effect of rapid cooling of shell eggs on microcrack development, penetration of salmonella...

EFFECT OF RAPID COOLING OF SHELL EGGS ON MICROCRACK DEVELOPMENT, PENETRATION OF

SALMONELLA ENTERITZDZS, AND EGGSHELL STRENGTH

HAIQIANG CHEN, RAMASWAMY C. ANANTHESWARAN' and STEPHEN J. KNABEL

Department of Food Science The Pennsylvania State University

University Park, PA 16802

Accepted for Publication January 18, 2002

ABSTRACT

Eggs were subjected to cryogenic cooling treatments using liquid CO, or liquid N,. In order to minimize the t h e m 1 stress in eggshells due to rapid cooling, a two-stage air-cooling method was also evaluated in this study. Eggs were cooled from an initial temperature of 25C to approximately 7C. It was found that cooling produced microcracks on eggshells. However, rapid cooling did not increase the penetration of Salmonella enterica serovar Enteritidis (Salmonella enteritidis) into egg contents. When egg contents alone were sampled for Salmonella enteritidis, extending the immersion timefrom 24 to 48 h signijkantly (P < 0.01) increased thepenetration of Salmonella enteritidisfrom 5.0 to 25.0%. When egg contents together with eggshells were sampled, Salmonella enteritidis was detected in 100% of the egg samples at the above two time intervals, There were no signflcant differences (P > 0.05) in the eggshell strength between control (no cooling) and cooling treatments, indicating that cooling did not weaken eggshell strength.

INTRODUCTION

Numerous studies indicate that temperature of storage is one of the most important factors affecting Salmonella enteritidis growth in eggs (Schoeni et al. 1995; Hammack et al. 1993; Saeed and Koons 1993; Kim et al. 1989). Humphrey (1990a) found that Salmonella enteritidis phage types 4, 8, and 13a did not grow in yolks stored at 8C, but grew relatively rapidly at 12C and above

' Author for correspondence: Department of Food Science, 11 1 Borland Lab, The Pennsylvania State University, University Park, PA 16802. TEL: (814) 865-3004; FAX: (814) 863-6132; E-mail: [email protected]

Journal of Food Processing Preservation 26 (2002) 57-73. All Rights Reserved. "Copyright 2002 by Food & Nutrition Press. Inc., Trumbull, Connecticut. 57

58 H. CHEN, R.C. ANANTHESWARAN and S.J. KNABEL

during three weeks of storage. Temperature of storage also affects the heat resistance of Salmonella enteritidis (Saeed and Koons 1993; Humphrey 199Ob). Humphrey (1990b) showed that storage of Salmonella enteritidis phage type 4 at either 4C or 8C before heating significantly reduced its heat resistance. These results clearly demonstrate the need to store eggs at refrigeration temperature to prevent Salmonella enteritidis growth. It has been estimated that an 8% reduction in illness associated with Salmonella enteritidis would occur if all eggs were maintained at an air temperature of 7C throughout shell egg processing and distribution (Anon. 2000). Storage of eggs at low temperature also improves egg quality because decline in Haugh units is slower when storage temperature is closer to OC (Stephenson et al. 1997; Williams 1992; Bornstein and Lipstein 1962; Proudfoot 1962; Fry and Newel1 1957; Dawson and Hall 1954).

Since the mid 1980's, egg-associated Salmonella enteritidis outbreaks have been a major cause of foodborne illness in the United States and several European countries (Humphrey 1994; Mason 1994; Gast and Beard 1993). In order to inhibit the growth of Salmonella enteritidis in eggs and reduce the risk associated with Salmonella enteritidis, the Food Safety and Inspection Service (FSIS) proposed a final rule that requires shell eggs packed for consumer use be stored and transported at an ambient temperature not to exceed 7.2C (FSIS 1998). Currently, egg-packing companies typically cool eggs by putting washed eggs on pallets and storing them in a cold room. Eggs at the center of a pallet can take up to 142 h or more to reach 7C under these conditions (Anderson 1993; Anderson et al. 1992). Czarick and Savage (1992) also reported that eggs in cardboard cases required nearly one week to cool from 27C to 7C. This slow rate of cooling may facilitate the multiplication of Salmonella enteritidis and pose a serious food safety problem. It also decreases egg quality. Rapid cooling (rapid cooling was defined in this study as cooling eggs from an initial center temperature of 25C to 7.2C within 30 min) of eggs before packaging has been proposed as a means to control Salmonella enteritidis growth in eggs (Keener et al. 2000a, b; Mermelstein 2000; Curtis ef al. 1995; Fajardo et al. 1995; Catalan0 and Knabel 1994). Curtis et al. (1995) developed a cryogenic cooling system using liquid C02 for rapid cooling of eggs. Eggs with initial temperature of 37C could be cooled to 7C in 3 min using the cryogenic cooling system set at -6OC. They found that the cryogenically cooled eggs had significantly higher Haugh unit values than traditionally cooled eggs after 30 days of refrigerated storage. Based on the inoculation studies, it was also shown that there were significantly fewer bacterial counts in the interior of cryogenically cooled eggs than in traditionally cooled eggs.

