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Factors Affecting the Vitelline Membrane of the Hen’s Egg1
Factors Affecting the Vitelline Membrane of the Hen's Egg1 J. L . HEATH
Department of Poultry Science, University of Maryland, College Park, Maryland 20742 (Received for publication August 6, 1975)
ABSTRACT Experimentation was conducted to determine the effect of egg size, length of storage, storage temperature, albumen pH and albumen viscosity on the vitelline membrane. The effect of changes in the vitelline membrane on water movement across it from the albumen to the yolk was also studied. The dry membrane weight decreased as a percentage of dry yolk weight as the egg weight decreased. Vitelline membrane weight increased under refrigerated (7° C.) conditions and decreased at room temperature (22° C ) . The length of storage from 0-9 days had no effect at 7 or 22° C. on membrane weight, although albumen breakdown occurred as evidenced by increased pH. Albumen pH affected the membrane weight only at 22° C. The membrane weight decrease at 22° C. was paralleled by a decrease in yolk index. Storing yolks in thick albumen under refrigerated conditions resulted in an increase in membrane weight. Storing in thin albumen produced no change. The increase in weight was thought to be due to the association of the thick albumen with the membrane. The increase in membrane weight did not increase the strength of the vitelline membrane. Movement of water into the yolk was decreased when membrane weight increased. POULTRY SCIENCE 55: 936-942, 1976
INTRODUCTION
T
HE vitelline membrane plays a very important role in egg quality. As the egg ages and decreases in quality, the vitelline membrane stretches and becomes weakened. This presents problems not only in table eggs but also those that are further processed through an egg breaking operation. The purpose of this experimentation was the determination of factors that influence the membrane and how the knowledge of these factors could lead to more optimum storage conditions. Various factors have been reported to influence the vitelline membrane. Aging has been reported to weaken the membrane by Fromm (1964), Fromm and Martone (1962), and Moran (1936) and cause it to lose weight (Fromm, 1964, 1967). The elasticity of the vitelline membrane has been reported by
1. Scientific Article No. A2137 Contribution No. 5100 of the Maryland Agricultural Experiment Station (Department of Poultry Science).
936
Moran (1936) to increase with age. Contrary to this, Fromm and Martone (1962) found no change in elasticity. Albumen pH has also been reported to play a role in vitelline membrane strength. Fromm (1966) reported that albumen pH controls the properties of the vitelline membrane and results in a weight loss as pH increases. Feeney et al. (1956) reported that deterioration of yolk strength and quality occurs under similar conditions to the breakdown of thick albumen. Bellairs et al. (1963) found the vitelline membrane consists of 2 layers, the inner layer was deposited in the ovary and the outer one in the oviduct. The two layers were distinctly different in structure and were separated by a very thin, continuous membrane. Fromm (1967) reported that the surface of the vitelline membrane has a f iberous network when fresh that disappears after 3 days at 35° C. The fibers were indicated to be a part of the chalaziferous layer. Earlier work by Fromm (1964) found that the membrane was strongest near the chalazae and weakened as movements were taken toward the equatorial sec-
VITELLINE MEMBRANE OF THE EGG
tion. The membrane in the small end of the egg was stronger than in the large end. The surface layer was found to dissipate with age and the rate of dissipation was influenced by storage temperature. The use of yolk index as a measure of vitelline membrane strength has been demonstrated by Fromm and Martone (1962) and Fromm (1964). The yolk index was not influenced by water absorption into the yolk (Fromm, 1966). Most of the previous work on the vitelline membrane has been done for long holding periods at elevated temperatures. These experiments were designed to investigate changes occurring during short-term storage under refrigerated and non-refrigerated environments. Factors which could affect the weight and strength of the vitelline membrane were also evaluated.
