D CONTROL (shell eggs)

Summary

Signiﬁcant hazards a r Salmonellae, especially Salmonella Enteritidis.

Control measures

Initial level (H 0 ) r Keep to appropriate farm measures (rearing of ﬂock controls, farm hygiene, elimination of contaminated ﬂock, etc.).

r Remove cracked eggs. Increase (ΣI)

r Keep eggs refrigerated (best below 8 ◦ C). r Avoid free water on the eggs: dry well if washed, store under suitable RH,

avoid condensation due to changes in temperature. Reduction (ΣR)

r Wash the eggs, where possible; chlorinated water can be used. Note: washing will reduce eggs from Grade A to Grade B in Europe.

Testing

r Salmonella monitoring program including environmental, line, ﬁnished product, and critical ingredients to verify effectiveness of preventive

measures such as zoning.

Spoilage

r Keep shell eggs dry and chilled to prevent spoilage.

Control measures. Control of bacteria in shell eggs requires an integrated effort that begins at the egg production facility and ends with the consumer. Laying hens should be raised under conditions that minimize stress and environmental contamination of the egg after it is laid. Cages, litter, and nesting materials should be clean and kept as free of feces as possible. Eggs should be collected at least daily; as often as every 4 h is ideal. Eggs should be kept dry at this and all other stages of handling, transport and sale. All eggs should be stored with the blunt end upward to prevent migration of the yolk. Measures taken in egg-production were reviewed by Humphrey (1994b).

While there continues to be debate over the need to refrigerate shell eggs, prompt chilling after collection to below 10 ◦

C retards growth of many spoilage and pathogenic bacteria. Cooling should be done to minimize damage to the cuticle and the shell, and only when the egg surface is dry to prevent aspiration of bacteria into the egg.

Eggs should be candled, using white-light and black-light candles, or in some other way inspected to remove as inedible spoiled, leaking, or otherwise unacceptable eggs. These techniques help segregation of eggs with punctured yolks, shell cracks, and have practical quality control applications.

C or higher, so that the wash water is at least 12 ◦ C warmer than the eggs. The water should be potable and low in iron content. It should contain an alkaline detergent, such as sodium metasilicate or trisodium phosphate, and should be continuously replenished to allow an overﬂow. The cleaned eggs should be rinsed in a spray of fresh water containing a suitable disin- fectant such as chlorine at 100–200 ppm, with this ﬁnal rinse being done at a temperature 1–2 ◦

If the eggs are washed, the water should be at 42 ◦

C warmer than the wash water. The washing machine should be emptied, cleaned, and reﬁlled with clean detergent solution at least daily. Washing should be performed in a manner that minimizes damage to the cuticle.

The shells should be dried immediately after washing, and recooled to below 15 ◦

C (preferably below

10 ◦ C). Shell eggs should not be frozen, since freezing can damage the shell. Movements in and out of storage should be in a manner that prevents condensation on the shell surface (sweating). All surfaces in contact with the shells should be clean and dry. The humidity of storage facilities should be maintained

617 between 70% and 80% RH, avoiding changes in temperature leading to condensation, to ensure that

EGGS AND EGG PRODUCTS

the surface of the egg remains dry without accelerating the loss of moisture from the egg and the concomitant loss of quality.

Special considerations. Salmonella is the primary pathogen of concern, and its control requires inter- ventions that disrupt both transovarian and trans-shell contamination of the egg. The general considerations outlined above have a beneficial impact on the control of salmonellae. There are a number of other potential means for reducing or controlling the incidence of Salmonella, including S. Enteritidis, particularly at the production level. The acquisition of birds from parent flocks that are salmonella-free is an important means of reducing the number of contaminated flocks. Other control measures include chlorination of drinking water, competitive exclusion (Seuna and Nurmi, 1979), cleaning and disinfec- tion of poultry houses between occupation by laying flocks (Schlosser et al., 1999), and vermin control programs. A number of control programs have included microbiological testing of the laying environment as a means of identifying infected flocks (ACMSF, 2001), but the efficacy of this approach and the

actions that should be taken once a positive flock has been identified are not agreed internationally. The multiplication of S. Enteritidis and other salmonellae in contaminated eggs can readily be controlled by refrigeration. Routine microbiological testing of shell eggs for S. Enteritidis, other Salmonella serovars, or other pathogens is not recommended due to the low frequency of contamination. However, occasional testing as part of a HACCP verification program can provide useful information concerning the adequacy of control programs over time. Control programs in Canada, Netherlands, Sweden, the United Kingdom, and the United States are summarized by Altekruse et al. (1993) and more recently in the United States by Schlosser et al. (1999).

Adequate cooking of shell eggs so that the yolk is no longer soft is a means by which the ﬁnal user can be assured that any salmonellae present are inactivated.

