D CONTROL (liquid eggs)

D CONTROL (liquid eggs)

Summary

Significant hazards a r Salmonellae, especially Salmonella Enteritidis. r L. monocytogenes for products with extended refrigerated shelf life.

Control measures

Initial level (H 0 ) r Same control measures as mentioned for shell-eggs apply. r Somewhat higher H 0 may be expected, due to common practice to use eggs of lesser quality for production of liquid egg products. H 0 up to

100 cfu/g Salmonella has been reported.

Increase (ΣI) r Limit time between lay and processing. r Use high quality, intact eggs. r Chill quickly to below 7 ◦

C after break-out and pasteurization. r Keep product at as low a temperature as possible, ideally below 3 ◦

C to

minimise pathogen proliferation. r Sanitize equipment; use proper process hygiene.

r Freezing promptly can be used for long shelf life products. Reduction (ΣR)

r Pasteurization aiming at a 4 to 5D reduction for salmonellae. r Pasteurization; 60 ◦

C for 3.5 min will achieve 2.1–2.7 D reduction

(Foegeding & Leasor, 1990). r Lower temperatures need to be used with liquid egg white products,

compared with liquid whole egg (yolk and egg white not separated) product.

Testing

r Periodic verification testing for salmonellae. r Coliform, E. coli, or Enterobacteriaceae may be useful indicators for

process control. r Environmental monitoring for salmonellae would be useful in post

pasteurization areas. r α − Amylase activity testing may be useful in certain situations; see

subsequent discussion.

Spoilage

r Freeze liquid eggs for prolonged shelf life. r Good Hygienic Practices are essential to control spoilage.

Hazards to be considered. Salmonellae are well recognized as a significant hazard in egg products. L. monocytogenes needs to be considered in products with extended refrigerated shelf life, although there is no epidemiological evidence of human listeriosis associated to liquid egg products.

Control measures. To prevent heavy bacterial contamination of liquid egg, eggs for breaking should

be washed, candled to remove rots, and examined for odor and appearance at breakout. Equipment that contacts contaminated eggs at breakout should be washed and sanitized before reuse (Forsythe, 1970). Ideally, automated equipment should be designed so that it is cleaned and sanitized on a continuous basis. All product-contact equipment such as pipes, churns, tanks, and pails should be thoroughly cleaned and sanitized at least daily. Equipment should be designed and installed for easy cleaning, as for instance described in E-3-A Sanitary Standards (IAMFES, 1976a–d).

MICROORGANISMS IN FOODS 6

C or below promptly after breakout and after pasteurizing. If the product is to be frozen, cans of eggs should be placed promptly into a freezer at −23 to −40 ◦

To slow microbial growth, liquid egg products should be chilled to 7 ◦

C and stored at or below −18 ◦ C.

Pasteurizers should have the following characteristics (Murdock et al., 1960; Lineweaver et al., 1969; Forsythe, 1970; Kaufman, 1969, 1972): r Automatic flow control, including flow diversion valves to divert inadequately heated egg. r Automatic control of temperature. r Higher pressure on the pasteurized side than on the unpasteurized to prevent raw melange from leaking

into the pasteurized material. r Recording thermometers at entrance and exit of the pasteurizer and the cooler.

r Leak detection devices. Because of the marginal nature of the thermal processing used to pasteurize liquid egg products, it

important that the levels of Salmonella and L. monocytogenes are as low as possible in the raw product. Excessively high levels of these pathogens will not be completely inactivated. This requires that eggs used have both a low numbers and prevalence of the two pathogens. Particular care is needed for products intended for extended refrigerated storage due to the inherent heat resistance of L. monocytogenes, its ubiquitous presence in processing environments, and its ability to grow at refrigeration temperatures. In addition to adequate heat treatment, such products should be kept as cold as possible, preferably close to freezing to avoid or minimise pathogen proliferation. Temperature abuse substantially increases the risk of growth of both pathogens.

