit surface area was calculated from the relationship published by Mueller and Scott (1940) in which surface area (SA) is estimated as a function of egg weight (EW), namely, SA (cm 2 ) = 4.67 (EW273). Specific strength is the ratio of CFS to shell thickness. The eggs collected each day in the second period were tested on the following day after being stored overnight at room temperature in 15 doz cardboard boxes. Before the test began each day, all eggs were candled and those eggs with broken or cracked shells were removed. This pretest candling was done by loading one tray of 30 eggs at a time onto the feed table 4 at C in Figure 1, using a vacuum lift 5 and then passing them through the candling unit. 6 Intact eggs were then passed through the washing and candling cycle three times; broken or cracked eggs were removed at the end of each cycle. Following the final cycle, the remaining eggs were hand candled for more subtle cracks. At
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generally cannot be used to establish breakage records and laboratory measurements. However, Thompson et al. (1983) have shown that one set of eggs from a bird provides a fairly accurate estimate of shell quality for subsequent eggs from that bird. The purpose of the present study was to determine how well records of shell strength, based on compression fracture strength and the nondestructive measures noted, reflected breakage of subsequent eggs laid by the same birds. Among others, Berry (1976) has observed that egg breakage in the egg grading process is high. Because little attention has been given to this area of egg handling, it was decided to restrict the scope of the study to examining breakage in the stages leading up to grading, that is, the initial loading, washing, and candling of the eggs.
EGG SHELL QUALITY AND BREAKAGE
Analyses of variance were applied to the laboratory measurements from both the intact and the cracked and broken eggs collected after each of the three test cycles of Period 2. However, the eggs of Periods 1 and 3 were used only to establish average values for each bird. In an attempt to take into account day-to-day variation (Hamilton and Thompson, 1984) and aging effects (Doyon et ah, 1985), means were adjusted both for day and period differences. For each of the measurements, the birds were allocated to quartiles, that is, the hens whose mean values ranked in the lowest 25% were included in the first quartile, the next 25% in the second quartile, and so on. Calculation of total breakage over the three test cycles was complicated by the removal of
7 Kuhl, Model HEW15, Kuhl Corporation, Flemington, NJ.
the samples of intact eggs at the end of the first and second cycles. If these eggs are ignored, one underestimates the number of eggs that would have survived the three test cycles. However, if they are included as intact over the three cycles, one ignores those that would have broken in subsequent cycles. As the number of such eggs was likely to be small relative to those remaining intact, it was decided to ignore the potential breakage and include all of these eggs as intact. RESULTS AND DISCUSSION Table 1 indicates the incidence of shell damage found at each of the five times eggs were candled during the test. Originally, it was decided to pass the eggs through the washing equipment three times to ensure that sufficient numbers of damaged eggs were obtained in case incidence was low. Although the incidence was higher than had been expected, it was still well within the range given by Berry (1976) for washing and candling in a commercial operation. The two strains used in the present study showed similar incidence patterns; the Shaver strain consistently had a slightly lower incidence of damage than the Ottawa 1 x 8 strain. The most unexpected result shown in Table 1 is the high level of cracked and broken eggs found after the second and third wash cycles. One might expect that most of the weaker eggs would crack or break during the first cycle, leading to less damage in the later cycles. Instead, the incidence of damage was higher, suggesting that some of the stronger eggs were breaking. Perhaps this was because of repeated insults to the shell during transfer to the test room from the laying house and earlier movement through the feed tables, washer, and candling units. Carter (1979) has pointed out that external insult to a shell may weaken it through cumulative and irreparable, although invisible, damage. The types of damage to the eggs ranged from very subtle cracks to total destruction of the shell. The subtle cracks accounted for most of the damage found when the eggs were hand candled after the third wash cycle. In many cases, these cracks could be seen only when pressure was applied at the poles of the egg. Such cracks are likely to escape detection during commercial candling but may account for some of the breakage in the later stages of the commercial process. A large proportion of
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each stage, including the prewash and postwash candling, a sample of intact eggs, numbering about the same as the cracked and broken eggs, was removed randomly. The cracked and broken eggs and the samples of intact eggs were moved to the laboratory where the various egg and shell quality measurements described above were obtained from the intact eggs and where possible, from the cracked and broken eggs. The egg washing and candling equipment is shown schematically in Figure 1. The eggs were transferred, one egg tray at a time, to the feed table (A) using a vacuum lift. The pressure for the prewash and the first wash cycle on the first day was 360 mm mercury, but this was judged excessive, and consequently, it was reduced to 80 mm mercury for the second and third cycles to minimize the effect, if any, on shell breakage. To maintain consistency, the same levels were used on the second and third days. Once the eggs had passed through the washer 7 (B) and candling equipment (D), they were collected, one at a time, by hand and placed back in the fiber egg trays at E. The broken and cracked eggs were removed at D whereas the sample of intact eggs was obtained at G before the next cycle began. The grading equipment (F) was not used in the test. The temperature of the wash water was about 33 C the first day and 45 C the second and third days. Wash water contained no detergent.
903
904
THOMPSON ET AL.