However, rapid cooling can generate thermal stresses and produce microcracks (Microcracks were defined in this study as cracks in eggshell in the magnitude of micron. They are too small to be visible during candling.) on eggshells (Lin et al. 1996; Rehkugler 1973; Manceau and Henderson 1970).

RAPID COOLING OF EGGS 59

Fajardo ef al. (1995) found that cooling produced microcracks on eggshells, which facilitated the penetration of Salmonella enterifidis into egg contents. Concerns arise that these microcracks might weaken the eggshell and affect the ability of eggs to defend against potential bacterial contamination. Eggshell strength is of significant importance to the egg industry. Shell breakage was estimated to cost egg producers in the United States about $60 to $73 million annually based on 5-876 loss or decrease in value (Rhorer 1987). Previous studies have shown that sound eggshells had a significantly higher strength than eggs with visible cracks (Carnarius ef al. 1996; Fajardo ef al. 1996). Carnarius ef al. (1996) found that the mean puncture forces of sound and cracked eggshells were 35.3 N and 30.4 N, respectively.

The objective of this study was to study the impact of rapid cooling using cryogenic systems on microcrack development on eggshell, the penetration of Salmonella enteritidis into egg contents, and eggshell strength. Thermal stresses are much higher under rapid cooling conditions than under slow cooling conditions (Lin ef al. 1996; Manceau and Henderson 1970). Therefore, a two- stage air-cooling method, which cooled eggs using air at OC for 6 min and -2OC for another 6 min, was also investigated in this study to alleviate the thermal stresses produced during cooling.

MATERIALS AND METHODS

Eggs Fresh nest-run eggs from the Pennsylvania State University Poultry

Research Farm were used for the experiments. The eggs were candled and any eggs with visible cracks were discarded. The weights of eggs used were 50-60 g. Eggs were stored at room temperature and used within 3 days after they were laid.

Cryogenic Cooling Systems

A laboratory scale cryogenic cooling system was developed and used in this study. Sixteen eggs were placed on a metal rack and the rack was put into a cryogenic cooling chamber made of Styrofoam for cooling. The dimensions of the cooling chamber were 550 X 330 X 275 mm. A small fan was installed in the chamber to promote uniform airflow (air speed: 0.85 m/s) and facilitate temperature equilibration within the chamber. Figure 1 shows the setup used for liquid CO, cooling. The liquid N, cooling chamber had a similar setup, except that approximately 12 L of liquid N, was poured directly into the cooling chamber before the cooling experiments began. During the cooling experiments, care was taken to make sure that eggs did not contact liquid C02 or N,.

60

r

-Fan

t Rack

H. CHEN, R.C. ANANTHESWARAN and S.J. KNABEL

~

Liquid co2 Cylinder

FIG. 1. SETUP FOR LIQUID C02 COOLING

Effect of Rapid Cooling on Microcrack Development

Cooling Treatments. Sixteen eggs were randomly assigned to each of the following four treatments: (1) Control (no cooling); (2) Two-stage air cooling: Eggs were cooled by air (air speed: 0.47 ds) at OC for 6 min and at -2OC for 6 min in a freezer chamber (CSZ Model 216, Cincinnati Sub-Zero Products, Inc., Cincinnati, OH); (3) Liquid CO, cooling: Eggs were cooled by liquid C02 at -6OC for 6 min; and (4) Liquid N,cooling: Eggs were cooled by liquid N2 at -8OC for 3.5 min. Eggs were cooled from an initial temperature of 25C to approximately 7C. During cooling, one egg from each treatment was monitored for temperature at an interval of 5 s using a data logger (Campbell Scientific, Inc., Logan, UT). After cooling, eggs were kept in the Styrofoam container for 2 h so that eggs could be warmed up slowly to avoid thermal stresses. Eggs were then removed from the Styrofoam container and stored at room temperature for 10 h for eggs to equilibrate to room temperature. Eggs were then candled again to make sure they did not have visible cracks.

Analysis of Microcrack Development Using Scanning Electron Microscope. A small hole was cut at the larger end of an egg and egg contents were carefully removed. The eggshells were then cut into two halves using a high speed drill (Dremel, Racine, WI) fitted with a 0.0635 cm thick grinding disc. These halves were cleaned using water at room temperature and air-dried. Shell samples were prepared and scanned around the shell center using a scanning electron microscope to analyze microcrack development (JSM 5400, Jeol, Peabody, MA) (Fajardo et al. 1995). Two replicates were conducted.