PROCEDURE Eggs were collected on day of lay from a flock of S.C. White Leghorns in their initial year of production. Except for the experiment in which the effect of egg size was evaluated, all eggs weighed between 60-65 g.; and were distributed by weight into the experimental groups so that average egg weights did not vary more than ±0.1 g. between treatments. This was necessary to reduce variation due to yolk size. All eggs were candled and those with unsound shells and interior quality below Grade A, as defined in Agriculture Handbook #75, were discarded. Eggs were not washed and only clean eggs were selected for experimentation. Measurements. Eggs used as fresh controls were evaluated immediately after gathering and were labeled fresh rather than control. Yolk index (YI), yolk moisture (YM), and albumen pH measurements were made as described by Heath (1975). To determine dry vitelline membrane weight (MW), the follow-
937
ing procedure was used. The egg contents were removed, yolk and albumen separated and yolk weighed. The yolk was washed in a stream of water, vitelline membrane (VM) carefully ruptured, a yolk sample removed for solids determination and the remaining yolk washed away from the membrane. The membrane was washed in a sequence of two solutions of chlorof orm-methanol (2:1 V. / V.) to remove any remaining yolk material. The membrane was then dried at 100° C. for 18 hours to determine MW. Statistical analysis was accomplished using the analysis of variance technique and Student-Newman-Keuls test presented in Steel and Torrie (1960). Egg Size. Eggs were collected at random over a six-month period, weighed, VM extracted, dried and membrane weighed. The MW was calculated as a percent of the dry yolk weight and recorded. All measurements and extractions were accomplished as previously described. Length of Storage Time and Storage Temperature. Eggs were stored large end up on egg flats for 0,3, 6 or 9 days under refrigerated (7° C.) and room (22° C.) conditions. The eggs were not disturbed for the duration of the storage period. The initial albumen pH, final albumen pH and MW were recorded. pH. Fromm (1967) demonstrated the alteration of albumen pH by oiling all or part of the shell. This technique was used to alter the pH of the shell eggs in study 1. Three shell treatments were used: (1) no shell coating, (2) shell completely coated with a food grade mineral oil, and (3) one-half of the shell coated with oil. These eggs were dipped so that the dividing line between the half oiled and the half unoiled was along the longitudinal axis of the egg. The eggs were placed horizontally on egg flats to prevent the unoiled portion of the eggs from becoming oiled. The
938
J. L. HEATH
oiled and unoiled eggs were also stored horizontally to prevent any differences attributable to position between treatment groups. The MW, YI and albumen pH were determined after storing the eggs for 7 days under refrigerated or room temperature conditions. The eggs in study 2 were removed from the shell, yolk and albumen separated and the yolks placed in either fresh or aged albumen for 7 days. The treatments were contained in 500 ml. Erlenmeyer flasks sealed with a cellophane wrapped rubber stopper, inverted and placed under refrigeration. The flasks were filled to within 2" of the top of the neck of the flask with yolks and albumen in quantities equivalent to the ratio in shell eggs. The fresh albumen was unblended and from day of lay eggs. Aged albumen was from eggs aged to produce an albumen pH of 9.0. The MW, YI and albumen pH were determined. Albumen Viscosity. Fresh yolks were placed in albumen of two different viscosities (thick and thin) and stored under refrigeration in flasks for 7 days (Study 1). The albumen was separated from fresh eggs into thick and thin fractions by gently pouring one from the other to prevent damage to the thick gel structure and maintain it in as near normal state as possible. The albumen and yolks were placed in flasks as previously described. The MW, YI and albumen pH were determined. In study 2, eggs stored in the shell were compared to those where the yolk was separated from the albumen. The chalazae was removed prior to storing in flasks so that the normal relationship of the chalaziferous layer to yolk was disrupted. The eggs were maintained under refrigerated or room conditions for 7 days. Moisture Content of the Yolks. The ture content of yolks stored for 7 days refrigerated and room conditions was mined in study 1. Shell eggs were stored
moisunder deterunder
the experimental conditions and after 7 days the yolk was removed from the albumen, rolled on damp filter paper to remove adherring albumen, placed in a weighing dish, vitelline membrane ruptured and percent water determined. The yolk contents were dried at 100° C. to a constant weight. In study 2, the yolks were removed from the albumen and placed in flasks in either thin or thick albumen under refrigerated conditions. These yolks were compared to fresh eggs and those stored in the shell. Yolk moisture was determined as outlined in study 1. RESULTS AND DISCUSSION Egg Size. The dry weight of the membrane, expressed as a percent of dry yolk weight, decreased as the egg weight increased (Figure 1). This was apparently due to an increase in size of the yolk but not accompanied by an increase in amount of membrane deposited. This could explain some of the decrease in egg quality and holding quality during the latter part of the laying year. This also points to the necessity of using eggs of a certain size range for experimental purposes. Length of Storage Time and Storage Temperature. The weight of the VM increased when the eggs were stored in refrigerated conditions 0.4
0.3 MEMBRANE WEIGHT AS O/O OF DRY YOIK
\ \ \ \
0.2
\
0.1
30
40
50
60
70
EGG WEIGHT (gl
FIG. 1. The relationship of egg size to membrane weight.
939
VITELLINE MEMBRANE OF THE EGG
TABLE 1.—The effect of storage time and temperature on the weight of the vitelline membrane Days of storage Storage temperature Refrigerator (7° C.) Room (22° C.)
PH
0
3
6
9
Initial
Final
12.8*b 12.8b
14.8a 11.4b
16.2a 12.1b
15.7a 11.6b
8.4 8.4
8.9 9.4
*Membrane weight in mg.—Each mean represents 2 replications of 30 membranes. Different letters indicate significant difference (P < 0.01).
ab
TABLE 2.—The effect of albumen pH on the vitelline membrane weight after 7 day storage Room (22° C.)
Refrigerated (7° C.) Membrane wt. (mg.)
Yolk index
No oil 1/2 oil Oiled
12.1*a 11.7a 11.4a
0.43c 0.44c 0.44c
Fresh albumen Aged albumen
12.4 12.8
0.43 0.43
Shell treatment
Albumen pH
Membrane wt. (mg.)