TABLE 1. The breakage of eggs from Period 2 during the test with the egg washing and candling equipment 1st wash
2nd wash
3rd wash
Postwash
Shaver No. of eggs Eggs damaged % Damaged
495 5 1.0
489 38 7.8
414 51 12.3
309 39 12.6
242 24 9.9
Ottawa 1 X 8 No. of eggs Eggs damaged % Damaged
425 5 1.2
413 34 8.2
347 49 14.1
257 36 14.0
175 20 11.4
Overall No. of eggs Eggs damaged % Damaged
920 10 1.1
902 72 7.8
761 100 13.0
566 75 13.3
417 44 10.6
the damage to the eggs in the test probably occurred at the two feed tables (A and C in Figure 1) when the flow of eggs was funnelled into two columns for passage through the washer and candling equipment. Eggs were observed to pile on top of each other at A, even though the eggs were loaded from at most two 30-egg trays at a time. Completely smashed eggs were found at the funnel of feed table A and in the egg washer. It is likely that some adjustments to the equipment could lower the incidence of damage to some degree; however, the equipment was used as installed by the vendor. The means of egg measurements of the two strains and of intact and damaged eggs that passed through the wash cycles are given in Table 2A. These means are pooled over the other factors (cycle and date) because there were only two significant interactions (P<.05), about the number to be expected by chance. Table 3 presents the corresponding ranges for intact and damaged eggs. The analyses of variance indicated all comparisons between intact eggs and damaged eggs to be significant (P<.01), except that of egg weight (P>.05). These results agree with trends observed by Siegel et al. (1978) and Belyavin and Boorman (1981), although the latter reported one strain where the intact eggs were significantly (P<.05) smaller than the damaged eggs. However, it can be seen that the range of the measurements for intact and damaged eggs overlapped (Table 3). In fact, the eggs with the highest shell weight, SG and SW/SA, that is, the supposedly "strongest" eggs by these measures, cracked during the test. Both Siegel et al. (1978) and Belyavin and
Boorman (1981) also reported an overlap in the ranges of the shell quality measurements. Means of shell thickness at each cycle are presented in Table 2B to show the changes in the differences between the cracked and broken eggs and the intact eggs. The changes are typical
DIAGRAM OF EGG GRADING SYSTEM
\
— c
FIG. 1. The layout of the equipment used in this study: A) feed table, B) egg washer, C) feed table, D) candling unit, E) eggs hand-collected, F) egg grader (not used in the experiment), and G) table.
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Prewash
EGG SHELL QUALITY AND BREAKAGE
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TABLE 2. Means of egg and shell quality measurements during the test with the egg washing and candling equipment Strain
Condition oi•egg Intact
Damaged
1X8
Shaver
SEM1
63.6 5.16 342 1.0771 8.12 69.3 36.8 70.1 103.3
64.8 5.39 350 1.0790 8.35 71.8 34.2 67.4 93.2
.29 .040 2.0 .00044 .053 .46 .77 1.66 1.84
A) Overall eggs submitted to at least one wash cycl e 64.4 5.52 359 1.0804 8.57 73.5 35.5 68.6 98.7
64.0 5.04 335 1.0756 7.90 67.6
_ ... —
B) Shell thickness (jum) for eggs removed at each cycle Cycle
Intact eggs No. Mean Damaged eggs No. Mean
1st wash
2nd wash
3rd wash
Postwash
69 352
94 361
74 361
42 360
72 326
100 329
76 342
44 347
1
Standard error of the mean, based on minimum number of observations contributing to a mean in the row. Shell weight per unit surface area. 3 Quasi-static compression fracture strength. 2
of those observed for the other variables considered in this study. Although the interactions between cycle and condition were not significant for all variables except SG (P<.05), there was a tendency for the differences between intact and damaged eggs to become smaller with additional cycles. For shell thickness, the difference dropped from 26 /Um after the first cycle to 19 /im after the third. The reduction was due mostly to an increase in the thickness of the damaged eggs, perhaps an indication that most of the weaker, thin-shelled eggs had been damaged in earlier cycles and, hence, were removed from subsequent cycles. The fact that the postwash difference was even smaller (13 fun) probably reflects a close similarity between intact eggs and those with subtle cracks. Differences between strains were found to be significant (P<.05) for all measurements except CFS. The shell quality measurements generally indicated the eggs from the Shaver hens had stronger shells than those from the
Ottawa 1 X 8 strain, a fact supported by the breakage data in Table 1 where the Shaver strain had fewer damaged eggs. It is interesting to note, however, that the CFS of the eggs from the Shaver hens was lower, although not significantly so, than that of the Ottawa hens; the means of the eggs in the first and third periods also showed the same trend. Hence, across strains, CFS was negatively correlated with measurements such as shell weight with which it is usually assumed to be positively correlated (Hamilton, 1982). Thompson et al. (1981) reported a similar result, finding that deformation and shell thickness were positively correlated across five avian species, even though the two were negatively correlated within each species. In the egg industry, it is likely that the shell measurements on a set of eggs from a bird, especially those that are destructive, are of more value as an indication (estimate) of the quality of the bird's eggs in general than for
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Egg weight, g Shell weight, g Shell thickness, mm Specific gravity % Shell SW/SA 2 , mg/cm 2 CFS 3 , N Deformation, Mm Specific strength, N/mm
THOMPSON ET AL.
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The percentage damage of eggs from the four quartiles based on the means of egg weight was similar. However, for egg weight unlike the other variables, the pattern of percent damaged seemed to change with successive cycles. The results in Table 4B show that in the initial cycle damage was similar in all quartiles. However, by the third cycle, more eggs were damaged in the Quartiles 1 and 2, which represented the smaller average egg weight. This trend is contrary to the theory of Carter (1978) who argued that the larger eggs should be more likely to break. However, he was studying the effects of the collision of two eggs. Perhaps the congestion of eggs, especially on the two feed tables (A and C of Figure 1), caused more
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these eggs specifically. The birds in the present study were ranked according to the means of the various measurements in Periods 1 and 3. The percentage of eggs from birds in each quartile, damaged during the three cycles, are shown in Table 4A. It was decided to omit from these results the eggs found damaged when they were hand candled postwash, because it was not clear if and when the damage had occurred during the three cycles. Nevertheless, the number of eggs found damaged at this step (Table 2B) was relatively small and would not alter the results of Table 4A appreciably. There is a remarkable similarity in the results of Table 4A among the various shell quality measurements. With the exceptions of deformation and specific strength, the poorest 25% of the birds, based on the means of these measurements, accounted for about 45% of the damage, whereas the best 25% accounted for only about 15%. The results for deformation appeared slightly poorer in the first quartile but the best results were in the fourth quartile with only 12% of the damaged eggs. As there was little difference between SG, CFS, and deformation, SG is probably the preferable measure simply because it is easiest to apply, especially if the flotation method is used. Although the results were based on SG by Archimedes' method, Thompson and Hamilton (1982) have shown that results by the flotation method are similar to those obtained by Archimedes' method. Holder and Bradford (1979) found the percentage of cracked eggs to be closely related to SG based on the flotation method. Wells (1967a) recommended SG (by Archimedes' method) after comparing it with SW/SA and two destructive measures.