Effect of Rapid Cooling on the Penetration of Salmonella enteritidis into Egg Contents

Preparation of Salmonella enteritidis Culture. A mutant strain of Salmonella enteritidis phage type 8 (SE PTSNSR) resistant to 0.1 mg/mL of nalidixic acid (Sigma Chemical Co., St. Louis, MO) and 0.1 mg/mL of streptomycin sulfate (Sigma) was isolated as previously described (Catalan0 and Knabel 1994). This mutant strain was used as an indicator organism to ensure that only those Salmonella enteritidis that were added would be detected. A loopful of SE PT8NSR was transferred from a tryptic soy agar plus yeast extract (TSAYE) (Difco Laboratories, Detroit, MI) plate containing 0.1 mg/mL of nalidixic acid and 0.1 mg/mL of streptomycin sulfate to 3 L of buffered peptone water (BPW) (Difco). The BPW culture was then incubated for 48 h at 37C and mixed with 25 L of BPW to yield a Salmonella ententidis level of approximately 3 x ~ O ~ C F U / ~ L .

Salmonella Enteritidis Inoculation. Two hundred and forty eggs were randomly assigned to each of the following four treatments: (1) Control (no cooling); (2) Two-stage air cooling for 6 min at OC and 6 min at -2OC; (3) Liquid C02 cooling for 6 min at -6OC; and (4) Liquid N2 cooling for 3.5 min at -8OC. After cooling, eggs were slowly brought to room temperature, and then immersed in the above BPW culture at room temperature (to avoid any thermal stresses). After 24 or 48 h of immersion, half of the eggs from each treatment were taken out and disinfected by soaking in 16 L of 300 ppm OC1- solution (sodium hypochlorite) containing 0.1% sodium dodecyl sulfate (SDS) (Sigma) for 30 min (FDA 1999). Then the eggs were again soaked in another fresh bath of OCl-/SDS solution for another 30 min. A higher OC1- concentration and an additional immersion in the OCl-/SDS solution were used in these experiments to ensure the eggshells were completely free of Salmonella enteritidis. The eggs were then rinsed with sterile distilled water.

Effectiveness of the Disinfection Procedure. A study was conducted to determine whether the disinfection procedure was effective in eliminating Salmonella enteritidis on the external surface of eggshells. Twenty eggs were randomly selected and each individual egg was put into an 18-oz whirlpack bag containing 50 mL of BPW and massaged vigorously by hand for 1 min. The eggs were then removed and the bags were incubated at 37C for 48 h. Then 0.1 mL of BPW from each bag was spread onto TSAYE plates. The plates were incubated at 37C for 48 h and checked for colonies.

Sampling of Salmonella enteritidis. Immediately after disinfection, eggs from each treatment were randomly divided into two groups. One group of eggs


was aseptically cracked, and egg contents were aseptically poured into jars (without eggshell). Another group of eggs were put into jars and cracked, and egg contents and eggshell were incubated together. The purpose of having the second group (egg contents plus eggshell) was to differentiate between Salmonella enrerifidis cells trapped in the shell pores and/or shell membranes and Salmonella enteritidis cells in the internal egg contents. Each egg was then supplemented with 25 mg of FeSO, (Chen e? al. 2001). Egg contents were mixed for 1 min and then incubated at 37C for 24 h. A loopful of egg contents from each jar was streaked onto XLD plates containing 0.05 mg/mL of nalidixic acid and 0.05 m g l d of streptomycin sulfate. The XLD plates were incubated at 37C for up to 72 h. Suspect colonies on XLD plates were randomly picked and used to inoculate triple sugar iron agar (TSI) (Difco) and lysine iron agar (LIA) (Difco) slants containing 0.1 mg/mL of nalidixic acid and 0.1 mg/mL of streptomycin sulfate. The slants were incubated at 35C for up to 48 h. To confirm isolates of Salmonella enteritidis, group D factor 9 slide-agglutination reactions were conducted on the colonies that had positive reactions in TSI and LIA slants.

Effect of Rapid Cooling on Eggshell Strength

Three experiments were conducted in this study to determine the type of cooling on eggshell strength. Numerous studies have shown that specific gravity is correlated with eggshell strength (Voisey e? al. 1979; Ahmad er al. 1976; Gaisford 1965; Frank et al. 1964). In order to reduce the error and increase the chance of detecting small differences between treatments, a randomized block design was used in this study. Specifically, eggs were grouped into different blocks according to their specific gravity and then randomly assigned to different cooling treatments. After cooling, eggs were kept in the Styrofoam container for 2 h, and then removed from the Styrofoam container and stored at room temperature for 10 h before eggshell strength was determined. The detailed experimental methods are explained in the following paragraphs.

Determination of the Specific Gravity of Eggs. The specific gravity of eggs was determined by immersing eggs at room temperature in a plastic basket in a series of brine solutions at room temperature in ascending specific gravities that ranged from 1.074 to 1.098 in increments of 0.004 (Hamilton 1982). The specific gravity of the brine solution, in which the egg first floated, was taken as the specific gravity of the egg. Eggs were then rinsed with water at room temperature and carefully dried with a paper towel. Eggs were stored at room temperature for at least 4 h for them to be completely dry. Then eggs were candled again and cracked eggs were removed.