IV Liquid eggs

Shell eggs ﬁt for human consumption may be separated from their shells to produce liquid, concentrated, dried, crystallized, frozen, quick frozen, coagulated, or reduced cholesterol products. Such products have been produced from hens’, ducks’, turkeys’, guinea fowls’, or quails’ eggs, but not a mixture of eggs from different species. The liquid egg may be homogenized as whole egg or separated into white and yolk. Salt, sugar, or acidulants may be added to liquid eggs destined for further processing. All liquid egg products should be pasteurized, chilled, ﬁlled into containers or tanks, and shipped refrigerated or frozen.

A Effects of processing on microorganisms Breaking, separating, and homogenizing. The initial microﬂora of liquid egg consists of a diverse

mixture of Gram-positive and Gram-negative bacteria that come from (i) the shell which is often contaminated with fecal and other matter; (ii) an occasional contaminated egg content, (iii) processing equipment (such as breaking utensils, pipes, pumps, filters, pails, churns, and holding tanks) and the processing environment, and (iv) food handlers. Unless carefully designed, equipment used continuously for long periods of time may be difficult to clean, and there can be pockets of liquid and semi-stagnant films where bacteria will accumulate and multiply.

Unless eggs are already clean, they should be washed immediately before the breaking operation. This operation should be done in a separate room in order to prevent cross-contamination. They need not be dried after washing so long as they drain enough that water from the shells does not run di- rectly into the liquid egg product (USDA, 1975b; EC, 1989). Washing dirty eggs before breaking can

MICROORGANISMS IN FOODS 6

Figure 15.2 Contamination of liquid egg by the machine-breaking of eggs infected with Serratia marcescens (Kraft, 1967b). Each sample represents the contents of 102 eggs. Sample 1–4 were from eggs infected with S. marcescens. Samples 5–10 were

from fresh uninfected eggs broken on the same machine after samples 1–4, without cleanup.

reduce the aerobic plate counts of liquid egg by several orders of magnitude (Penniston and Hedrick, 1947).

A single spoiled egg can contaminate breaking equipment and add millions of bacteria to the liquid egg. Candling before the breaking operation can detect most spoiled eggs, but some types of spoilage are hard to detect. For example, fluorescent rots by Pseudomonas spp. are difficult to see using only a white- light candler. Their mild odors are also deceiving (Johns and Berard, 1945; Elliott, 1954; Mercuri et al., 1957). Similarly, the Acinetobacter–Moraxella group can produce colorless rots that can enter the liquid egg undetected. With automatic breaking machines, examination for rots is even more difficult. Efficient use of such machines depends on a uniform supply of sound, clean, unspoiled eggs (Forsythe, 1970). The use of cracked eggs increases the rate of contamination by salmonellae and spoilage organisms in egg mixes (Baker, 1974). The use of incubator rejects eggs, meaning eggs that have been subjected to incubation but removed from it as infertile or unhatchable, is not allowed in some countries (EU, the United States, and Canada). A spoiled egg can contaminate breaking equipment and subsequent liquid egg product (Figure 15.2).

Crushing the entire egg, followed by separation of the shell by centrifugation is a technique that has been used to produce liquid egg, although this procedure is not allowed in the EU. The mixing of the melange with shell can lead to heavy contamination, even when the eggs are surface disinfected, rinsed, and dried immediately before crushing. Several countries have regulations that disallow the use of crushing technology.

Homogenization of liquid eggs is usually accomplished in a large mixing vat called a churn or in continuous homogenizer systems. Here the individual eggs, white, or yolks are thoroughly mixed. During this process, microbial contamination is uniformly distributed throughout the batch. Egg mixes should be further processed immediately. If this is not possible, the product should be immediately put into short-term storage at temperatures of not >4 ◦ C.

Pasteurization. Salmonella is the target pathogen for which egg pasteurization treatments were designed. Fortunately, salmonellae are not particularly heat resistant. However, in egg products, surrounded by proteins and fats, their heat resistance increases. The times and temperatures that kill salmonellae are at or near temperatures that adversely affect the physical and functional properties of egg products. The albumen is the most sensitive; it is denatured in a very few minutes at or above 60 ◦

C. Homogenized whole egg and yolk are reasonably stable at this temperature.

EGGS AND EGG PRODUCTS

Table 15.13 Pasteurization temperatures and times for liquid whole egg required by regulations in various countries (Cunningham, 1990)

Country Time (s) a Temperature ( ◦ C) Australia

United States

a Times for average particle. With higher temperatures turbulence within the mix must be increased to minimize heat damage to the eggs.

Recommended pasteurization temperatures for liquid eggs that have received no chemical additives vary from 55.6 ◦

C, and the times of exposure vary from 10 to 1.5 min. Lower temperatures and shorter times increase the risk of survival of salmonellae, whereas higher temperatures and longer times increase the damage to functional properties (whipping, emulsifying, binding, coagulation, ﬂavor, texture, color, and nutrition) (Forsythe, 1970). The minimum pasteurization temperatures and times required by various countries vary substantially (Table 15.13).