EGG SHELL QUALITY AND BREAKAGE
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TABLE 4. Percentage egg shell damage of eggs in the second period for birds classified in quartiles according to their average egg and shell characteristics in the first and third periods Quartile 1 Characteristic A) Over the three test cycles 32.4 36.9 45.2 45.5 44.2 47.8 44.6 45.8 38.3
28.1 38.2 27.1 25.8 30.0 27.0 32.7 30.0 28.8
23.1 22.9 21.1 21.6 20.6 22.3 21.8 22.4 22.4
25.5 12.2 17.4 18.6 16.4 14.8 13.8 14.8 20.5
7.1 13.0 16.5
5.9 12.3 10.4
12.9 9.6
B) At each cycle, based on egg weight means Cycle 1 2 3
9.8 14.6 16.1
1 For all but deformation, the first quartile represents the lowest means; for deformation, quartiles have been reversed. 2 Compression fracture strength. 3 Shell weight per unit surface area.
insults to the lighter eggs, leading to a weakening of these eggs and an increase in their subsequent breakage (Carter, 1979). It was noted,
for example, that the eggs tended to pile on top of each other at A, and it seems reasonable to suppose it was the smaller ones that were
TABLE 5. Percentage shell damage for birds ranked in the same quartile, by each pair of three shell quality measurements, and the counts of birds by quartile for each ranking A) Damage over the three test cycles
Quartiles
Measurements
1
2
3
4
CFS 1 and specific gravity CFS and shell thickness Specific gravity and shell thickness
48.9 46.6 47.6
29.8 31.7 29.4
21.9 22.0 20.6
13.9 15.8 14.5
B) Counts of birds in the quartiles for each pair of measurements CFS 1 quartiles
Specific gravity quartiles
1
2
3
4
1
2
3
4
88 16 7 0
24 62 32 3
2 32 47 32
2 4 30 80
Shell thickness
1 2 3 4
77 25 10 3
20 52 34 9
12 31 39 33
1 13 30 71
Specific gravity
1 2 3 4
81 26 6 2
20 53 28 14
11 28 46 30
1 9 36 69
Compression fracture strength.
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Egg weight Deformation CFS 2 Shell weight Shell thickness SW/SA3 Specific gravity % Shell Specific strength
THOMPSON ET AL.
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T h e high frequency of damaged eggs, a m o n g t h e b e t t e r birds, m a y be explained in t w o ways. First, as Wells ( 1 9 6 7 a ) n o t e d , " w i t h a c o m p l e x p h e n o m e n o n like shell strength, a perfect m e a s u r e m e n t would appear t o be a physical impossibility." F o r example, Carter ( 1 9 7 8 ) found t h a t in 9 0 i m p a c t s of pairs of eggs, t h e thicker shelled egg cracked in 13 cases. Second, it is possible t h a t t h e e q u i p m e n t used in this study subjected some eggs t o stress t h a t n o egg could survive. In such cases, n o shell strength measure would predict p o t e n t i a l breakage accurately, because c h a n c e determines which eggs suffer damaging insults. Whatever t h e explanation, it is clear from t h e results of this s t u d y t h a t t h e shell strength m e a s u r e m e n t s c o m m o n l y used will fail t o predict a substantial p r o p o r t i o n of breakage in t h e commercial process of egg handling, and in particular, during loading, washing, and handling. ACKNOWLEDGMENTS The a u t h o r s t h a n k Dina D ' E r m o and J. W. Dickie for their c o o p e r a t i o n and technical assistance. REFERENCES Belyavin, C. G., and K. N. Boorman, 1981. Physical characteristics of intact and cracked eggs. Br.
Poult. Sci. 2 2 : 9 - 1 5 . Berry, J. G., 1976. Extending egg shell damage survey results into the field. Poultry Sci. 5 5 : 7 5 8 - 7 6 1 . Bowman, J. C , and N. I. Challender, 1964. Egg shell strength. A comparison of two laboratory tests and field results. Br. Poult. Sci. 4 : 1 0 3 - 1 1 6 . Carter, T. C , 1978. The hen's egg: Fracture of shells loaded very slowly. Br. Poult. Sci. 19:669-679. Carter, T. C , 1979. The hen's egg: Evidence of the mechanism relating shell strength to loading rate. Br. Poult. Sci. 20:175-183. Doyon, G., R.M.G. Hamilton, M. B. Cardou, F. Castaigne, and H. MacLean, 1985. Shell and interior egg quality. I. Changes in shell strength of eggs from five commercial strains of White Leghorn hens during their first laying cycle. Poultry Sci. 64: (in press). Fairfull, R. W., R. S. Gowe, and J.A.B. Emsley, 1983. Diallel cross of six long-term selected Leghorn strains with emphasis on heterosis and reciprocal effects. Br. Poult. Sci. 24:133-158. Hamilton, R.M.G., 1978. Observations on the changes in physical characteristics that influence egg shell quality in ten strains of White Leghorns. Poultry Sci. 57:1192-1197. Hamilton, R.M.G., 1982. Methods and factors that affect the measurement of egg shell quality. Poultry Sci. 61:2022-2039. Hamilton, R.M.G., and B. K. Thompson, 1981. Effects of the sequence of measuring nondestructive deformation and specific gravity on the quasistatic compression and impact strength of eggs from White Leghorn hens. Poultry Sci. 60: 1798-1801. Hamilton, R.M.G., and B. K. Thompson, 1984. Observations on daily variation in feed intake and shell strength of eggs from White Leghorn pullets and force-molted hens. Poultry Sci. 6 3 : 2335-2344. Holder, D. P., and M. V. Bradford, 1979. Relationship of specific gravity of chicken eggs to number of cracked eggs observed and percent shell. Poultry Sci. 58:250-251. Mueller, C. D., and H. M. Scott, 1940. The porosity of the egg-shell in relation to hatchability. Poultry Sci. 19:163-166. Schoorl, P., and H. Y. Boersma, 1962. Research on the quality of the egg shell. (A new method of determination.) Pages 432—435 in Proc. 12th World's Poult. Congr. Shrimpton, D. H., and C. M. Hann, 1967. Shell deformation in predicting breakage due to transport and handling. Br. Poult. Sci. 8:317-320. Siegel, P. B., J. H. Van Middelkoop, and P.R.K. Reddy, 1978. Comparisons of frequencies and egg shell characteristics of broken and intact eggs within diverse populations of chickens. Br. Poult. Sci. 19:411-476. Thompson, B. K., A. A. Grunder, R.M.G. Hamilton, and K. G. Hollands, 1983. Repeatability of egg shell quality measurements within individual hens. Poultry Sci. 62:2309-2314. Thompson, B. K., and R.M.G. Hamilton, 1982. Comparison of the precision and accuracy of the flotation and Archimedes' methods for measuring specific gravity of eggs. Poultry Sci. 6 1 : 1599-1605.