Effect of Two-stage Air Cooling on Eggshell Strength. Thirty eggs were grouped into different blocks according to their specific gravity and then randomly assigned to each of the following two treatments: (1) Control (no cooling), and (2) Two-stage air cooling for 6 min at OC and 6 min at -2OC. After cooling, eggs were slowly brought to room temperature and eggshell strength was determined using an Instron Universal Testing Machine (Model 1000, Instron Corp., Canton, Mass.) by the quasi-static compression test. During compression, each egg was compressed between two parallel flat steel plates with the major axis horizontal so that force was applied at the equator. The diameter of the loading head was 57.35 mm and the loading speed was 12.7 mm/min during compression. The force and deformation data were obtained at a rate of 9 readingds from a Data Acquisition/Control Unit (3497 A, Hewlett Packard Co., San Diego, Calif.) connected to a personal computer.

Effect of Liquid COz Cooling on Eggshell Strength. To determine the effect of liquid CO, cooling and the time of cooling on eggshell strength, 28 eggs were grouped into different blocks according to their specific gravity and randomly assigned to each of the following four treatments: (1) Control (no cooling); (2) Liquid CO, cooling for 3 min; (3) Liquid C02 cooling for 6 min; and (4) Liquid C02 cooling for 9 min. After cooling, eggs were slowly brought to room temperature and eggshell strength was determined as described above.

Effect of Liquid N2 Cooling on Eggshell Strength. To determine the effect of liquid N, cooling and the time of cooling on eggshell strength, 28 eggs were grouped into different blocks according to their specific gravity and randomly assigned to each of the following four treatments: (1) Control (no cooling); (2) Liquid N, cooling for 3 min; (3) Liquid N2 cooling for 4 min; and (4) Liquid N, cooling for 5 min. After cooling, eggs were slowly brought to room temperature and eggshell strength was determined as described above.

Statistical Analysis

Statistical analysis was conducted using Minitab 11.2 (Minitab Inc., University Park, PA). Because of the unequal sample size in the eggshell strength experiments, analysis of variance was performed on the data using General Linear Model. For penetration experiments, the two time intervals, 24 h and 48 h, were treated as two blocks, and the data were analyzed using General Linear Model.


RESULTS AND DISCUSSION

Effect of Rapid Cooling on Microcrack Development

The typical time-temperature profiles at the centers of eggs cooled by two- stage air cooling for 6 min at OC and 6 min at -2OC, liquid C02 cooling for 6 min, and liquid N, cooling for 3.5 min are shown in Fig. 2. The temperatures at the egg centers reached 14.7, 16.0, and 16.7C at the end of cooling for two- stage air cooling, liquid CO, cooling, and liquid N, cooling respectively, and then equilibrated to approximately 7C in approximately 20 min after being put into a Styrofoam container.

s v

30

25

20

15

10

5

" I

0 5 10 I5 20 25

Time (min)

FIG. 2. THE TEMPERATURES AT THE CENTERS OF EGGS COOLED BY TWO-STAGE AIR COOLING FOR 6 MIN AT OC AND ANOTHER 6 MIN AT -2OC. LIQUID CO,

COOLING FOR 6 MIN AT -6OC. AND LIQUID NZ COOLING AT -8OC FOR 3.5 MIN AND THEN STORED IN A STYROFOAM CONTAINER

Thermal stresses occur when there is a difference between egg temperature and ambient temperature (Rehkugler 1973; Manceau and Henderson 1970). Thermal stresses produced, due to the constrained shape of the eggshell, are relieved by crack formation. If a crack already exists, then the energy is used to expand the crack (Fajardo er al. 1996). Figure 3 shows the effect of rapid cooling on microcrack development on the eggshell. Prior to cooling, eggshells had cracks of approximately 0.4 pm wide. However, wider cracks and many


offshoots were found on the eggshells of cooled eggs. Eggs in the liquid N, cooling treatment had the largest microcracks. Some microcracks in this treatment had openings as large as 2.0 pm.

FIG. 3. EFFECT OF RAPID COOLING ON MICROCRACK DEVELOPMENT ON EGGSHELLS

(A) Control (B) Two-stage air cooling for 6 min at OC and 6 min at -2OC (C) Liquid C02 cooling for 6 min at -6OC (D) Liquid N2 cooling for 3.5 min at -8OC.