Yolk index
Albumen pH
8.6b 10.8a lO.Oab
0.34a 0.39b 0.42bc
9.4 9.0 7.9
Study 1 9.0 8.7 8.1 Study 2 8.3 9.0
*Each mean represents 2 replications of 20 membranes. Different letters indicate significant difference (P < 0.01) for each parameter.
abc
TABLE 3.—The effect of albumen viscosity on the vitelline membrane during 7 day storage
Albumen Fresh Thin Thick
Membrane wt. (mg.) 12.0*a 12.1a 15.8b
Study 1 Initial Yolk albumen index pH 8.2 0.44 8.2 0.43 8.2 0.43 Study 2 Shell ori
Storage condition Fresh Refrigerated (7° C.) Room (22° C.)
Membrane wt. (mg.) 12.0*a 13.2b 10.6c
Shell removed
Yolk index
Final albumen pH
Membrane wt. (mg.)
Yolk index
Final albumen pH
0.44a 0.41b 0.35c
8.3 8.9 9.2
12.0a 11.lab 10.8b
0.44a 0.43a 0.40b
8.3 8.0 8.0
*Each mean represents 4 replications of 12 membranes. Different letters indicate significant difference (P < 0.01) for each parameter.
abc
(Table 1). Storing at room temperature produced no differences except in albumen pH where an increase from pH 8.4 to 9.4 was recorded. The membrane weight did not change as the storage time increased from 3 to 9 days. As expected, the final albumen pH of the refrigerated eggs (8.9) was less
than the pH of the albumen of eggs stored at room temperature. The effect of refrigeration on the VM was verified in subsequent experiments. In Table 2, unoiled, refrigerated eggs had membranes that were heavier than unoiled, unref rigerated eggs. In Table 3, eggs stored for 7 days in
940
J. L. HEATH
refrigerated conditions with the shell on had membranes which increased in weight and were significantly heavier than those stored at room temperature. Fromm (1964) found a more rapid decrease in MW at 40° C. than 25° C. during a holding period of 0-16 days. Both of the temperatures were higher than the ones used in this experiment and the lower temperatures could have been the reason no decrease in MW was found. The increase in MW under refrigerated conditions and the lack of change under non-refrigerated conditions could be important in the relationship between albumen and yolk quality. The increase in weight could be due to several factors. In previous work, Heath (1975) found that under refrigerated conditions water movement into the yolk from the albumen could be slowed and under the proper conditions reversed. This could result in the movement of yolk protein into the fiber network (Bellairs et al., 1963) but not through. The weight increase could also be attributable to the chalaziferous layer becoming more closely associated with the membrane such that during removal and cleaning this albumen increased the weight of the membrane. The reduction in albumen degradation as evidenced by the slower pH increase could allow the chalaziferous layer to become more closely associated with the membrane than the remainder of the albumen. The lack of effect due to storage time, especially at 22° C., was interesting when the amount of change in pH was noted. This indicated albumen pH did not play an important role in VM degredation during short-term storage. pH. Because albumen pH plays such an important role in albumen breakdown and in the previous experiment had no effect on the VM, the following two studies were conducted. Fromm (1967) demonstrated the alteration
of albumen pH by oiling all or part of the shell. This technique was used successfully and differences in pH were obtained at both room and refrigerated temperatures. A change in MW attributable to albumen pH was found only at room temperature and this was when the oiled eggs were compared to those unoiled (Table 2, Study 1). The room temperature eggs had a more alkaline pH and larger differences between treatments. The effect on MW was reflected by the YI. Significant differences in YI were found when oiled and unoiled eggs were compared at the same point where the MW differences were found. This demonstrated an effect attributable to pH on the membrane but only at relatively high pH values. Because the pH was allowed to change over a period of time, its effect could have been reduced especially under refrigerated conditions. When fresh albumen (pH 8.3) was compared to aged albumen (pH 9.0) in its effect on yolks removed from the shell (Table 2, Study 2), no effect was found attributable to pH on YI or MW. When the eggs were broken into the jars, disruption of the yolk-chalaziferous layer relationship was unavoidable. This would then be a measure of pH effect on the membrane and not on the membrane chalaziferous layer relationship. The advantage of this approach was that the pH was TABLE 4.—The effect of storage and albumen conditions on yolk moisture Study 1 Storage condition Fresh Refrigerated (7° C.) Room (22° C.)
% Yolk moisture 45.3*a 44.5a 46.5b
Study 2 Fresh 46.0**a Thin albumen 46.8a Thick albumen 44.9b Shell 45.2ab *Each mean represents 30 yolks. **Each mean represents 3 replications of 12 yolks. ab Different subscripts indicate significant difference (P < 0.01).
VITELLINE MEMBRANE OF THE EGG
established prior to exposing the yolk, the jars were sealed under refrigeration to prevent or slow normal reactions and migration of constituents. All yolks in one replication were placed in the same media. The pH was widely different, pH 8.3 vs. 9.0, but no effect due to pH was found, indicating that if pH had an effect on the VM it was minimal. The main effect on the membrane appears to be at high pH under room temperature conditions. If the eggs are maintained under proper conditions for periods of short-term storage, little degradation of the VM can be expected. Albumen Viscosity. The previous study used albumen of two viscosities (aged-thin, fresh-thick) which may have prevented any effect of pH being found. Albumen viscosity may play a role if an increase in MW is attributable to albumen-membrane relationships. Eggs were removed from the shell and placed in flasks of thick or thin albumen (Table 3, Study 1). The chalaziferous layer was removed from each yolk in the process of separating thick from thin albumen so that the albumen in the flask was in direct contact with the VM. An increase in MW was recorded for the yolks in the thick albumen. No significant change was recorded in yolk index. The increase in MW was apparently a result of the association of the VM and albumen. Since the washing technique used to clean the membranes was the same, it appears that the firm albumen remains more firmly attached to the VM. No change in pH was found, therefore, this effect could be disregarded as causing the differences. The influence of the thick chalaziferous layer was demonstrated by comparing shell egg VM with those where the yolks were removed from the shell, separated from the albumen and stored in flasks (Table 3, Study 2). The eggs refrigerated in the shell had heavier MW than eggs at room temperature.