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forced t o t h e t o p , w h e r e t h e y could strike parts of t h e e q u i p m e n t before being d r o p p e d t o the belt. A l t h o u g h t h e results of Table 4A show t h a t shell quality m e a s u r e m e n t s on a set of eggs from a bird do reflect p o t e n t i a l breakage of eggs from t h a t bird, it was disconcerting to find such a high incidence of shell damage a m o n g birds in t h e f o u r t h quartile, t h a t is, a m o n g birds ranked highly according t o their m e a n shell quality m e a s u r e m e n t s . It seemed possible t h a t t h e percentages could be reduced b y ranking according t o pairs of quality m e a s u r e m e n t s jointly. Therefore, t h e calculations were repeated including only hens whose m e a n shell quality was allocated t o t h e same quartiles b y pair-wise c o m b i n a t i o n s of t h r e e m e a s u r e m e n t s . T h e results are given in Table 5. It can be seen (Table 5B) t h a t t h e majority of birds were allocated t o t h e same quartiles b y t h e t h r e e variables considered, although there were a n u m b e r of cases w h e r e birds were allocated t o t h e first quartile b y o n e m e a s u r e a n d t h e f o u r t h b y a n o t h e r . In spite of t h e deletion of these debatable allocations, t h e percentages of damaged eggs in each quartile changed very little (Table 4 A vs. Table 5A).
EGG SHELL QUALITY AND BREAKAGE Thompson, B. K., R.M.G. Hamilton, and P. W. Voisey, 1981: Relationships among various egg traits relating to shell strength among and within five avian species. Poultry Sci. 60:2388-2394. Voisey, P. W., and R.M.G. Hamilton, 1975. Behaviour of egg shell under compression in relation to deformation measurements. Br. Poult. Sci. 16:461-470. Voisey, P. W., and J. R. Hunt, 1974. Measurement of eggshell strength. J. Texture Stud. 5:135-182. Voisey, P. W., and D. C. MacDonald, 1978. Laboratory measurements of egg shell strength. 1. An
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instrument for measuring shell strength by quasi-static compression, puncture, and nondestructive deformation. Poultry Sci. 57: 860-869. Wells, R. G., 1967a. Egg shell strength. 1. The relationship between egg breakage in the field and certain laboratory assessments of shell strength. Br. Poult. Sci. 8:131-139. Wells, R. G., 1967b. Egg shell strength. 2. The relationship between egg specific gravity and egg shell deformation and their reliability as indicators of shell strength. Br. Poult. Sci. 8:193-199.
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1985 Poultry Science64:901-909 INTRODUCTION
A variety of methods have been proposed for measuring egg shell quality (Hamilton, 1982). However, as Bowman and Challender (1964) noted, these methods are of value to industry only if they relate closely to shell breakage under commercial conditions. Information concerning such relationships seems to be generally lacking. Bowman and Challender (1964) and Shrimpton and Hann (1967) considered shell thickness and shell deformation, respectively, as they relate to egg breakage from laying house to the packing station. In proposing their method for measuring shell deformation, Schoorl and Boersma (1962) conducted a test for breakage during transportation. Holder and Bradford (1979) looked at the possibility of using specific gravity deter-
1 Contribution numbers: 1-589 Engineering and Statistical Research Institute and 1234 Animal Research Centre. 2 Mention of a trade name, proprietary product, or specific equipment does not imply its official endorsement by Agriculture Canada to the exclusion of other products that may be suitable. 3 Animal Research Centre.
mined by flotation as a method of predicting shell breakage in an egg-sizing machine. Other researchers have restricted their attention to breakage in the laying house. Wells (1967a,b) examined egg specific gravity, shell weight per unit area, and shell deformation as indicators of breakage. More recently, Siegel et al. (1978) and Belyavin and Boorman (1981) considered shell thickness and its relationship to breakage, with the latter also including egg specific gravity and shell weight in their study. However, of all these studies, only Wells (1967a) and Siegel et al. (1978) relate breakage and laboratory measurements of shell quality to individual bird data, the area usually of primary concern in breeding programs. One problem in examining laboratory measurements of shell quality, as they relate to breakage, is that methods such as quasi-static compression fracture strength, and impact fracture destroy the eggs (Hamilton, 1982). Even nondestructive methods may weaken the egg (Voisey and Hamilton, 1975; Carter, 1979; Hamilton and Thompson, 1981), thereby influencing subsequent records of breakage. Moreover, some methods, both destructive and nondestructive, cannot be applied to cracked or broken eggs. Hence, the same set of eggs
901
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ABSTRACT Eggs from 494 hens of two White Leghorn strains were passed through egg-washing and candling equipment three times. At each cycle, the cracked and broken eggs were removed and a sample of the intact eggs obtained. Records were kept of the hen that laid each egg. Egg and shell quality measurements were obtained from additional eggs of the same hens; they were used to rank the birds as to the quality of their eggs by each measure. Breakage of eggs was high for the three cycles of washing and candling (7.8, 13, and 13.3%, respectively). The shell weight, percentage shell, shell thickness, specific gravity (by Archimedes' method), and shell weight per unit surface area were significantly (P<.05) higher for the intact than for the cracked and broken eggs. However, in each case, the range of values almost completely overlapped, indicating that factors other than shell quality influence shell breakage. For each of these variables, as well as quasi-static compression fracture strength and nondestructive deformation, the patterns of breakage in the four quartiles, obtained by ranking the birds, were remarkably similar. The incidence of shell damage over the three cycles was about 45% in the lowest quartile and 15% in the highest, whatever the variable used to define the quartiles. (Key words: Specific gravity, deformation, compression strength, shell weight, shell thickness, eggs, breakage)
902
THOMPSON ET AL.