Effect of Rapid Cooling on the Penetration of Salmonella enteritidis Into Egg Contents

The Salmonella enteritidis in the BPW culture in which eggs were immersed grew to 2.4 x lo8 and 3.3 x lo8 CFU/mL at 24 and 48 h, respectively. After eggshells were disinfected by the OCl-/SDS solution, no Salmonella enteritidis could be recovered from the external eggshell surface, indicating that the disinfection procedure was effective in eliminating Salmnella enteritidis on the external surface of the eggshells. The effect of rapid cooling on the penetration


of Salmonella enteritidis into egg contents is shown in Table 1. There were no significant differences (P > 0.05) in Salmonella enteritidis penetration between control and cooling treatments at the immersion time of 24 or 48 h, indicating that the microcracks developed on the eggshells during rapid cooling did not weaken the egg defense system against Salmonella enteritidis penetration. Two types of defenses, physical and chemical defenses, are involved in protection of egg contents from microbial penetration (Bruce and Drysdale 1994; Mayes and Takeballi 1983). The physical defenses include cuticle, eggshell, and shell membranes (Sauter and Petersen 1974; Vadehra e? al. 1970; Board 1966). The shell membranes act as bacterial filters and are more impenetrable to bacteria than the shell (Lifshitz ef al. 1964; Kraft e? al. 1958; Haines and Moran 1940). Garibaldi and Stokes (1958) proved the above statement by replacing the egg contents with bacteria suspension and then applying suction to the outside of the shell. The filtrates drawn from the shell without shell membranes contained essentially the same number of bacteria as the original suspension. In contrast, the filtrates from the shell with shell membranes were completely free of bacteria. Lifshitz ef al. (1964) found that the inner shell membrane was the most effective barrier against bacterial penetration, the shell ranked second, and the outer shell membrane was the least important. All these results clearly indicate that shell membranes are very effective mechanical barriers to bacteria penetration and the shell only plays a minor role in hindering the entrance of bacteria into the egg contents. Therefore, the microcracks developed on the shell should not affect the capability of egg defense system against bacterial penetration.

TABLE 1 . EFFECT OF RAPID COOLING ON THE PENETRATION OF SALMONELu E " I D I S

(SE) INTO EGG CONTENTS

Treatments Number of SE positive eggs (1 5 eggsitreatment)

Egg contents with eggshell

24 h' 48 hb 24 hb 48 hb

Egg contents

Control I5 15 0 4 TAC' 15 I5 2 4 LCC' for 6 min 15 15 0 3 LNC' for 3.5 min 14' 1s 1 4

SE Penetration (Mean) 100% 100% 5.0% 25.0%

TAC = Two-stage air cooling for 6 min at OC and 6 min at -2OC: LCC = Liquid CO, cooling at -6OC; LNC = Liquid N, cooling at -8OC. The time eggs were immersed in the Salmnella enteritidis culture. One egg was broken during cooling and was removed from the penetration treafment.


When egg contents alone were sampled for Salmonella enteritidis, extending the immersion time from 24 to 48 h significantly (P<O.Ol) increased the penetration of Salmonella enteritidis into egg contents from 5.0 to 25.0% (Table 1). These data are in agreement with the results of other researchers. Garibaldi and Stokes (1958) reported that the time taken for bacteria to penetrate the eggshell with membranes ranged between 1 and 4 days. Wong Liong et al. (1997) used confocal scanning laser microscopy to visualize the penetration process of Salmonella enteritidis into shell membranes. They found that Salmonella enteritidis readily move through the eggshell and accumulate in the outer shell membrane, and that shell membranes could provide mechanical barriers for at least 40 h. When egg contents together with eggshells were sampled, Salmonella enteritidis was detected in 100% of the egg samples at both the immersion times (Table 1). At immersion time of 24 h or 48 h, the percentage detection of Salmonella enteritidis was significantly higher (P< 0.01) when egg contents with eggshell were sampled for Salmonella enteritidis than when egg contents alone were sampled. The results indicated that, in most eggs, Salmonella enteritidis accumulated in the shell pores and/or shell membranes, and did not penetrate through shell membranes. In view of this, the direct plating method recommended by the United States Department of Agriculture to detect Salmonella enteritidis in eggs, which only samples egg contents (USDA 1993), should be reevaluated. Transovarian and trans-shell transmission are two ways in which eggs can become contaminated (Bruce and Drysdale 1994). In transovarian transmission, eggs become contaminated before eggs are laid, with the source of contamination originating in the ovary or oviduct of hens. Trans- shell transmission occurs when microorganisms penetrate the eggshell and contaminate the internal egg contents after eggs are laid (Bruce and Drysdale 1994). Therefore, egg contents should be sampled for Salmonella enteritidis together with eggshell to detect Salmonella enteritidis contamination through the trans-shell transmission route. In this study, OCl/SDS solution was very effective in removing Salmonella enteritidis from the external surface of the eggshells, but could not remove Salmonella enteritidis which had penetrated into the pores of shells or shell membranes. These trapped Salmonella enteritidis cells may finally penetrate into the egg contents and multiply during storage. This raises the question about the effectiveness of the washing technique used in the current egg industry. Therefore, studies should be conducted to determine the effectiveness of the commercial washing technique in removing Salmonella enteritidis from shell pores and shell membranes.