941
At both temperatures, the albumen pH increased and the YI decreased. No increase in MW was found when the yolks stored in the flasks were considered. This indicated the involvement of the chalaziferous layer in MW increase. The yolks in the flasks decreased in MW and YI at room temperature. The albumen pH decreased during storage, again indicating albumen pH was not a prerequisite for VM weakening. The increase in MW did not give additional strength to the VM as indicated by the YI but may slow the weakening process and may influence movement of other constituents across the membrane. Movement of Water into Yolk. Previous work (Heath, 1975) indicated that a difference in movement of water across the VM occurred under various conditions of temperature and environment. This difference was accounted for primarily as differences in permeability of the VM and/or osmotic differences. Water migration was studied under conditions similar to the conditions that increased MW. These two studies were designed to duplicate the conditions causing an increase in MW and determine the influence on water movement across the VM. The percent H 2 0 did not change under refrigerated conditions but increased at room temperature (Table 4, Study 1). The refrigerated eggs had an increase in MW in the previous experiments. A comparison was made in water movement in yolks stored in thin albumen, thick albumen and in the shell (Table 4, Study 2). The yolks stored in the shell and thin albumen did not change in H 2 0 content during the holding period. The yolks in the thick albumen had a decrease in the amount of yolk water and were the ones that had previously had heavier MW. It was apparent from this research that the association of the thick albumen with the VM did influence the movement of water between the albumen and yolk.
942
J. L. HEATH
REFERENCES Bellairs, R., M. Harkness and R. D. Harkness, 1963. The vitelline membrane of the hen's egg: A chemical and electron microscopial study. J. Ultrastructure Res. 8: 339-359. Feeney, R. E., J. M. Weaver, J. R. Jones and M. B. Rhodes, 1956. Studies of the kinetics and mechanisms of yolk deterioration in shell eggs. Poultry Sci. 35: 1061-1066. Fromm, D., 1964. Strength distribution, weight and some histological aspects of the vitelline membrane of the hen's egg yolk. Poultry Sci. 43: 1240-1247. Fromm, D., 1966. The influence of ambient pH on moisture content and yolk index of the hen's yolk. Poultry Sci. 45: 374-379.
Fromm, D., 1967. Some physical and chemical changes in the vitelline membrane of the hen's egg during storage. J. Food Sci. 32: 52-56. Fromm, D., and G. Martone, 1962. A rapid method for evaluating the strength of the vitelline membrane of the hen's egg yolk. Poultry Sci. 41: 1516-1521. Heath, J. L., 1975. Investigation of changes in yolk moisture. Poultry Sci. 54: 2007-2013. Moran, T., 1936. Physics of the hen's egg. II. The bursting strength of the vitelline membrane. J. Exp. Biol. 13: 41-47. Steel, R. F. D., and J. H. Torrie, 1960. Principles and Procedures of Statistics. McGraw-Hill Book Co., New York.
Egg Usage in Restaurants and Mass Feeding Institutions12 E . M. A R E N D T A N D D . M. GRAHAM
Department of Food Science and Nutrition, AND
Department of Agricultural Economics,
L . A. Voss University of Missouri, Columbia, Missouri 65201
(Received for publication August 6, 1975)
ABSTRACT A study of egg usage in Missouri institutions indicated a strong preference for eggs. Shell eggs are preferred to frozen or dried eggs because respondents think they are the best. Frozen eggs were used by one-fourth of the institutions. Dried eggs were not purchased and not even considered a satisfactory product by most institutions. Menu planners included eggs primarily because of consumer demand and nutritional value. Hospitals had the most formally trained menu planners while restaurants had the most trained by experience. Respondents had almost no suggestions for more uses or product changes for frozen eggs. More films and demonstrations were suggested as additional technical services. POULTRY SCIENCE 55: 942-949, 1976
INTRODUCTION
E
use about 15% of the total shell egg supply
GGS have been and continue to be an important item in the American diet.
According to the U.S.D.A. Food
Service
annually. Other than this U.S.D.A. work, there was not evidence of previous studies on egg usage.
Study (1966), it was estimated that institutional users, which include hospitals, college dormitories, nursing homes, and restaurants,
METHODS AND PROCEDURE The first step was to develop a questionnaire for the interviewers purpose. The use
1. Contribution from the Missouri Agricultural Experiment Station. Journal Series Number 7349. 2. Partially supported by a grant from the American Egg Board.
of several questionnaires and contacts were considered. The questionnaire included 39 questions (Table 1). A stratified random sample was chosen.