MATERIALS AND METHODS Eggs were collected from 494 Single Comb White Leghorn hens for three 3-day periods, the first beginning when the birds were 535 days of age, the second when they were 592 days of age, and the third when they were 596 days of age. The eggs from the first and third periods were used to obtain laboratory measurements of shell quality, and those in the second were passed through the egg-handling equipment. The flock included almost equal numbers of two strains, one commercial (Shaver Starcross 288) and the other a two-way cross (1 X 8) between Ottawa Strains 1 and 8, as described by Fairfull et al. (1983). The birds were between 60 and 65% egg production during the experiment. Older birds were used to ensure less than optimal shell quality with the hope that breakage would be fairly high in the egg-handling equipment. After the eggs had been properly identified by bird, they were placed in fiber Keyes trays and stored overnight in 15 doz cardboard egg boxes at about 10 C. The eggs of the first and
4
"Egomatic" Model SCF stainless steel spring cushion feed table, Otto Niederer Sons, Incorp. Pennington, NJ. 5 ST Manufacturing Corporation, P. O. Box 97, Benton Harbour, MI. 6 "Egomatic" Model SR-2 with two row spring cushion feed and mercury vapor lamps.
third periods were then moved to the laboratory for egg and shell quality measurements, and those of the second period were moved to the room containing the egg washing and grading equipment. Before any measurements were taken in the laboratory, the eggs were warmed to room temperature. Each egg was weighed to the nearest .1 g, then submerged in tap water at room temperature and reweighed to enable the determination of the egg's specific gravity (SG) by Archimedes' principle. After all eggs had been weighed, they were carefully dried with a paper towel and two measures of shell strength were obtained using the eggshell tester described by Voisey and MacDonald (1978). Each egg was compressed at 20 mm/min, and the nondestructive deformation (DFM) was measured as the applied force increased from .98 to 10.79 Newtons (N). The quasi-static compression fracture strength (CFS) was determined at the time of fracture. After the eggs were broken, the contents of the eggs were discarded and the shells, including membranes, were washed with warm tap water, dried, and weighed, according to the procedures described by Hamilton (1978). Shell thickness was measured on three pieces of shell taken near the point of fracture with a dial gauge comparator (see Voisey and Hunt, 1974). Subsequently, three additional measures of shell quality were derived using these data: percentage shell, shell weight per unit surface area (SW/SA), and specific strength. Shell weight per unit surface area was calculated from the relationship published by Mueller and Scott (1940) in which surface area (SA) is estimated as a function of egg weight (EW), namely, SA (cm 2 ) = 4.67 (EW273). Specific strength is the ratio of CFS to shell thickness. The eggs collected each day in the second period were tested on the following day after being stored overnight at room temperature in 15 doz cardboard boxes. Before the test began each day, all eggs were candled and those eggs with broken or cracked shells were removed. This pretest candling was done by loading one tray of 30 eggs at a time onto the feed table 4 at C in Figure 1, using a vacuum lift 5 and then passing them through the candling unit. 6 Intact eggs were then passed through the washing and candling cycle three times; broken or cracked eggs were removed at the end of each cycle. Following the final cycle, the remaining eggs were hand candled for more subtle cracks. At
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generally cannot be used to establish breakage records and laboratory measurements. However, Thompson et al. (1983) have shown that one set of eggs from a bird provides a fairly accurate estimate of shell quality for subsequent eggs from that bird. The purpose of the present study was to determine how well records of shell strength, based on compression fracture strength and the nondestructive measures noted, reflected breakage of subsequent eggs laid by the same birds. Among others, Berry (1976) has observed that egg breakage in the egg grading process is high. Because little attention has been given to this area of egg handling, it was decided to restrict the scope of the study to examining breakage in the stages leading up to grading, that is, the initial loading, washing, and candling of the eggs.
EGG SHELL QUALITY AND BREAKAGE
Analyses of variance were applied to the laboratory measurements from both the intact and the cracked and broken eggs collected after each of the three test cycles of Period 2. However, the eggs of Periods 1 and 3 were used only to establish average values for each bird. In an attempt to take into account day-to-day variation (Hamilton and Thompson, 1984) and aging effects (Doyon et ah, 1985), means were adjusted both for day and period differences. For each of the measurements, the birds were allocated to quartiles, that is, the hens whose mean values ranked in the lowest 25% were included in the first quartile, the next 25% in the second quartile, and so on. Calculation of total breakage over the three test cycles was complicated by the removal of