Testing. The efficacy of integrated processes used to produce and pasteurize liquid egg products should

be validated and periodically verified by appropriate laboratory testing to assure that they are capable of achieving the degree of microbial inactivation needed. The egg products industry has traditionally relied heavily on microbiological testing programs that have focused on testing for salmonellae and either coliforms, E. coli, or Enterobacteriaceae. These organisms should not be found in the pasteurized material. The rationale for testing for salmonellae is obvious, i.e. the pasteurization is designed to elim- inate this organism. However, most of the egg entering the pasteurizer does not contain salmonellae, so coliforms, E. coli, or Enterobacteriaceae have been used as process integrity indicators (i.e. heating sufficiency). These indicator organisms are virtually always present in raw egg melange and their heat resistance is similar to that of the salmonellae. Escherichia coli is also an indicator of temperature abuse since it would also not be expected to reach substantial numbers unless the product was temperature abused. However, commercially produced egg products sometimes (though rarely) contain salmonellae in 25 g portions when other Enterobacteriaceae are not detected in 0.1 or 1 g samples (van Schothorst and van Leusden, 1977). A positive finding for Enterobacteriaceae or coliforms could mean either that pasteurization was inadequate or that post-pasteurization contamination occurred. While microbiolog- ical testing of this type helps verify process adequacy, the level of testing is insufficient to assure safety on a lot-by-lot basis. The resources that have traditionally been expended in testing of this type would

be better used to develop, validate, and implement better process controls. In some countries (e.g. the United Kingdom), an enzyme assay for α-amylase is used to verify the efficacy of pasteurization (Brooks and Shrimpton, 1962; Murdock et al., 1960; Shrimpton et al., 1962). At the temperatures and times preferred in the United Kingdom (64.4 ◦

C for 2.5 min for liquid whole egg), α-amylase is destroyed. The α-amylase test is rapid, accurate, convenient, and inexpensive, whereas tests for microorganisms are slow, expensive, and often give variable results among workers and among laboratories. However, α-amylase is not destroyed by the lower heating temperatures preferred in the United States (60 ◦

C for 3.5 min; Lineweaver et al., 1969) and other countries, and is not applicable

629 in such instances. The α-amylase test cannot be used with salted or sugared eggs, and does not detect

EGGS AND EGG PRODUCTS

post-pasteurization contamination. Two methods are employed to detect the use of incubator reject eggs (Robinson et al., 1975):

r An enzyme assay comparable to the α-amylase test; r

A test for 3-hydroxy butyric acid, formed upon inhibition of embryonic growth within the egg.

V Dried eggs

A Effects of processing on microorganisms Three methods are widely used for the dehydration of liquid egg products. r Spray drying, where the liquid is atomized by squirting through a nozzle.

r Drying on a heated surface (pan or drum drying). r Freeze drying.

Water may be removed by ultra filtration or reverse osmosis before final drying. The microbiology of all three is essentially the same. Drying kills many of the bacteria initially

present in the liquid egg. However, once the egg material is dry, the microbial population is stabilized, and further declines occur only slowly with extended storage, even at ambient temperatures. Only rarely is there a complete extinction of surviving strains. The predominant microorganisms in the dried product are enterococci and aerobic spore-forming bacilli, the most resistant members of the original microflora. The number of salmonellae may be reduced as much as 4 logs during drying (Gibbons and Moore, 1944). Fermented albumen or temperature-abused whole egg can have high initial bacterial levels, such that survival of some bacteria is likely (Ayres and Slosberg, 1949; Gibbons et al., 1944). Salmonellae are the principal microbial problem in dried eggs, and the problem becomes more serious if they grow during fermentation for glucose removal.

Glucose removal (fermentation). Dried whites normally contain about 0.6% free carbohydrate, pri- marily as glucose. On storage, particularly above 15 ◦

C, the aldehyde group of the glucose combines with the amino groups of the proteins, reducing their solubility, causing off flavors, and forming insoluble brown compounds (Maillard reaction products). Removal of glucose from the liquid egg white before drying prevents these reactions. Glucose removal also improves to a lesser extent the stability of dried whole egg and egg yolk (Stewart and Kline, 1941; Paul et al., 1957; Forsythe, 1970; Kilara and Shahini, 1973).