Department of Poultry Science, University of Maryland, College Park, Maryland 20742 (Received for publication August 6, 1975)
ABSTRACT Experimentation was conducted to determine the effect of egg size, length of storage, storage temperature, albumen pH and albumen viscosity on the vitelline membrane. The effect of changes in the vitelline membrane on water movement across it from the albumen to the yolk was also studied. The dry membrane weight decreased as a percentage of dry yolk weight as the egg weight decreased. Vitelline membrane weight increased under refrigerated (7° C.) conditions and decreased at room temperature (22° C ) . The length of storage from 0-9 days had no effect at 7 or 22° C. on membrane weight, although albumen breakdown occurred as evidenced by increased pH. Albumen pH affected the membrane weight only at 22° C. The membrane weight decrease at 22° C. was paralleled by a decrease in yolk index. Storing yolks in thick albumen under refrigerated conditions resulted in an increase in membrane weight. Storing in thin albumen produced no change. The increase in weight was thought to be due to the association of the thick albumen with the membrane. The increase in membrane weight did not increase the strength of the vitelline membrane. Movement of water into the yolk was decreased when membrane weight increased. POULTRY SCIENCE 55: 936-942, 1976
INTRODUCTION
T
HE vitelline membrane plays a very important role in egg quality. As the egg ages and decreases in quality, the vitelline membrane stretches and becomes weakened. This presents problems not only in table eggs but also those that are further processed through an egg breaking operation. The purpose of this experimentation was the determination of factors that influence the membrane and how the knowledge of these factors could lead to more optimum storage conditions. Various factors have been reported to influence the vitelline membrane. Aging has been reported to weaken the membrane by Fromm (1964), Fromm and Martone (1962), and Moran (1936) and cause it to lose weight (Fromm, 1964, 1967). The elasticity of the vitelline membrane has been reported by
1. Scientific Article No. A2137 Contribution No. 5100 of the Maryland Agricultural Experiment Station (Department of Poultry Science).
936
Moran (1936) to increase with age. Contrary to this, Fromm and Martone (1962) found no change in elasticity. Albumen pH has also been reported to play a role in vitelline membrane strength. Fromm (1966) reported that albumen pH controls the properties of the vitelline membrane and results in a weight loss as pH increases. Feeney et al. (1956) reported that deterioration of yolk strength and quality occurs under similar conditions to the breakdown of thick albumen. Bellairs et al. (1963) found the vitelline membrane consists of 2 layers, the inner layer was deposited in the ovary and the outer one in the oviduct. The two layers were distinctly different in structure and were separated by a very thin, continuous membrane. Fromm (1967) reported that the surface of the vitelline membrane has a f iberous network when fresh that disappears after 3 days at 35° C. The fibers were indicated to be a part of the chalaziferous layer. Earlier work by Fromm (1964) found that the membrane was strongest near the chalazae and weakened as movements were taken toward the equatorial sec-
VITELLINE MEMBRANE OF THE EGG
tion. The membrane in the small end of the egg was stronger than in the large end. The surface layer was found to dissipate with age and the rate of dissipation was influenced by storage temperature. The use of yolk index as a measure of vitelline membrane strength has been demonstrated by Fromm and Martone (1962) and Fromm (1964). The yolk index was not influenced by water absorption into the yolk (Fromm, 1966). Most of the previous work on the vitelline membrane has been done for long holding periods at elevated temperatures. These experiments were designed to investigate changes occurring during short-term storage under refrigerated and non-refrigerated environments. Factors which could affect the weight and strength of the vitelline membrane were also evaluated.