Effect of Rapid Cooling on Eggshell Strength

Table 2 shows the effect of different cooling treatments on eggshell strength. There was no significant difference (P>0.05) in fracture force


between control and two-stage air-cooling treatment. No visible cracked eggs were found in this experiment. The equilibrated temperatures for the eggs cooled by liquid CO, for 3, 6 and 9 min were 17.4, 8.3, and 2.3C, respectively. There were no significant differences (P>0.05) in fracture force between control and the three liquid C02 cooling treatments. No visible cracked eggs were found in any of the cooling treatments. The equilibrated temperatures for the eggs cooled by liquid N2 for 3, 4 and 5 min were 9.2, 2.4, and 0.4C, respectively. There were also no significant differences (b0.05) in fracture force between control and the three liquid N, cooling treatments. There were no cracked eggs found in the 3-min and 4-min liquid N, cooling treatments. However, 3 out of 28 eggs were cracked in the 5-mi0 cooling treatment. It was found that a thin layer of albumen just below the eggshell was frozen for 3-min treatment, but a thick layer of albumen was frozen for the 5-min treatment. The effect of specific gravity oneggshell strength was significant (P< 0.01). Eggs with higher specific gravity had higher eggshell strength.

TABLE 2. EFFECT OF DIFFERENT COOLING TREATMENTS ON EGGSHELL STRENGTH

Experiments Eggshell strength (N) at different specific gravities Number of

1.078 1.082 1.086 1.090 1.094 cracked eggs

TAC EXLU. Control 30.4 (3.2) 31.5 (2.9) 33.5 (4.3) 37.6 (6.7) 35.9 (2.3) 0 TAC 31.1 (2.9) 30.6 (2.2) 37.3 (3.0) 35.9 (0.3) 36.8 (2.2) 0

LCc &of. Control 30.1 (5.5) 30.4 (3.7) 32.5 (2.8) 34.2 (4.1) 41.2 (2.8) 0 LCC for 3 rnin 27.5 (2.8) 31.7 (1.4) 33.6 (5.5) 34.8 (4.2) 38.1 (0.6) 0 LCC for 6 rnin 28.5 (5.3) 30.9 (4.5) 33.3 (3.6) 35.4 (4.5) 38.7 (1.8) 0 LCC for 9 rnin 27.1 (1.1) 27.8 (4.9) 31.6 (4.9) 36.6 (4.5) 37.1 (3.4) 0

LNC Exor. Control 27.8 (2.1) 31.6 (3.5) 34.8 (4.2) 36.2 (5.2) 41.1 (0.8) 0 LNC for 3 rnin 28.6 (3.2) 31.6 (5.5) 35.2 (4.4) 35.2 (5.6) 41.7 (3.1) 0 LNC for 4 rnin 29.4 (3.5) 31.7 (1.6) 34.6 (4.6) 36.6 (3.1) 38.3 (5.9) 0 LNC for 5 rnin 29.1 (4.4) 30.7 (5.2) 33.1 (5.4) 36.9 (2.4) 43.4 (1.7) 3

' TAC = Two-stage air cooling for 6 min at OC and 6 min at -2OC; LCC = Liquid CO, cooling at -6OC; LNC = Liquid N, cooling at -80C. Standard deviations are listed in parentheses.

fracture force between control and cooling treatments (l90.05). ' In each experiment PAC. LCC, or LNC experiment), there were no significant differences in

' There were significant differences in fracture force among specific gravities (P<O.Ol).


These results clearly demonstrate that overcooling of eggs froze the eggs and this might crack the eggs. However, when these cracked eggs were removed from the eggshell strength measurement, rapid cooling did not affect eggshell strength. Numerous funnel-shaped pores are distributed unevenly over the shell surface, ranging from 6,000 to 10,OOO pores per egg. The diameter of the pores varies throughout the shell with a diameter of approximately 13 pm at the top and 6 pm at the bottom (Haines and Moran 1940; Bruce and Drysdale 1994). The largest size of microcracks found in this study was approximately 2.0 pm. Compared to the size of pores, these microcracks are too small to affect eggshell strength. The two-stage air-cooling method was originally thought to minimize the temperature change on the eggshell during cooling and alleviate the thermal stresses and the associated microcrack development. However, no significant differences in eggshell strength among the four treatments indicated that this method was not practically important. Maximum thermal stresses are created during the first few seconds of cooling due to the large temperature difference between the inside and outside of the shell (Lin et al. 1996). Therefore, if thermal stresses generated during rapid cooling can crack eggs, the cracks should appear at the beginning of the cooling experiment. None of the eggs in the liquid N2 cooling for 3 min treatment cracked during cooling; cracked eggs were only found in the 5 min treatment, indicating that thermal stresses generated by these cooling methods were not large enough to crack eggs. However, the excessive expansion of the internal egg contents due to ice formation in the outer albumen may cause fractures on eggshells.

CONCLUSIONS

Rapid cooling of eggs produced microcracks on eggshells. However, the microcracks developed on eggshells did not weaken the capability of the egg defense system against Salmonella enteritidis penetration and did not affect eggshell strength. Therefore, rapid cooling of eggs to approximately 7C before packaging is highly recommended for egg-packing companies. The air-cooling method is more preferable than cryogenic cooling due to its low cost and controllability. Moreover, the cooling rate can be easily increased by increasing air speed andlor decreasing air temperature.