C to 69 ◦

The thermal resistance of salmonellae, including S. Enteritidis, in liquid egg products is dependent on the physical and chemical characteristics of the individual products. Furthermore, thermal resistance varies among Salmonella serovars and strains. Examining 17 strains of S. Enteritidis, Shah et al. (1991)

reported that the D 57.2 ◦ C and D 60 ◦ C values in liquid whole egg ranged from 1.21 to 2.81 min and 0.20 to 0.52 min, respectively. There was no increased resistance in strains from egg-associated outbreaks of gastroenteritis. However, Humphrey et al. (1993, 1995) reported that among S. Enteritidis phage type 4 isolates, stationery phase cultures of clinical isolates had substantially greater resistance to heat, acid, and hydrogen peroxide than chicken or egg isolates. Stationary phase cells were ∼10-fold

more heat resistant than log phase cells. Garibaldi et al. (1969b) reported that the D 60 ◦ C value for S. Typhimurium in liquid whole egg and yolk was 0.27 and 0.40 min, respectively. There are differences in thermal resistance among isolates and phage types of Salmonella Enteritidis. Palumbo et al. (1995)

found that the D 60 ◦ C value for four strains of S. Enteritidis in yolk ranged from 0.5 to 0.75 min, with z-values ranging from 4 ◦

C. Single strains of S. Senftenberg and S. Typhimurium had D 60 ◦ C values of 0.73 and 0.67, respectively, with z-values of 4.1 ◦

C to 6 ◦

C. Salmonella Enteritidis PT4 was somewhat more heat resistant than some poultry associated Salmonella isolates, but not to an extent that would impact pasteurization effectiveness (Humphrey et al., 1990). Heat resistance and acid tolerance of S. Enteritidis increase if cells are pre-exposed to elevated temperatures (37–48 ◦ C) (Shah et al., 1991; Humphrey et al., 1993). Garibaldi et al. (1969b) reported that in liquid egg white

C and 3.2 ◦

S. Typhimurium had D 54.8 ◦ C and D 56.7 ◦ C values of 0.64 and 0.25 min, respectively. Palumbo et al. (1996) used a mixture of the six Salmonella isolates to evaluate thermal resistance in liquid egg white. The

C. Palumbo et al. (1995, 1996) found reasonable agreement between the thermal resistance values from sealed tube studies and plate pasteurizer studies. When evaluating the heat resistance of six strains of Salmonella (including Enteritidis, Heidelberg,

D 56.6 ◦ C value was 1.44 min with a z-value of 4.0 ◦

and Typhimurium) in liquid whole egg and shell eggs, Brackett et al. (2001) found that D 57.6 -values ranged from 3.05 to 4.09 min, with signiﬁcant differences between the strains (alpha = 0.05). When ∼ 7log 10 cfu/g of a six-strain cocktail was inoculated into the geometric center of raw shell eggs and the eggs was heated at 57.2 ◦

C, the D-values of the pooled salmonellae ranged from 5.49 to 6.12; a heating period of 70 min or more resulted in no surviving salmonellae being detected (i.e. an 8.7-log reduction per egg).

An atypical strain that is not destroyed by current egg pasteurization practices is S. Senftenberg 775W. Originally isolated from eggs in 1946, this strain has a heat resistance that is 10–20 times greater

MICROORGANISMS IN FOODS 6

Table 15.14 Reduction of numbers of various bacteria during pasteurization of liquid egg white Pasteurization scheme

Log 10 reduction of bacterial counts

◦ C Holding time (min)

> 3 Ayres and Slosberg (1949)

> 3 Ayres and Slosberg (1949) 60 “ﬂash”

> 3 Ayres and Slosberg (1949)

Kline et al. (1966)

> 5.4 Barnes and Corry (1969)

Table 15.15 Reduction of numbers of various bacteria during pasteurization of liquid whole egg Pasteurization scheme

Log 10 reduction of bacterial counts

◦ C Holding time (min)

Aerobic plate count

Gibbons et al.(1946)

> 4.8 Goresline et al. (1951)

> 4 Murdock et al. (1960)

Mulder and van der Hulst (1973)

> 2 Murdock et al. (1960) 66.1–62.8

2.5 > 5.1 > 6 > 3 Heller et al. (1962) 67–68

> 2 Murdock et al. (1960)

than other salmonellae (Osborne et al., 1954). In one study with albumen at pH 9.1, S. Senftenberg 775W had a D 57.8 ◦ C value at of 2.1–2.4 min, while that for S. Typhimurium was 0.125 min (Corry and Barnes, 1968). Among hundreds of isolates tested over a period of more than 30 years, no one has re-isolated this strain or found another strain with comparable heat resistance. Therefore, pasteurization times and temperatures have been designed to destroy the typical, less-resistant strains.