PROCEDURE Eggs were collected on day of lay from a flock of S.C. White Leghorns in their initial year of production. Except for the experiment in which the effect of egg size was evaluated, all eggs weighed between 60-65 g.; and were distributed by weight into the experimental groups so that average egg weights did not vary more than ±0.1 g. between treatments. This was necessary to reduce variation due to yolk size. All eggs were candled and those with unsound shells and interior quality below Grade A, as defined in Agriculture Handbook #75, were discarded. Eggs were not washed and only clean eggs were selected for experimentation. Measurements. Eggs used as fresh controls were evaluated immediately after gathering and were labeled fresh rather than control. Yolk index (YI), yolk moisture (YM), and albumen pH measurements were made as described by Heath (1975). To determine dry vitelline membrane weight (MW), the follow-
937
ing procedure was used. The egg contents were removed, yolk and albumen separated and yolk weighed. The yolk was washed in a stream of water, vitelline membrane (VM) carefully ruptured, a yolk sample removed for solids determination and the remaining yolk washed away from the membrane. The membrane was washed in a sequence of two solutions of chlorof orm-methanol (2:1 V. / V.) to remove any remaining yolk material. The membrane was then dried at 100° C. for 18 hours to determine MW. Statistical analysis was accomplished using the analysis of variance technique and Student-Newman-Keuls test presented in Steel and Torrie (1960). Egg Size. Eggs were collected at random over a six-month period, weighed, VM extracted, dried and membrane weighed. The MW was calculated as a percent of the dry yolk weight and recorded. All measurements and extractions were accomplished as previously described. Length of Storage Time and Storage Temperature. Eggs were stored large end up on egg flats for 0,3, 6 or 9 days under refrigerated (7° C.) and room (22° C.) conditions. The eggs were not disturbed for the duration of the storage period. The initial albumen pH, final albumen pH and MW were recorded. pH. Fromm (1967) demonstrated the alteration of albumen pH by oiling all or part of the shell. This technique was used to alter the pH of the shell eggs in study 1. Three shell treatments were used: (1) no shell coating, (2) shell completely coated with a food grade mineral oil, and (3) one-half of the shell coated with oil. These eggs were dipped so that the dividing line between the half oiled and the half unoiled was along the longitudinal axis of the egg. The eggs were placed horizontally on egg flats to prevent the unoiled portion of the eggs from becoming oiled. The
938
J. L. HEATH
oiled and unoiled eggs were also stored horizontally to prevent any differences attributable to position between treatment groups. The MW, YI and albumen pH were determined after storing the eggs for 7 days under refrigerated or room temperature conditions. The eggs in study 2 were removed from the shell, yolk and albumen separated and the yolks placed in either fresh or aged albumen for 7 days. The treatments were contained in 500 ml. Erlenmeyer flasks sealed with a cellophane wrapped rubber stopper, inverted and placed under refrigeration. The flasks were filled to within 2" of the top of the neck of the flask with yolks and albumen in quantities equivalent to the ratio in shell eggs. The fresh albumen was unblended and from day of lay eggs. Aged albumen was from eggs aged to produce an albumen pH of 9.0. The MW, YI and albumen pH were determined. Albumen Viscosity. Fresh yolks were placed in albumen of two different viscosities (thick and thin) and stored under refrigeration in flasks for 7 days (Study 1). The albumen was separated from fresh eggs into thick and thin fractions by gently pouring one from the other to prevent damage to the thick gel structure and maintain it in as near normal state as possible. The albumen and yolks were placed in flasks as previously described. The MW, YI and albumen pH were determined. In study 2, eggs stored in the shell were compared to those where the yolk was separated from the albumen. The chalazae was removed prior to storing in flasks so that the normal relationship of the chalaziferous layer to yolk was disrupted. The eggs were maintained under refrigerated or room conditions for 7 days. Moisture Content of the Yolks. The ture content of yolks stored for 7 days refrigerated and room conditions was mined in study 1. Shell eggs were stored
moisunder deterunder
the experimental conditions and after 7 days the yolk was removed from the albumen, rolled on damp filter paper to remove adherring albumen, placed in a weighing dish, vitelline membrane ruptured and percent water determined. The yolk contents were dried at 100° C. to a constant weight. In study 2, the yolks were removed from the albumen and placed in flasks in either thin or thick albumen under refrigerated conditions. These yolks were compared to fresh eggs and those stored in the shell. Yolk moisture was determined as outlined in study 1. RESULTS AND DISCUSSION Egg Size. The dry weight of the membrane, expressed as a percent of dry yolk weight, decreased as the egg weight increased (Figure 1). This was apparently due to an increase in size of the yolk but not accompanied by an increase in amount of membrane deposited. This could explain some of the decrease in egg quality and holding quality during the latter part of the laying year. This also points to the necessity of using eggs of a certain size range for experimental purposes. Length of Storage Time and Storage Temperature. The weight of the VM increased when the eggs were stored in refrigerated conditions 0.4
0.3 MEMBRANE WEIGHT AS O/O OF DRY YOIK
\ \ \ \
0.2
\
0.1
30
40
50
60
70
EGG WEIGHT (gl
FIG. 1. The relationship of egg size to membrane weight.
939
VITELLINE MEMBRANE OF THE EGG
TABLE 1.—The effect of storage time and temperature on the weight of the vitelline membrane Days of storage Storage temperature Refrigerator (7° C.) Room (22° C.)
PH
0
3
6
9
Initial
Final
12.8*b 12.8b
14.8a 11.4b
16.2a 12.1b
15.7a 11.6b
8.4 8.4
8.9 9.4
*Membrane weight in mg.—Each mean represents 2 replications of 30 membranes. Different letters indicate significant difference (P < 0.01).
ab
TABLE 2.—The effect of albumen pH on the vitelline membrane weight after 7 day storage Room (22° C.)
Refrigerated (7° C.) Membrane wt. (mg.)
Yolk index
No oil 1/2 oil Oiled
12.1*a 11.7a 11.4a
0.43c 0.44c 0.44c
Fresh albumen Aged albumen
12.4 12.8
0.43 0.43
Shell treatment
Albumen pH
Membrane wt. (mg.)
Yolk index
Albumen pH
8.6b 10.8a lO.Oab
0.34a 0.39b 0.42bc
9.4 9.0 7.9
Study 1 9.0 8.7 8.1 Study 2 8.3 9.0
*Each mean represents 2 replications of 20 membranes. Different letters indicate significant difference (P < 0.01) for each parameter.
abc
TABLE 3.—The effect of albumen viscosity on the vitelline membrane during 7 day storage
Albumen Fresh Thin Thick
Membrane wt. (mg.) 12.0*a 12.1a 15.8b
Study 1 Initial Yolk albumen index pH 8.2 0.44 8.2 0.43 8.2 0.43 Study 2 Shell ori
Storage condition Fresh Refrigerated (7° C.) Room (22° C.)