ACKNOWLEDGMENTS

This research was funded by the Pennsylvania Department of Agriculture. The authors thank Mr. Robert Guyer for his technical assistance with the microbiological experiments.

70 H. CHEN. R.C. ANANTHESWARAN and S.J. KNABEL

REFERENCES

AHMAD, M.M., FRONINIG. G. W., MATHER, F.B. and BASHFORD, L.L. 1976. Relationships of egg specific gravity and shell thickness to quasi- static compression tests. Poult. Sci. 55, 1282-1289.

ANDERSON, K.E. 1993. Refrigeration and removal of heat from eggs. Misset- World Poult. 9(11), 40-43.

ANDERSON, K.E., JONES, F.T. and CURTIS, P.A. 1992. Legislation ignores

ANON. 2000. Egg safety activities. Food Technol. 54(6), 101-102. BOARD, R.G. 1966. Review article: The course of microbial infection of the

hen’s egg. J. Appl. Bacteriol. 29, 319-341. BORNSTEIN, S. and LIPSTEIN, B. 1962. Some characteristics of measures

employed for determining the interior quality of chicken eggs. Br. Poult. Sci. 3, 127-138.

BRUCE, J. and DRYSDALE, E.M. 1994. Trans-shell transmission. In Microbiology of the Avian Egg, (R.G. Board and R. Fuller, eds.) pp. 133-214, Chapman and Hall, New York.

CARNARIUS, K.M., CONRAD, K.M., MAST, M.G. and MACNEIL, J.H. 1996. Relationship of eggshell ultrastructure and shell strength to the soundness of shell eggs. Poult. Sci. 75, 656-663.

CATALANO, C.R. and KNABEL, S.J. 1994. Destruction of Salmonella enteritidis by high pH and rapid chilling during simulated commercial egg processing. J. Food hot . 57, 592-595.

CHEN, H., ANANTHESWARAN, R.C. and KNABEL, S.J. 2001. Optimiza- tion of iron supplementation for enhanced detection of Salmonella enterifidis

CURTIS, P. A. , ANDERSON, K.E. and JONES, F.T. 1995. Cryogenic gas for rapid cooling of commercially processed shell eggs before packaging. J.

CZARICK, M. and SAVAGE, S. 1992. Egg cooling characteristics in commercial egg coolers. J. Appl. Poult. Res. 1, 258-270.

DAWSON, L.E. and HALL, C.W. 1954. Relationship between rate of cooling, holding container and egg albumen quality. Poult. Sci. 33, 624-628.

FAJARDO, T.A., ANANTHESWARAN, R.C. and PURI, V.M. 1996. Effect of cooling on crack development and mechanical behavior of eggshells. Appl. Eng. in Agric. 12, 49-55.

FAJARDO, T.A., ANANTHESWARAN, R.C., PURI, V.M. and KNABEL, S.J. 1995. Penetration of Salmonella enterifidis into eggs subjected to rapid cooling. J. Food Prot. 58, 473-477.

Food Safety and Inspection Service, USDA. 1998. Final rule: refrigeration and labeling requirements for shell eggs. Fed. Reg. 63( 166), 45663-45675.

techno lo^. Egg Ind. (9/10), 11-13.

in Eggs. J. Food Prot. 64, 1279-1285.

Fwd ho t . 58, 389-394.


FRANK, F.R., SWANSON, M.H. and BURGER, R.E. 1964. The relationships between selected physical characteristics and the resistance to shell failure of Gallus Domesticus eggs. Poult. Sci. 43, 1228-1235.

FRY, J.L. and NEWELL, G.W. 1957. Management and holding conditions as they affect the interior quality of eggs. Poult. Sci. 36, 240-246.

GAISFORD, M . J . 1965. The application of shell strength measurements in egg shell quality determination. Br. Poult. Sci. 6, 193-196.

GARIBALDI, J.A. and STOKES, J.L. 1958. Protective roleof shell membranes in bacterial spoilage of eggs. Food Res. 23, 283-290.

GAST, R.K. and BEARD, C.W. 1993. Research to understand and control Salmonella enteritidis in chickens and eggs. Poult. Sci. 72, 1157-1 163.

HAINES, R.B. and MORAN, T. 1940. Porosity of, and bacteria invasion through the shell of the hen’s egg. J. Hyg. 40, 453-461.

HAMILTON, R.M.G. 1982. Methods and factors that affect the measurement of egg shell quality. Poult. Sci. 61, 2022-2039.

HAMMACK, T.S., SHERROD, P.S., BRUCE, V.R., JUNE, G.A., SATCHELL, F.B. and ANDREWS, W.H. 1993. Growth of Salmonella enteritidis in grade A eggs during prolonged storage. Poult. Sci. 72, 373-377.

HUMPHREY, T.J. 1990a. Growth of Salmonellas in intact shell eggs: Influence of storage temperature. Vet. Rec. 126, 292.