The degree of protection offered by pasteurization is related to the numbers of salmonellae originally present. Most recommendations for egg pasteurization reduce the levels of salmonellae in inoculated eggs by 1000–10000-fold (Tables 15.14 and 15.15). Generally, adequate pasteurization (adhering to recommended pasteurization times and temperatures) would give an adequate safety margin as it eliminate virtually all salmonellae present in unpasteurized liquid egg under hygienic processing operation conditions. Considering that a single contaminated egg would be mixed with a large number of Salmonella-free eggs, it is unlikely that a batch of egg mix with high levels of salmonellae would be encountered with a hygienic processing operation. However, this does emphasize the im- portance of using only raw eggs with an incidence and prevalence of salmonellae that is as low as possible.

Pasteurization reduces the aerobic plate count in liquid eggs by 100–1000-fold, usually to about 100 cfu/g (Tables 15.14 and 15.15). Survivors are mostly Micrococcus, Staphylococcus, Bacillus spp., and a few Gram-negative rods (Shaﬁ et al., 1970). Payne et al. (1979) found that the major organisms surviving after heating at 65 ◦

C for 3 min was Microbacterium lacticum and Bacillus spp. None of the isolates were capable of growth at 5 ◦

C, but several were capable of relatively rapid growth at 10 ◦

C and

15 ◦ C. Although the transmission of L. monocytogenes to humans via pasteurized egg products has not been documented as yet, the pathogens and other Listeria spp. have been isolated from liquid whole egg (Leasor and Foegeding, 1989; Moore and Madden, 1993) Characterizations of the heat resistance of

621 Table 15.16 The heat resistance characteristics of Listeria monocytogenes in liquid egg products

EGGS AND EGG PRODUCTS

Product D ◦ C value (min)

Reference Whole egg

z-value( ◦ C)

System/strains

D 51 14.3–22.6;

Foegeding and Leasor (1990) D 55.5 5.3–8.0; D 60 1.3–1.7; D 66 0.06–0.20

Sealed capillary tube, one strain

Yolk D 61.1 0.7–2.3;

Palumbo et al. (1995) D 63.3 0.35–1.28;

Sealed tubes, ﬁve individual

strains of L. monocytogenes

D 64.4 0.19–0.82

and one of L . innocua

White D 55.5 13.0;

Palumbo et al. (1996) D 56.6 12.0;

11.3 Sealed tubes, mixture of ﬁve

strains of L. monocytogenes D 57.7 8.3 and one L. innocua

L. monocytogenes (Table 15.16) indicate that current U.S. minimum pasteurization requirement (60 ◦ C for 3.5 min) would achieve a 2.1–2.7log 10 reduction in the pathogen in liquid whole egg (Foegeding and Leasor, 1990). A similar estimate (2.5log 10 reduction) was observed for the current minimum pasteurization requirement for liquid yolk (Palumbo et al., 1995). The extent of inactivation of L. innocua was <10-fold in 3.5 min when egg white was heated using both sealed ampoules and a plate pasteurizer at 56.6 ◦

C (Palumbo et al., 1996). It has been concluded that current minimum pasteurization requirements would be sufﬁcient to control L. monocytogenes in extended shelf life egg products only if the initial levels of the pathogen were low (Foegeding and Leasor, 1990; Foegeding and Stanley, 1990; Palumbo et al., 1995, 1996). Moore and Madden (1993) considered that current pasteurization practices were adequate based on the absence of Listeria spp. in 500 daily samples of the pasteurized product.

C and 57.7 ◦

Continuous pasteurization systems heat the liquid egg to the target temperature and then maintain it at that temperature for a specified amount of time through the use of holding tubes of appropriate lengths. Such pasteurizers require adequate control systems that can assure a constant rate of flow. This includes automatic equipment for temperature monitoring and recording, automatic control devices to prevent insufficient heating, and a safety system (including appropriate recording devices) that diverts inadequately heated product and prevents it from mixing with fully pasteurized product.

Humectants added to liquid egg products to decrease the water activity also increase the thermal resistance of salmonellae and other pathogens. For example, Palumbo et al. (1995) reported the D 63.3 ◦ C values for salmonellae in yolk, yolk + 10% sucrose, and yolk + 10% NaCl (a w s 0.989, 0.978, and 0.965) to be 0.21, 0.72, and 11.50 min. Inactivation of L. monocytogenes in these products gave D 63.3 ◦ C values for yolk and yolk + 10% sucrose of 0.81 and 1.05 min, while that for yolk + 10% NaCl was

10.5 min after an initial lag period of 14.8 min during which time there was no decline in pathogen levels. If salted yolks are destined for use in high acid salad dressing or high acid mayonnaise production, no pasteurization is necessary. However, greater care is needed to prevent post-processing contamination. The yolks can be pasteurized before the salt is added, taking precautions to avoid contamination. Acetic acid or other organic acids can be added to lower the pH to ≤4.6, where salmonellae die more rapidly. Salmonella Enteritidis may be more acid tolerant than S. Typhimurium (Humphrey et al., 1993). Acidiﬁed salted yolks can be pasteurized in 1 min at 60 ◦

C (Garibaldi, 1968).