7 Kuhl, Model HEW15, Kuhl Corporation, Flemington, NJ.
the samples of intact eggs at the end of the first and second cycles. If these eggs are ignored, one underestimates the number of eggs that would have survived the three test cycles. However, if they are included as intact over the three cycles, one ignores those that would have broken in subsequent cycles. As the number of such eggs was likely to be small relative to those remaining intact, it was decided to ignore the potential breakage and include all of these eggs as intact. RESULTS AND DISCUSSION Table 1 indicates the incidence of shell damage found at each of the five times eggs were candled during the test. Originally, it was decided to pass the eggs through the washing equipment three times to ensure that sufficient numbers of damaged eggs were obtained in case incidence was low. Although the incidence was higher than had been expected, it was still well within the range given by Berry (1976) for washing and candling in a commercial operation. The two strains used in the present study showed similar incidence patterns; the Shaver strain consistently had a slightly lower incidence of damage than the Ottawa 1 x 8 strain. The most unexpected result shown in Table 1 is the high level of cracked and broken eggs found after the second and third wash cycles. One might expect that most of the weaker eggs would crack or break during the first cycle, leading to less damage in the later cycles. Instead, the incidence of damage was higher, suggesting that some of the stronger eggs were breaking. Perhaps this was because of repeated insults to the shell during transfer to the test room from the laying house and earlier movement through the feed tables, washer, and candling units. Carter (1979) has pointed out that external insult to a shell may weaken it through cumulative and irreparable, although invisible, damage. The types of damage to the eggs ranged from very subtle cracks to total destruction of the shell. The subtle cracks accounted for most of the damage found when the eggs were hand candled after the third wash cycle. In many cases, these cracks could be seen only when pressure was applied at the poles of the egg. Such cracks are likely to escape detection during commercial candling but may account for some of the breakage in the later stages of the commercial process. A large proportion of
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each stage, including the prewash and postwash candling, a sample of intact eggs, numbering about the same as the cracked and broken eggs, was removed randomly. The cracked and broken eggs and the samples of intact eggs were moved to the laboratory where the various egg and shell quality measurements described above were obtained from the intact eggs and where possible, from the cracked and broken eggs. The egg washing and candling equipment is shown schematically in Figure 1. The eggs were transferred, one egg tray at a time, to the feed table (A) using a vacuum lift. The pressure for the prewash and the first wash cycle on the first day was 360 mm mercury, but this was judged excessive, and consequently, it was reduced to 80 mm mercury for the second and third cycles to minimize the effect, if any, on shell breakage. To maintain consistency, the same levels were used on the second and third days. Once the eggs had passed through the washer 7 (B) and candling equipment (D), they were collected, one at a time, by hand and placed back in the fiber egg trays at E. The broken and cracked eggs were removed at D whereas the sample of intact eggs was obtained at G before the next cycle began. The grading equipment (F) was not used in the test. The temperature of the wash water was about 33 C the first day and 45 C the second and third days. Wash water contained no detergent.
903
904
THOMPSON ET AL.
TABLE 1. The breakage of eggs from Period 2 during the test with the egg washing and candling equipment 1st wash
2nd wash
3rd wash
Postwash
Shaver No. of eggs Eggs damaged % Damaged
495 5 1.0
489 38 7.8
414 51 12.3
309 39 12.6
242 24 9.9
Ottawa 1 X 8 No. of eggs Eggs damaged % Damaged
425 5 1.2
413 34 8.2
347 49 14.1
257 36 14.0
175 20 11.4
Overall No. of eggs Eggs damaged % Damaged
920 10 1.1
902 72 7.8
761 100 13.0
566 75 13.3
417 44 10.6
the damage to the eggs in the test probably occurred at the two feed tables (A and C in Figure 1) when the flow of eggs was funnelled into two columns for passage through the washer and candling equipment. Eggs were observed to pile on top of each other at A, even though the eggs were loaded from at most two 30-egg trays at a time. Completely smashed eggs were found at the funnel of feed table A and in the egg washer. It is likely that some adjustments to the equipment could lower the incidence of damage to some degree; however, the equipment was used as installed by the vendor. The means of egg measurements of the two strains and of intact and damaged eggs that passed through the wash cycles are given in Table 2A. These means are pooled over the other factors (cycle and date) because there were only two significant interactions (P<.05), about the number to be expected by chance. Table 3 presents the corresponding ranges for intact and damaged eggs. The analyses of variance indicated all comparisons between intact eggs and damaged eggs to be significant (P<.01), except that of egg weight (P>.05). These results agree with trends observed by Siegel et al. (1978) and Belyavin and Boorman (1981), although the latter reported one strain where the intact eggs were significantly (P<.05) smaller than the damaged eggs. However, it can be seen that the range of the measurements for intact and damaged eggs overlapped (Table 3). In fact, the eggs with the highest shell weight, SG and SW/SA, that is, the supposedly "strongest" eggs by these measures, cracked during the test. Both Siegel et al. (1978) and Belyavin and
Boorman (1981) also reported an overlap in the ranges of the shell quality measurements. Means of shell thickness at each cycle are presented in Table 2B to show the changes in the differences between the cracked and broken eggs and the intact eggs. The changes are typical
DIAGRAM OF EGG GRADING SYSTEM
\
— c
FIG. 1. The layout of the equipment used in this study: A) feed table, B) egg washer, C) feed table, D) candling unit, E) eggs hand-collected, F) egg grader (not used in the experiment), and G) table.
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Prewash
EGG SHELL QUALITY AND BREAKAGE
905
TABLE 2. Means of egg and shell quality measurements during the test with the egg washing and candling equipment Strain
Condition oi•egg Intact
Damaged
1X8
Shaver
SEM1
63.6 5.16 342 1.0771 8.12 69.3 36.8 70.1 103.3
64.8 5.39 350 1.0790 8.35 71.8 34.2 67.4 93.2
.29 .040 2.0 .00044 .053 .46 .77 1.66 1.84
A) Overall eggs submitted to at least one wash cycl e 64.4 5.52 359 1.0804 8.57 73.5 35.5 68.6 98.7
64.0 5.04 335 1.0756 7.90 67.6
_ ... —
B) Shell thickness (jum) for eggs removed at each cycle Cycle
Intact eggs No. Mean Damaged eggs No. Mean
1st wash
2nd wash
3rd wash
Postwash
69 352
94 361
74 361
42 360
72 326
100 329
76 342
44 347
1
Standard error of the mean, based on minimum number of observations contributing to a mean in the row. Shell weight per unit surface area. 3 Quasi-static compression fracture strength. 2
of those observed for the other variables considered in this study. Although the interactions between cycle and condition were not significant for all variables except SG (P<.05), there was a tendency for the differences between intact and damaged eggs to become smaller with additional cycles. For shell thickness, the difference dropped from 26 /Um after the first cycle to 19 /im after the third. The reduction was due mostly to an increase in the thickness of the damaged eggs, perhaps an indication that most of the weaker, thin-shelled eggs had been damaged in earlier cycles and, hence, were removed from subsequent cycles. The fact that the postwash difference was even smaller (13 fun) probably reflects a close similarity between intact eggs and those with subtle cracks. Differences between strains were found to be significant (P<.05) for all measurements except CFS. The shell quality measurements generally indicated the eggs from the Shaver hens had stronger shells than those from the
Ottawa 1 X 8 strain, a fact supported by the breakage data in Table 1 where the Shaver strain had fewer damaged eggs. It is interesting to note, however, that the CFS of the eggs from the Shaver hens was lower, although not significantly so, than that of the Ottawa hens; the means of the eggs in the first and third periods also showed the same trend. Hence, across strains, CFS was negatively correlated with measurements such as shell weight with which it is usually assumed to be positively correlated (Hamilton, 1982). Thompson et al. (1981) reported a similar result, finding that deformation and shell thickness were positively correlated across five avian species, even though the two were negatively correlated within each species. In the egg industry, it is likely that the shell measurements on a set of eggs from a bird, especially those that are destructive, are of more value as an indication (estimate) of the quality of the bird's eggs in general than for
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Egg weight, g Shell weight, g Shell thickness, mm Specific gravity % Shell SW/SA 2 , mg/cm 2 CFS 3 , N Deformation, Mm Specific strength, N/mm
THOMPSON ET AL.