HUMPHREY, T.J. 1990b. Heat resistance in Salmonella enreriridis phage type 4: The influence of storage temperatures before heating. J. Appl. Bacteriol.

HUMPHREY, T.J. 1994. Contamination of eggshell and contents with Salmonella enteritidis: A review. Intern. J. Food Microbiol. 21, 31-40.

KEENER, K.M., LACROSSE, J.D., FARKAS, B.E., CURTIS, P.A. and ANDERSON, K.E. 2000a. Gas exchange into shell eggs from cryogenic cooling. Poult. Sci. 79, 275-280.

KEENER, K.M., LACROSSE, J.D., CURTIS, P.A., ANDERSON, K.E. and FARKAS, B.E. 2000b. The influence of rapid air cooling and carbon dioxide cooling and subsequent storage in air and carbon dioxide on shell egg quality. Poult. Sci. 79, 1067-1071.

KIM, C.J., EMERY, D.A., RINKE, H., NAGARAJA, K.V. and HALVORON, D.A. 1989. Effect of time and temperature on growth of Salmonella enteritidis in experimentally inoculated eggs. Avian Dis. 33,

KRAFT, A.A., ELLIOTT, L.E. and BRANT, A.W. 1958. The shell membrane as a barrier to bacterial penetration of eggs. Poult. Sci. 37, 238-240.

LIFSHITZ, A., BAKER, R.C. and NAYLOR, H.B. 1964. The relative importance of chicken egg exterior structures in resisting bacterial penetration. J. Food Sci. 29, 94-99.

69, 493-497.

735-742.


LIN, J., PURI, V.M. and ANANTHESWARAN, R.C. 1996. Strains in eggshell during cooling of eggs-measurement and prediction using the finite element method. Trans. ASAE. 39, 1005-1012.

MANCEAU, J.R. and HENDERSON, J.M. 1970. Stress analysis of eggshell. Trans. ASAE. 13, 440-443.

MASON, J. 1994. Salmonella enteritidis control programs in the United States. Intern. J. Food Microbiol. 21, 155-169.

MAYES, F.J. and TAKEBALLI, M.A. 1983. Microbial contamination of the hen's egg: a review. J. Food h t . 46, 1092-1098.

MERMELSTEIN, N.H. 2000. Cryogenic system rapidly cools eggs. Food Technol. 54(6), 100-102.

PROUDFOOT, F.G. 1962. The decline of internal egg quality during storage at 30F and 70F among six strains of Leghorns reared in confinement and on range. Poult. Sci. 41, 98-103.

REHKUGLER, G.E. 1973. Thermally induced stresses in eggshells. J. Agric. Eng. Res. 18, 275-279.

RHORER, A. 1987. Troubleshooting egg breakage in processing plants. Egg

SAEED, A.M. and KOONS, C.W. 1993. Growth and heat resistance of Salmonella enteritidis in refrigerated and abused eggs. J. Food Prot. 56,

SAUTER, E.A. and PETERSEN, C.F. 1974. The effect of egg shell quality on penetration by various Salmonellae. Poult. Sci. 53, 2159-2162.

SCHOENI, J.L., GLASS, K.A., MCDERMO'IT, J.L. and WONG, A.C.L. 1995. Growth and penetration of Salmonella enteritidis, Salmonella heidelberg and Salmonella typhimurium in eggs. Intern. J. Food Microbiol.

STEPHENSON, H.P., DAVIS, B.M., MAYER, R.J. and REID, D.J. 1997. Egg quality under tropical conditions in north Queensland 3. effect of oiling and storage temperature on egg quality - further experiments. Food Aust.

U.S. Department of Agriculture. 1993. Proposed rules: chicken disease caused by Salmonella enteritidis. Fed. Reg. 58, 41048-41061.

U.S. Food and Drug Administration. 1999. Bacteriological Analytical Manual, 8th Ed., AOAC Intern., Gaithersburg, MD.

VADEHRA, D.V., BAKER, R.C. and NAYLOR, H.B. 1970. Role of cuticle in spoilage of chicken eggs. J. Food Sci. 35, 5-6.

VOISEY, P.W., HAMILTON, R.M.G. and THOMPSON, B.K. 1979. Laboratory measurements of eggshell strength. 2. The quasi-static compression, puncture, non-destructive deformation, and specific gravity methods applied to the same egg. Poult. Sci. 58, 288-294.

Ind. 11, 20-24.

927-93 1.

24, 385-396.

49, 411-414.


WILLIAMS, K.C. 1992. Some factors affecting albumen quality with particular reference to Haugh unit score. World’s Poult. Sci. J. 48, 5-16.

WONG LIONG, J.W., FRANK, J.F. and BAILEY, S. 1997. Visualization of eggshell membranes and their interaction with Salmonella enteritidis using confocal scanning laser microscopy. J . Food Prot. 60, 1022-1028.

effect of rapid cooling of shell eggs on microcrack development, penetration of salmonella...

Documents