Pasteurizer temperatures often cause egg material to coagulate on the hot surfaces of the heating plates (Ling and Lund, 1978), which adversely affects both the functional quality of the product and the effectiveness of the microbial inactivation. Therefore, research has centered on means to:

r Repair damaged quality by adding chemicals such as whipping aids at point of use. r Increase the sensitivity of salmonellae to heat by adding chemicals or altering pH r Prevent adverse effects on quality by adding chemicals before pasteurization.

MICROORGANISMS IN FOODS 6

Egg white is the most heat sensitive egg component. Heating unaltered egg white at 62 ◦

C for

3.5 min alters 3–5 % of the ovomucin, 90–100% of the lysozyme, and >50% of the conalbumin (Lineweaver et al., 1967). The US minimum heat treatment of 56.7 ◦

C for 3.5 min increases markedly the whipping time to prepare meringue. Even a minimal heat treatment of 3 min at 54.4 ◦

C doubled the whipping time, but whipping aids such as triethyl citrate or triacetin restored this function to near normal (Kline et al., 1965). In this temperature range, a 2 ◦

C rise in temperature increases the damage 2.5–3-fold, whereas it increases the destruction rate of salmonellae only 2-fold. The kinetics of quality loss vs. destruction of salmonellae, in combination with the build-up of coagulated material on pasteurizer plates at higher temperatures, makes it unlikely that unaltered egg white would be pasteurized at temperatures >60 ◦

C (Kline et al., 1965; Lineweaver et al., 1967). Authorities in the United Kingdom have recommended 57 ◦

C for 2.5–3 min (Corry and Barnes, 1968; Hobbs and Gilbert, 1978). Unaltered homogenized whole egg is less sensitive to damage, in part because iron from the yolk satisﬁes the chelating capacity of the conalbumin, and in doing so, stabilizes it (Cunningham, 1966). Egg yolk is relatively stable, but is difﬁcult to handle because of its high viscosity.

Lower heating temperatures can be used if H 2 O 2 is added to eggs at ∼0.1–1.0% when it increases the heat sensitivity of salmonellae. For example, Palumbo et al. (1996) reported that the

D 56.6 ◦ C value in liquid egg white was 1.44 min, while D 53.2 ◦ C value in liquid egg white treated with 0.875% H 2 O 2 was 1.54 min. Treatment with catalase after pasteurization breaks down excess H 2 O 2 (Lloyd and Harriman, 1957; Rogers et al., 1966). Palumbo et al. (1996) reported that the addition of H 2 O 2 did not enhance destruction of L. monocytogenes in liquid egg white. “Electroheating” has been used commercially as an alternative means of heating liquid egg products (Reznik and Knipper, 1994).

The thermal resistance of bacteria is also affected by the pH of the heating menstruum. Typically, bacteria are most resistant near their optimal pH for growth, and become increasingly less resistant as the pH deviates from this optimum. A freshly laid hen’s egg has a pH in the range 7.6–7.8, rising as the egg ages to 9.1–9.6. Adjusting the pH of egg white from its normal level near 9 to between 6.5–6.7 increases the stability of both the egg white and salmonellae to heat, but salmonellae less so than the egg white (Lineweaver et al., 1967). At pH values below 7, salmonellae become more susceptible to heating, especially in the presence of organic acids. The heat required to kill salmonellae in eggs can

be reduced substantially by adjusting the pH to 5.5 or 6 with citric, lactic, acetic, formic, or propionic acids (Lategan and Vaughn, 1964). When the pH of egg white is ≥9, salmonellae do not grow, but if the pH is adjusted to pH 6.8, they grow well (Banwart and Ayres, 1957). Care must be taken to ensure that pasteurized, acidiﬁed egg white is not recontaminated with salmonellae and then temperature abused.

As more eggs are produced speciﬁcally for liquid egg products, an increasing percentage of eggs are entering the breaking facility before the pH of the egg white has reached pH 9.1–9.6. Cotterill (1968) reported that the temperature to achieve a 99.99% (4-D) reduction of S. Oranienburg in egg white within a speciﬁed time increased from 55.0 ◦

C at pH 8.5. Garibaldi et al. (1969a) found that the D-value for S. Typhimurium in egg white was 4.6 times greater at pH 7 than at pH 9. Palumbo et al. (1996) reported that reducing the pH of egg white to 7.8 approximately triples the thermal resistance of S. Enteritidis (Figure 15.3). Interestingly, the opposite relationship was observed with L. monocytogenes, which Palumbo et al. (1996) attributed to the effect of pH on the inactivation of lysozyme.