The earliest method used to remove glucose was simply to permit the natural egg microflora to grow at temperatures of 21–29 ◦

C for 2–7 days. The length of time was judged by observations of bubbling, consistency, and clarity of samples. Scums and sediments were discarded and ammonia added to clear the liquid. The reactions were not well controlled, and often led to objectionable odors and proteolysis. This procedure allowed the growth of enterococci, Enterobacter aerogenes, and other bacteria. In addition, salmonellae could grow, presenting a health hazard (Ayres, 1958).

Some manufacturers add bacterial starter cultures to rapidly ferment egg whites. The temperature is raised to 35 ◦

C so that the glucose is metabolized within 12–24 h. Enterococci are favored; Enterobacter spp. and lactic organisms are not as competitive at the natural pH of egg white (>9.0), although all three organisms are used by different manufacturers. Enterococci do not cause proteolysis or off-odors, but do acidify the whites to approximately pH 6. This fermentation method may permit the rapid growth of salmonellae after the pH drops below 8.

MICROORGANISMS IN FOODS 6

Table 15.19 Commercial and experimental methods for the removal of glucose from liquid eggs Microorganism or agent

Reference Natural flora

Comment

Ayres (1958) Coliforms

Enterobacter, enterococci, and other bacteria

Stuart and Goresline (1942a, 1942b) Saccharomyces apiculatis

Early pure culture studies

Hawthorne and Brooks (1944) Saccharomyces cerevisiae

1% inoculum gave yeasty flavor

Yeasty odor from large inocula can be eliminated by

Ayes and Stewart (1947),

0.1% yeast extract which stimulates activity of

Hawthorne (1950), Carlin and

small inocula. Can centrifuge to remove yeast.

Ayres (1953), Kline and Sonoda (1951), Ayres (1958)

Streptococcus lactis and

Kaplan et al. (1950), Ayres Streptococcus faecalis

Resting cells, 37 ◦

C for 3 h, yeast extract at 0.1%

(1958), Galuzzo et al. (1994) subsp. liquefaciens Glucose oxidase and catalase

inhibits acid production.

Glucose oxidized to gluconic acid, catalase destroys

Baldwin et al. (1953), Carlin and

the H 2 O 2 that is formed.

Ayres (1953), Scott (1953), Paul et al. (1957), Ayres (1958)

Enterobacter aerogenes

Acetyl methyl carbinol production can be

Ayres (1958)

minimized by using small inocula with 0.1% yeast extract for stimulation.

Cell-free yeast extracts Desugars in 4–5 h at 5 ◦ C. Niewiarowicz et al. (1967) Escherichia coli

Mickelson and Flippin (1960), Flippin and Mickelson (1960) Lactobacillus brevis,

Shows antagonism for Salmonella.

Mulder and Bolder (1988) Lactobacillus casei,

C optimal temperature, lactobacilli eliminated

by pH adjustment prior to hot room treatment

Lactobacillus fermenti, Lactobacillus plantarum

It is generally recommended that manufacturers employ pure culture starters. The first step is to reduce the pH of the egg from its normal 9.0 to 7.0–7.5 with an organic acid such as lactic acid. The starter culture is then added, and allowed to ferment the available carbohydrate for 12–24 h at 30–33 ◦

C. It has been claimed that bacterial cultures are best because the finished product has high whipping quality, good odor, and solubility (Forsythe, 1970). However, others prefer alternative yeast or enzyme treatments (Table 15.19). Glucose oxidase treatment has been investigated as a potential means of inactivating Salmonella Enteritidis and other bacteria in liquid whole egg (Dobbenie et al., 1995).