Membrane wt. (mg.) 12.0*a 13.2b 10.6c
Shell removed
Yolk index
Final albumen pH
Membrane wt. (mg.)
Yolk index
Final albumen pH
0.44a 0.41b 0.35c
8.3 8.9 9.2
12.0a 11.lab 10.8b
0.44a 0.43a 0.40b
8.3 8.0 8.0
*Each mean represents 4 replications of 12 membranes. Different letters indicate significant difference (P < 0.01) for each parameter.
abc
(Table 1). Storing at room temperature produced no differences except in albumen pH where an increase from pH 8.4 to 9.4 was recorded. The membrane weight did not change as the storage time increased from 3 to 9 days. As expected, the final albumen pH of the refrigerated eggs (8.9) was less
than the pH of the albumen of eggs stored at room temperature. The effect of refrigeration on the VM was verified in subsequent experiments. In Table 2, unoiled, refrigerated eggs had membranes that were heavier than unoiled, unref rigerated eggs. In Table 3, eggs stored for 7 days in
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J. L. HEATH
refrigerated conditions with the shell on had membranes which increased in weight and were significantly heavier than those stored at room temperature. Fromm (1964) found a more rapid decrease in MW at 40° C. than 25° C. during a holding period of 0-16 days. Both of the temperatures were higher than the ones used in this experiment and the lower temperatures could have been the reason no decrease in MW was found. The increase in MW under refrigerated conditions and the lack of change under non-refrigerated conditions could be important in the relationship between albumen and yolk quality. The increase in weight could be due to several factors. In previous work, Heath (1975) found that under refrigerated conditions water movement into the yolk from the albumen could be slowed and under the proper conditions reversed. This could result in the movement of yolk protein into the fiber network (Bellairs et al., 1963) but not through. The weight increase could also be attributable to the chalaziferous layer becoming more closely associated with the membrane such that during removal and cleaning this albumen increased the weight of the membrane. The reduction in albumen degradation as evidenced by the slower pH increase could allow the chalaziferous layer to become more closely associated with the membrane than the remainder of the albumen. The lack of effect due to storage time, especially at 22° C., was interesting when the amount of change in pH was noted. This indicated albumen pH did not play an important role in VM degredation during short-term storage. pH. Because albumen pH plays such an important role in albumen breakdown and in the previous experiment had no effect on the VM, the following two studies were conducted. Fromm (1967) demonstrated the alteration
of albumen pH by oiling all or part of the shell. This technique was used successfully and differences in pH were obtained at both room and refrigerated temperatures. A change in MW attributable to albumen pH was found only at room temperature and this was when the oiled eggs were compared to those unoiled (Table 2, Study 1). The room temperature eggs had a more alkaline pH and larger differences between treatments. The effect on MW was reflected by the YI. Significant differences in YI were found when oiled and unoiled eggs were compared at the same point where the MW differences were found. This demonstrated an effect attributable to pH on the membrane but only at relatively high pH values. Because the pH was allowed to change over a period of time, its effect could have been reduced especially under refrigerated conditions. When fresh albumen (pH 8.3) was compared to aged albumen (pH 9.0) in its effect on yolks removed from the shell (Table 2, Study 2), no effect was found attributable to pH on YI or MW. When the eggs were broken into the jars, disruption of the yolk-chalaziferous layer relationship was unavoidable. This would then be a measure of pH effect on the membrane and not on the membrane chalaziferous layer relationship. The advantage of this approach was that the pH was TABLE 4.—The effect of storage and albumen conditions on yolk moisture Study 1 Storage condition Fresh Refrigerated (7° C.) Room (22° C.)
% Yolk moisture 45.3*a 44.5a 46.5b
Study 2 Fresh 46.0**a Thin albumen 46.8a Thick albumen 44.9b Shell 45.2ab *Each mean represents 30 yolks. **Each mean represents 3 replications of 12 yolks. ab Different subscripts indicate significant difference (P < 0.01).
VITELLINE MEMBRANE OF THE EGG
established prior to exposing the yolk, the jars were sealed under refrigeration to prevent or slow normal reactions and migration of constituents. All yolks in one replication were placed in the same media. The pH was widely different, pH 8.3 vs. 9.0, but no effect due to pH was found, indicating that if pH had an effect on the VM it was minimal. The main effect on the membrane appears to be at high pH under room temperature conditions. If the eggs are maintained under proper conditions for periods of short-term storage, little degradation of the VM can be expected. Albumen Viscosity. The previous study used albumen of two viscosities (aged-thin, fresh-thick) which may have prevented any effect of pH being found. Albumen viscosity may play a role if an increase in MW is attributable to albumen-membrane relationships. Eggs were removed from the shell and placed in flasks of thick or thin albumen (Table 3, Study 1). The chalaziferous layer was removed from each yolk in the process of separating thick from thin albumen so that the albumen in the flask was in direct contact with the VM. An increase in MW was recorded for the yolks in the thick albumen. No significant change was recorded in yolk index. The increase in MW was apparently a result of the association of the VM and albumen. Since the washing technique used to clean the membranes was the same, it appears that the firm albumen remains more firmly attached to the VM. No change in pH was found, therefore, this effect could be disregarded as causing the differences. The influence of the thick chalaziferous layer was demonstrated by comparing shell egg VM with those where the yolks were removed from the shell, separated from the albumen and stored in flasks (Table 3, Study 2). The eggs refrigerated in the shell had heavier MW than eggs at room temperature.