C at pH 9.4 to 58.6 ◦

Altering the pH of egg white to increase the thermal destruction of bacteria has adverse effects on the egg white proteins, primarily conalbumin. The adjustment of the pH to 7 with lactic acid enhances the stability of ovalbumin, lysozyme, and ovomucoid (Cunningham and Lineweaver, 1965). The addition

EGGS AND EGG PRODUCTS

Figure 15.3 Effect of pH on the D 56.6 ◦ C value for Salmonella Enteritidis and Listeria monocytogenes heated in liquid egg white (adapted from Palumbo et al., 1996).

of a metal salt satisﬁes the chelating activity of the conalbumin, and makes it relatively stable to heat. Either Fe 3+ or Al 3+ will work, but because Fe 3+ turns the albumen pink, Al 3+ as aluminum sulfate is the metal of choice (Cunningham, 1966). With the adjustment of pH to 7 and the addition of Al 3+ egg white

can be pasteurized at 60 ◦ –62 ◦

C for 3.5–4 min. The total amount of protein altered is <1%; however, whipping time is still increased and the white requires the addition of a whipping aid (Lineweaver et al., 1967). Aluminum sulfate is not permitted as a food additive in some countries.

Other compounds also stabilize conalbumin or decrease Salmonella resistance. The addition of 0.5– 0.75% sodium polyphosphate in egg white permits effective destruction of salmonellae at 52.2–55 ◦

C for

3.5 min, without damage to functional properties (Chang et al., 1970; Kohl, 1971). Disodium ethylene- diamine tetra-acetic acid (EDTA) at 7 mg/mL of egg white killed 10 6 salmonellae in >24 h at 28 ◦

C, and

C. When the egg white was adjusted with lactic acid to pH 5.3, and EDTA added at 7 mg/mL, Salmonella heat resistance decreased 100-fold. These sequestrants make Ca 2+ and Mg 2+ unavailable to the microorganisms. Microorganisms then become susceptible to attack by lysozyme (Garibaldi et al., 1969a).

70 mg of sodium polyphosphate per mL of egg white killed 10 6 salmonellae in 60 h at 28 ◦

Filling and chilling. Once an egg product has been pasteurized, it must be handled with care to prevent recontamination from unpasteurized egg, insanitary equipment, containers, dust, or human or animal sources. It must also be chilled quickly, preferably using a heat exchanger, or if not, then ﬁlled into

cans and cooled within 1.5 h to 7 ◦

C or below to prevent growth of any surviving microorganisms. The temperature of the egg product should pass quickly through the temperature range that supports rapid microbial growth (50–7 ◦ C).

Salt. The addition of 10% salt to yolks decreases the water activity (a w ) of the egg yolk to about 0.90 (i.e. 20.3 g salt in 100 g of water phase). Salmonellae will not grow at this a w regardless of temperature, and die off within in a matter of weeks (Banwart, 1964; Cotterill and Glauert, 1972; Ijichi et al., 1973). However, when unpasteurized 10% salted yolks are shipped and used immediately after manufacture, die-off may not be complete and the product contain salmonellae at point of use.

Freezing and thawing. Containers of liquid eggs to be frozen should be placed in a freezer at −23 to −40 ◦

C immediately after chilling and should be stacked in such a manner that they will freeze

MICROORGANISMS IN FOODS 6

Table 15.17 Effect of freezing on the microﬂora of pasteurized and unpasteurized liquid whole egg (Wrinkle et al., 1950)

Pasteurized Genera

Unpasteurized

Before freezing After freezing Acinetobacter–Moraxella

Before freezing

After freezing

29 27 4 0 Gram cocci

– a Percent of isolates.

promptly. Temperatures for prolonged storage should be at or below −18 ◦ C; a few microorganisms can grow slowly at or above −10 ◦

C (Michener and Elliott, 1964).

Freezing and frozen storage reduce the numbers of microorganisms but usually not to the point of extinction of a given strain, particularly not in the protective proteinaceous egg matrix. While freezing and frozen storage reduce salmonellae, these cannot be relied on to eliminate the pathogen. L. monocytogenes levels remained unchanged in frozen (−18 ◦

C) liquid whole egg during 6 months of storage (Brackett and Beuchat, 1991). An example of the effects of freezing on pasteurized and unpasteurized whole egg is depicted in Table 15.17.