In many of the fermentation procedures listed in Table 15.19, salmonellae can grow if the pH range of the egg white is reduced to 6–8, but not if the pH is maintained at ≥9 (Banwart and Ayres, 1957). After fermentation, pasteurization is essential to kill salmonellae (Kline and Sonoda, 1951; Ayres and Stewart, 1947), so that any present are not carried into the dried product. In the United States, whites are usually fermented, dried, and then pasteurized.

Destruction of salmonellae by hot storage. Despite the rather efficient procedures developed to pasteur- ize liquid eggs prior to drying, salmonellae are occasionally present in the final dried packaged product. This can be due to insufficient pasteurization, excessively high initial numbers, or post-pasteurization

contamination. After the product has been dried, microorganisms can still be destroyed by hot storage (hot room treatment).

Microorganisms are most heat sensitive when wet; their heat resistance increasing as the environment becomes dry. While the pasteurization times for a wet product are typically in the range of 2–5 min, those for dried egg products are several days. Examples of times and temperatures for “pasteurization” of dried egg white are listed in Table 15.20. These exposures have no serious effect on functional qualities.

631 Table 15.20 Times and temperatures of hot room storage to destroy salmonellae in dried egg albumen

EGGS AND EGG PRODUCTS

Pretreatment

Reference Fermented, pan dried

Temperature ( ◦ C)

Time (days)

48.9 20 Ayres and Slosberg (1949)

3% Moisture 50 9 Banwart and Ayres (1956) 6% Moisture

50 6 Banwart and Ayres (1956) Spray dried

54.4 7 USDA (1975b) Pan dried

51.7 5 USDA (1975b) Adjusted to pH 9.8 with ammonia, pan dried

49 14 Northolt et al. (1978) Treated with citric acid

55 14 Northolt et al. (1978) Spray dried

49 14 Northolt et al. (1978)

The advantage of hot storage over the standard wet pasteurization is that there is little possibility of recontamination as long as the containers remain closed during and after the treatment period. The bactericidal effect of the hot room treatment depends on the moisture content, the temperature, and the nature of treatments preceding the heat treatment (e.g. method of fermentation, use of ammonia or citric acid, method of drying) (Banwart and Ayres, 1956; Carlson and Snoeyenbos, 1970; Northolt et al., 1978). As the time required for inactivation of all the organisms depends upon the number originally present, potentially a shorter treatment could be applied to materials with low levels of contamination.

Investigators have demonstrated that salmonellae in egg powders can also be inactivated by irradiation (Matic et al., 1990; Narvaiz et al., 1992). The irradiation resistance of a mixture of S. Enteritidis, S. Typhimurium, and S.Lille was 0.8 kGy; a reduction of 10 3 required 2.4 kGy. A similar degree of inactivation was achieved by irradiating egg powder at 1.0 kGy, and holding the product for 3 weeks.

B Spoilage Potentially, microorganisms could remain alive indefinitely, but not grow. However, most bacteria die

slowly over time depending on a variety of factors such as species, temperature, pH, water activity, and atmosphere. On reconstitution with water or on accidental wetting during storage, surviving bacteria will grow resuscitate and spoil the product.

C Pathogens Salmonella-contaminated dried egg has caused numerous outbreaks of food-borne illness by direct

ingestion, and additional outbreaks by the way of other foods cross-contaminated by dried egg in the kitchen. Before 1965, dried eggs were often contaminated. Subsequently, low levels (0.1–0.01 cfu/g) of S. Enteritidis, S. Typhimurium, and S. Lille were isolated from whole egg powder produced in Yugoslavia (Matic et al., 1990).

Since that time, pasteurization, either wet or dry, has come into general use. National governments have also established and enforced regulations requiring pasteurization. The incidence of salmonellae in dried egg products is now typically at a low, but largely undetermined level.

L. monocytogenes can survive essentially unchanged for extended periods in dried powdered whole egg and egg yolk stored at refrigeration temperatures, but declines over time when stored at 20 ◦ C (Brackett and Beuchat, 1991).

MICROORGANISMS IN FOODS 6