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The percentage damage of eggs from the four quartiles based on the means of egg weight was similar. However, for egg weight unlike the other variables, the pattern of percent damaged seemed to change with successive cycles. The results in Table 4B show that in the initial cycle damage was similar in all quartiles. However, by the third cycle, more eggs were damaged in the Quartiles 1 and 2, which represented the smaller average egg weight. This trend is contrary to the theory of Carter (1978) who argued that the larger eggs should be more likely to break. However, he was studying the effects of the collision of two eggs. Perhaps the congestion of eggs, especially on the two feed tables (A and C of Figure 1), caused more
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these eggs specifically. The birds in the present study were ranked according to the means of the various measurements in Periods 1 and 3. The percentage of eggs from birds in each quartile, damaged during the three cycles, are shown in Table 4A. It was decided to omit from these results the eggs found damaged when they were hand candled postwash, because it was not clear if and when the damage had occurred during the three cycles. Nevertheless, the number of eggs found damaged at this step (Table 2B) was relatively small and would not alter the results of Table 4A appreciably. There is a remarkable similarity in the results of Table 4A among the various shell quality measurements. With the exceptions of deformation and specific strength, the poorest 25% of the birds, based on the means of these measurements, accounted for about 45% of the damage, whereas the best 25% accounted for only about 15%. The results for deformation appeared slightly poorer in the first quartile but the best results were in the fourth quartile with only 12% of the damaged eggs. As there was little difference between SG, CFS, and deformation, SG is probably the preferable measure simply because it is easiest to apply, especially if the flotation method is used. Although the results were based on SG by Archimedes' method, Thompson and Hamilton (1982) have shown that results by the flotation method are similar to those obtained by Archimedes' method. Holder and Bradford (1979) found the percentage of cracked eggs to be closely related to SG based on the flotation method. Wells (1967a) recommended SG (by Archimedes' method) after comparing it with SW/SA and two destructive measures.
EGG SHELL QUALITY AND BREAKAGE
907
TABLE 4. Percentage egg shell damage of eggs in the second period for birds classified in quartiles according to their average egg and shell characteristics in the first and third periods Quartile 1 Characteristic A) Over the three test cycles 32.4 36.9 45.2 45.5 44.2 47.8 44.6 45.8 38.3
28.1 38.2 27.1 25.8 30.0 27.0 32.7 30.0 28.8
23.1 22.9 21.1 21.6 20.6 22.3 21.8 22.4 22.4
25.5 12.2 17.4 18.6 16.4 14.8 13.8 14.8 20.5
7.1 13.0 16.5
5.9 12.3 10.4
12.9 9.6
B) At each cycle, based on egg weight means Cycle 1 2 3
9.8 14.6 16.1
1 For all but deformation, the first quartile represents the lowest means; for deformation, quartiles have been reversed. 2 Compression fracture strength. 3 Shell weight per unit surface area.
insults to the lighter eggs, leading to a weakening of these eggs and an increase in their subsequent breakage (Carter, 1979). It was noted,
for example, that the eggs tended to pile on top of each other at A, and it seems reasonable to suppose it was the smaller ones that were
TABLE 5. Percentage shell damage for birds ranked in the same quartile, by each pair of three shell quality measurements, and the counts of birds by quartile for each ranking A) Damage over the three test cycles
Quartiles
Measurements
1
2
3
4
CFS 1 and specific gravity CFS and shell thickness Specific gravity and shell thickness
48.9 46.6 47.6
29.8 31.7 29.4
21.9 22.0 20.6
13.9 15.8 14.5
B) Counts of birds in the quartiles for each pair of measurements CFS 1 quartiles
Specific gravity quartiles
1
2
3
4
1
2
3
4
88 16 7 0
24 62 32 3
2 32 47 32
2 4 30 80
Shell thickness
1 2 3 4
77 25 10 3
20 52 34 9
12 31 39 33
1 13 30 71
Specific gravity
1 2 3 4
81 26 6 2
20 53 28 14
11 28 46 30
1 9 36 69
Compression fracture strength.
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Egg weight Deformation CFS 2 Shell weight Shell thickness SW/SA3 Specific gravity % Shell Specific strength
THOMPSON ET AL.
908
T h e high frequency of damaged eggs, a m o n g t h e b e t t e r birds, m a y be explained in t w o ways. First, as Wells ( 1 9 6 7 a ) n o t e d , " w i t h a c o m p l e x p h e n o m e n o n like shell strength, a perfect m e a s u r e m e n t would appear t o be a physical impossibility." F o r example, Carter ( 1 9 7 8 ) found t h a t in 9 0 i m p a c t s of pairs of eggs, t h e thicker shelled egg cracked in 13 cases. Second, it is possible t h a t t h e e q u i p m e n t used in this study subjected some eggs t o stress t h a t n o egg could survive. In such cases, n o shell strength measure would predict p o t e n t i a l breakage accurately, because c h a n c e determines which eggs suffer damaging insults. Whatever t h e explanation, it is clear from t h e results of this s t u d y t h a t t h e shell strength m e a s u r e m e n t s c o m m o n l y used will fail t o predict a substantial p r o p o r t i o n of breakage in t h e commercial process of egg handling, and in particular, during loading, washing, and handling. ACKNOWLEDGMENTS The a u t h o r s t h a n k Dina D ' E r m o and J. W. Dickie for their c o o p e r a t i o n and technical assistance. REFERENCES Belyavin, C. G., and K. N. Boorman, 1981. Physical characteristics of intact and cracked eggs. Br.