Improper thawing can lead to unacceptable increases in microbial content (Forsythe, 1970). For example, when a can of liquid eggs is allowed to thaw completely in a warm room, the temperature of the outside portion will have been in the temperature range that supports bacterial growth for many hours. Frozen products should be thawed under conditions that permit the temperature of the thawed material to rise to >4 ◦

C for only short periods. Some manufacturers thaw in a refrigerator at about

4 ◦ C; others immerse the cans in cold running water; others have installed crushers to break up the partly frozen mass; still others remove by centrifuge or otherwise, the liquid egg material as it thaws (Lawler, 1965).

Special treatment with alcohol. When used for the production of egg containing liqueurs, egg products receive less than the recommended heat treatment. The preservative effect of alcohol kills salmonellae within 6 days of storage when the alcohol content is 13% and above (Bolder et al., 1987; Warburton et al., 1993).

Irradiation. It is technically feasible to control salmonellae by using ionizing radiation (Comer et al., 1963; Schaffner et al., 1989; Slater and Sanderson, 1989; Kijowski et al., 1994). In the United States, ionizing radiation in doses up to 3.0 kGy may be used on shell eggs. A dose of 3.0 kGy is sufﬁcient to eliminate salmonellae and Enterobacteriaceae from liquid egg white, egg yolk, and whole egg with and without added 50% sugar or 11% salt. Salmonella Enteritidis appears to be somewhat more radiation resistant than S. Typhimurium (Thayer et al., 1990). The combination of irradiation and thermal processing has been shown to reduce the requirements of both treatments to eliminate S. Enteritidis in liquid whole egg (Schaffner et al., 1989).

EGGS AND EGG PRODUCTS

B Spoilage The contaminating organisms at time of breaking are primarily those on the egg-shell (Table 15.10) and

within the occasional spoiled egg. If not pasteurized immediately, the broken out eggs must be cooled promptly to 7 ◦

C or below, especially if cracked or dirty eggs were used. After the liquid eggs are pasteurized, they should again be cooled promptly. Although organisms that spoil eggs under refrigeration have largely been destroyed (Speck and Tarver, 1967), a delay in cooling will permit mesophilic bacteria to grow and will permit the few psychrotrophs present to build up rapidly. In one study, unpasteurized whole eggs held at 4 ◦

C for 8–10 days had bacterial levels of

3 × 10 6 cfu/g (Steele et al., 1967). Pasteurized eggs, frozen immediately and then stored at 4 ◦

C for

C for 24 h before freezing, followed by storage at 4 6 ◦ C, developed 3 × 10 cfu/g in 24 days.

45 days, had fewer than 100 cfu/g. Pasteurized eggs held at 13 ◦

The refrigerated shelf life of pasteurized egg products (i.e. time until spoilage is evident) is remarkably long. Clean eggs that are broken, cooled, mixed, pasteurized, and cooled under ideal sanitary conditions, remain edible for 20–22 days in the refrigerator (Wilkin and Winter, 1947; Kraft et al., 1967a). If made from dirty eggs, even though the egg product is pasteurized, the refrigerated shelf life may be only 2 or

3 days; more microorganisms will survive because of the high original level of contamination (Baker, 1974). Most samples of pasteurized eggs from commercial operations in the United States had a shelf life of 12–15 days at 2 ◦

C (Vadehra et al., 1969; York and Dawson, 1973). Before pasteurization came into general use in Europe and North America during the 1960s and early 1970s, the shelf life of refrigerated whole egg was only 5–7 days (Wilkin and Winter, 1947; Wrinkle et al., 1950). Ultrapasteurization systems (i.e. heating over 60 o

C for <3.5 min) in combination with aseptic packaging systems can produce whole egg products with signiﬁcantly extended shelf lives, e.g. 3–6 months at 4 ◦

C (Ball et al., 1987). It was concluded that ultrapasteurization could be used to effectively produce liquid whole egg that is free from L. monocytogenes (Foegeding and Stanley, 1990).

Salted yolks, particularly when batch pasteurized and ﬁlled hot into cans, have a long shelf life, even without rapid cooling, and stored at room temperature, because the bacterial vegetative cells capable of growth in 10% salt have been killed by the heat. Eventually a few spores may germinate and grow (Cotterill et al., 1974).

Pasteurization destroys microorganisms such as Pseudomonas, Acinetobacter, and Enterobacter spp.,which grow in raw albumen. This leaves mesophilic organisms like micrococci, staphylococci, Bacillus spp., enterococci, and catalase-negative rods able to grow if the product is temperature abused (Barnes and Corry, 1969; Shaﬁ et al., 1970). In one investigation, the primary microorganisms surviving ultrapasteurization were Bacillus circulans, a Pseudomonas isolate, and Enterococcus faecalis (Foegeding and Stanley, 1987). The pseudomonad and the enterococcus were capable of growth at 4 ◦ C and 10 ◦

C, respectively. Table 15.18 lists the changes produced by different genera when growing in pure culture in liquid whole egg. Most of the genera would not be expected to survive pasteurization, and their presence would indicate post-pasteurization contamination. The odors of spoilage are much more intense in yolk

or whole egg than in white. In white, isolated strains caused no putrid odor or H 2 S (Imai, 1976). Most spoilage organisms reduced the pH of the white slightly and produce a small amount of trimethylamine. On the other hand, the yolk develops ﬁshy, moldy, and ammoniacal odors, with high levels of H 2 S and trimethylamine and a high total volatile base level.