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At both temperatures, the albumen pH increased and the YI decreased. No increase in MW was found when the yolks stored in the flasks were considered. This indicated the involvement of the chalaziferous layer in MW increase. The yolks in the flasks decreased in MW and YI at room temperature. The albumen pH decreased during storage, again indicating albumen pH was not a prerequisite for VM weakening. The increase in MW did not give additional strength to the VM as indicated by the YI but may slow the weakening process and may influence movement of other constituents across the membrane. Movement of Water into Yolk. Previous work (Heath, 1975) indicated that a difference in movement of water across the VM occurred under various conditions of temperature and environment. This difference was accounted for primarily as differences in permeability of the VM and/or osmotic differences. Water migration was studied under conditions similar to the conditions that increased MW. These two studies were designed to duplicate the conditions causing an increase in MW and determine the influence on water movement across the VM. The percent H 2 0 did not change under refrigerated conditions but increased at room temperature (Table 4, Study 1). The refrigerated eggs had an increase in MW in the previous experiments. A comparison was made in water movement in yolks stored in thin albumen, thick albumen and in the shell (Table 4, Study 2). The yolks stored in the shell and thin albumen did not change in H 2 0 content during the holding period. The yolks in the thick albumen had a decrease in the amount of yolk water and were the ones that had previously had heavier MW. It was apparent from this research that the association of the thick albumen with the VM did influence the movement of water between the albumen and yolk.
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REFERENCES Bellairs, R., M. Harkness and R. D. Harkness, 1963. The vitelline membrane of the hen's egg: A chemical and electron microscopial study. J. Ultrastructure Res. 8: 339-359. Feeney, R. E., J. M. Weaver, J. R. Jones and M. B. Rhodes, 1956. Studies of the kinetics and mechanisms of yolk deterioration in shell eggs. Poultry Sci. 35: 1061-1066. Fromm, D., 1964. Strength distribution, weight and some histological aspects of the vitelline membrane of the hen's egg yolk. Poultry Sci. 43: 1240-1247. Fromm, D., 1966. The influence of ambient pH on moisture content and yolk index of the hen's yolk. Poultry Sci. 45: 374-379.
Fromm, D., 1967. Some physical and chemical changes in the vitelline membrane of the hen's egg during storage. J. Food Sci. 32: 52-56. Fromm, D., and G. Martone, 1962. A rapid method for evaluating the strength of the vitelline membrane of the hen's egg yolk. Poultry Sci. 41: 1516-1521. Heath, J. L., 1975. Investigation of changes in yolk moisture. Poultry Sci. 54: 2007-2013. Moran, T., 1936. Physics of the hen's egg. II. The bursting strength of the vitelline membrane. J. Exp. Biol. 13: 41-47. Steel, R. F. D., and J. H. Torrie, 1960. Principles and Procedures of Statistics. McGraw-Hill Book Co., New York.
Egg Usage in Restaurants and Mass Feeding Institutions12 E . M. A R E N D T A N D D . M. GRAHAM
Department of Food Science and Nutrition, AND
Department of Agricultural Economics,
L . A. Voss University of Missouri, Columbia, Missouri 65201
(Received for publication August 6, 1975)
ABSTRACT A study of egg usage in Missouri institutions indicated a strong preference for eggs. Shell eggs are preferred to frozen or dried eggs because respondents think they are the best. Frozen eggs were used by one-fourth of the institutions. Dried eggs were not purchased and not even considered a satisfactory product by most institutions. Menu planners included eggs primarily because of consumer demand and nutritional value. Hospitals had the most formally trained menu planners while restaurants had the most trained by experience. Respondents had almost no suggestions for more uses or product changes for frozen eggs. More films and demonstrations were suggested as additional technical services. POULTRY SCIENCE 55: 942-949, 1976
INTRODUCTION
E
use about 15% of the total shell egg supply
GGS have been and continue to be an important item in the American diet.
According to the U.S.D.A. Food
Service
annually. Other than this U.S.D.A. work, there was not evidence of previous studies on egg usage.
Study (1966), it was estimated that institutional users, which include hospitals, college dormitories, nursing homes, and restaurants,
METHODS AND PROCEDURE The first step was to develop a questionnaire for the interviewers purpose. The use
1. Contribution from the Missouri Agricultural Experiment Station. Journal Series Number 7349. 2. Partially supported by a grant from the American Egg Board.
of several questionnaires and contacts were considered. The questionnaire included 39 questions (Table 1). A stratified random sample was chosen.