Poult. Sci. 2 2 : 9 - 1 5 . Berry, J. G., 1976. Extending egg shell damage survey results into the field. Poultry Sci. 5 5 : 7 5 8 - 7 6 1 . Bowman, J. C , and N. I. Challender, 1964. Egg shell strength. A comparison of two laboratory tests and field results. Br. Poult. Sci. 4 : 1 0 3 - 1 1 6 . Carter, T. C , 1978. The hen's egg: Fracture of shells loaded very slowly. Br. Poult. Sci. 19:669-679. Carter, T. C , 1979. The hen's egg: Evidence of the mechanism relating shell strength to loading rate. Br. Poult. Sci. 20:175-183. Doyon, G., R.M.G. Hamilton, M. B. Cardou, F. Castaigne, and H. MacLean, 1985. Shell and interior egg quality. I. Changes in shell strength of eggs from five commercial strains of White Leghorn hens during their first laying cycle. Poultry Sci. 64: (in press). Fairfull, R. W., R. S. Gowe, and J.A.B. Emsley, 1983. Diallel cross of six long-term selected Leghorn strains with emphasis on heterosis and reciprocal effects. Br. Poult. Sci. 24:133-158. Hamilton, R.M.G., 1978. Observations on the changes in physical characteristics that influence egg shell quality in ten strains of White Leghorns. Poultry Sci. 57:1192-1197. Hamilton, R.M.G., 1982. Methods and factors that affect the measurement of egg shell quality. Poultry Sci. 61:2022-2039. Hamilton, R.M.G., and B. K. Thompson, 1981. Effects of the sequence of measuring nondestructive deformation and specific gravity on the quasistatic compression and impact strength of eggs from White Leghorn hens. Poultry Sci. 60: 1798-1801. Hamilton, R.M.G., and B. K. Thompson, 1984. Observations on daily variation in feed intake and shell strength of eggs from White Leghorn pullets and force-molted hens. Poultry Sci. 6 3 : 2335-2344. Holder, D. P., and M. V. Bradford, 1979. Relationship of specific gravity of chicken eggs to number of cracked eggs observed and percent shell. Poultry Sci. 58:250-251. Mueller, C. D., and H. M. Scott, 1940. The porosity of the egg-shell in relation to hatchability. Poultry Sci. 19:163-166. Schoorl, P., and H. Y. Boersma, 1962. Research on the quality of the egg shell. (A new method of determination.) Pages 432—435 in Proc. 12th World's Poult. Congr. Shrimpton, D. H., and C. M. Hann, 1967. Shell deformation in predicting breakage due to transport and handling. Br. Poult. Sci. 8:317-320. Siegel, P. B., J. H. Van Middelkoop, and P.R.K. Reddy, 1978. Comparisons of frequencies and egg shell characteristics of broken and intact eggs within diverse populations of chickens. Br. Poult. Sci. 19:411-476. Thompson, B. K., A. A. Grunder, R.M.G. Hamilton, and K. G. Hollands, 1983. Repeatability of egg shell quality measurements within individual hens. Poultry Sci. 62:2309-2314. Thompson, B. K., and R.M.G. Hamilton, 1982. Comparison of the precision and accuracy of the flotation and Archimedes' methods for measuring specific gravity of eggs. Poultry Sci. 6 1 : 1599-1605.
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forced t o t h e t o p , w h e r e t h e y could strike parts of t h e e q u i p m e n t before being d r o p p e d t o the belt. A l t h o u g h t h e results of Table 4A show t h a t shell quality m e a s u r e m e n t s on a set of eggs from a bird do reflect p o t e n t i a l breakage of eggs from t h a t bird, it was disconcerting to find such a high incidence of shell damage a m o n g birds in t h e f o u r t h quartile, t h a t is, a m o n g birds ranked highly according t o their m e a n shell quality m e a s u r e m e n t s . It seemed possible t h a t t h e percentages could be reduced b y ranking according t o pairs of quality m e a s u r e m e n t s jointly. Therefore, t h e calculations were repeated including only hens whose m e a n shell quality was allocated t o t h e same quartiles b y pair-wise c o m b i n a t i o n s of t h r e e m e a s u r e m e n t s . T h e results are given in Table 5. It can be seen (Table 5B) t h a t t h e majority of birds were allocated t o t h e same quartiles b y t h e t h r e e variables considered, although there were a n u m b e r of cases w h e r e birds were allocated t o t h e first quartile b y o n e m e a s u r e a n d t h e f o u r t h b y a n o t h e r . In spite of t h e deletion of these debatable allocations, t h e percentages of damaged eggs in each quartile changed very little (Table 4 A vs. Table 5A).
EGG SHELL QUALITY AND BREAKAGE Thompson, B. K., R.M.G. Hamilton, and P. W. Voisey, 1981: Relationships among various egg traits relating to shell strength among and within five avian species. Poultry Sci. 60:2388-2394. Voisey, P. W., and R.M.G. Hamilton, 1975. Behaviour of egg shell under compression in relation to deformation measurements. Br. Poult. Sci. 16:461-470. Voisey, P. W., and J. R. Hunt, 1974. Measurement of eggshell strength. J. Texture Stud. 5:135-182. Voisey, P. W., and D. C. MacDonald, 1978. Laboratory measurements of egg shell strength. 1. An
909
instrument for measuring shell strength by quasi-static compression, puncture, and nondestructive deformation. Poultry Sci. 57: 860-869. Wells, R. G., 1967a. Egg shell strength. 1. The relationship between egg breakage in the field and certain laboratory assessments of shell strength. Br. Poult. Sci. 8:131-139. Wells, R. G., 1967b. Egg shell strength. 2. The relationship between egg specific gravity and egg shell deformation and their reliability as indicators of shell strength. Br. Poult. Sci. 8:193-199.
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