C Pathogens Salmonellae, the most important pathogens in liquid egg, are discussed in detail in sections describ-

ing the effect of processing procedures on microorganisms and methods of control. Since the heat

MICROORGANISMS IN FOODS 6

Table 15.18 Changes produced by different genera of bacteria originally isolated from liquid egg and then inoculated in pure culture into sterile egg (Wrinkle et al., 1950)

Number of

Genus strains inoculated Changes produced Acinetobacter–Moraxella

1 No change in 72 h

Enterobacter

4 Slight acid odor after 72 h

Alcaligenes 17 12 very sour odor in 60 h, 1 musty odor in 48 h, 4 no detectable change Bacillus

8 6 coagulation within 18 h, 1 very sour odor in 24 h, 1 no detectable change Chromobacterium

2 No change in 72 h

Escherichia 15 4 slight acid odor in 60 h, 8 sour odor after 60 h, 3 no detectable change Flavobacterium

11 2 coagulation in 120 h, 1 fecal odor after 60 h, 4 slight odor in 120 h, 4 no change in 72 h

Proteus 10 3 very ﬂat-sour odor in 60 h, 2 coagulation in 18 h, 4 very sour odor in

60 h, 1 no detectable change

Pseudomonas 7 3 very sour odour in 60 h, 2 gas within 60 h,1 sour odor in 60 h, 1 no detectable change

Gram + cocci 3 2 produced gas in 18 h, 1 no detectable change

treatments used for liquid egg products do not produce shelf stable products, they should be held refrigerated (Gibbons et al., 1944). Salmonellae grow rapidly in yolks either aerobically or anaerobically (Lineweaver et al., 1969; Lawler, 1965).

C. Staphylococci are a potential hazard in salted yolks because they grow readily at the reduced a w (0.90) of the product. If they survive pasteurization or are re-introduced post-pasteurization, they must reach levels of 10 5 cfu/mL, or more, before toxin forms. This would require severe temperature abuse, e.g. storage of the egg product for several days at ambient temperatures (Ijichi et al., 1973). It would also require the presence of sufﬁcient oxygen since staphylococcal enterotoxins synthesis would not be expected at this a w under anaerobic conditions.

Staphylococcus aureus grows well in liquid whole egg held above 15.6 ◦

While pasteurized liquid egg products have not been associated with cases of human listeriosis to date, the presence of L. monocytogenes in unpasteurized processed eggs (Leasor and Foegeding, 1989; Nitcheva et al., 1990; Moore and Madden, 1993) has become a significant concern, particularly in products with extended refrigerated shelf life. A survey of commercially broken raw eggs from 11 establishments in the United States detected Listeria spp. in 36% of the samples (Leasor and Foegeding, 1989). Listeria innocua was found in each of the 15 positive samples, whereas L. monocytogenes was present in only 2 (5%). However, it is often difficult to isolate low levels of L. monocytogenes when present with L. innocua. In another survey, Listeria spp. were isolated from 72% of in-line filters from the processing of raw blended whole egg, with L. innocua isolated from 62% and L. monocytogenes from 38% (Moore and Madden, 1993). The mean level of Listeria spp. in the raw liquid whole egg was one organism per mL.

L. monocytogenes has been observed to grow in raw and pasteurized liquid whole egg and yolk at temperatures ranging from 5 ◦

C, but was inactivated in unpasteurized liquid albumen at pH 7.0–8.9, presumably due to the action of lysozyme (Khan et al., 1975; Foegeding and Leasor, 1990; Sionkowski and Shelef, 1990). Schuman and Sheldon (2003) investigated the use of nisin in pH-adjusted liquid whole egg, and found that the bacteriocin could delay or prevent the growth of L. monocytogenes. The generation times among different L. monocytogenes strains in liquid whole egg ranged from 24–51 h at 4 ◦

C to 30 ◦

C (Foegeding and Leasor, 1990). The pathogen survived for extended periods in liquid whole egg stored at 0 ◦

C and 8–31 h at 10 ◦

C (Brackett and Beuchat, 1991). Liquid egg products appear to have a low potential for becoming a source of enterohemmorrhagic Escherichia coli, since its prevalence in eggs appears low and current pasteurization temperatures are sufﬁcient to inactivate the microorganism (Erickson et al., 1995).

EGGS AND EGG PRODUCTS

D CONTROL (shell eggs)