For many years, products prepared from raw shell eggs have been implicated in foodborne disease outbreaks. It is now known that there may be the possible presence of Salmonella enteritidis within the yolk of a small percentage of intact, USDA-graded shell eggs. Food microbiologists and public health authorities have named homemade mayonnaise, Caesar dressing, and Hollandaise and Béarnaise sauces as items that pose risks of a hazard if prepared from raw shell eggs. The safety of these products can be assured if the cook takes specific steps, as presented in this paper, to reduce any possible Salmonella spp. in raw eggs (egg yolks) to a safe level. This can be accomplished when cooks use a preparation method for egg yolks that includes the addition of acid and a pasteurization step that provides a 100,000-to-1 CFU/g (5D) Salmonella reduction before the acidified/pasteurized egg yolks are used in the preparation of these products. Recipe formulations, calculations, directions for preparation, and pH of final products are given.
Raw eggs are a functional ingredient in many sauces and salad dressings. In the past, the FDA considered the contents of whole, uncooked shell eggs to be pathogen-free and did not consider the contents of fresh eggs to be a high risk food. However, possible contamination of intact shell eggs by S. enteritidis was recognized in Europe and the United States during the latter 1980s when it became known that this pathogen could be transferred from the infected ovaries of laying hens to the egg yolk before the shell was formed. USDA grading of shell eggs does not detect the presence of this pathogen. Today, a small percentage of raw shell eggs are contaminated, perhaps 1 in 10,000. This is true throughout the U. S. Unfortunately, one cannot look at the intact shell egg and have any idea about its safety. Therefore, food microbiologists and public health authorities have named homemade mayonnaise, Caesar dressing and Hollandaise and Béarnaise sauces as items that pose risks of a hazard if prepared from raw shell eggs. However, the safety of these products can be assured if the cook takes specific steps as presented in this paper to reduce any possibleSalmonella spp. in raw eggs to a safe level. This can be accomplished when cooks use a preparation method for egg yolks that includes pasteurization which provides the same 100,000-to-1 CFU/g (5D) Salmonella reduction, or greater than is specified for cooking hamburgers.
To prevent foodborne illness, public health authorities recommend: (1) using pasteurized intact shell eggs (available in only a few areas of the U.S.), (2) using commercial liquid egg products pasteurized according to USDA specifications (USDA 1969) that are normally only available frozen, in large containers, (3) purchasing supplier-certified, salmonellae-free eggs (which are available from only a few suppliers), and (4) cooking eggs until all parts of the egg reach a temperature of at least 145F (63C) for 15 seconds according to the FDA 1995, 1997 Food Codes. (This really is not an adequate pasteurization.) None of these options are practical for a normal retail food operation preparing fresh sauces and dressings in small quantity.
The microbiological hazard can be eliminated if certain precautions are taken, such as those used in the commercial preparation of mayonnaise. Acid ingredients in mayonnaise, if in sufficient concentration or amount, can eliminate salmonellae from raw egg yolks if given an adequate amount of holding time at room temperature after manufacture. Federal regulations assume the presence of salmonellae in raw eggs used for salad dressings and require that commercially manufactured dressings such as mayonnaise and salad dressing made with unpasteurized eggs must have a pH of less than or equal to 4.1, an acetic acid level of the aqueous phase of greater than or equal to 1.4%, and a holding period of 72 hours before the product is shipped (CFR Title 21 Part 101.100 and 169.140). These conditions were established to assure destruction of Salmonella and were based on studies of Wethington and Fabian (1950). The use of unpasteurized eggs in mayonnaise or salad dressing by commercial manufacturers was discontinued in the early 1970s and they began using USDA-certified, pasteurized eggs. However, there still may be low contamination of these pasteurized products with low levels of Salmonella spp. Therefore, the acetic/citric acid level in these products remains critical. Commercial mayonnaise in the United States produced in accordance with the FDA Standard of Identity actually contains enough acid to destroy Salmonella spp. and inhibit the growth of other foodborne pathogenic bacteria such as Listeria monocytogenes (Smittle, 1977; Glass and Doyle, 1991; Erickson and Jenkins, 1991; Radford and Board, 1993).
This paper will describe a simple egg yolk pasteurization method that incorporates this acid-safety concept. Restaurant chefs and cooks in homes can use this method to assure the safety of sauces and dressings made from fresh egg yolks. This method can be used to prepare a few pasteurized egg yolks in advance for use in sauces and dressings throughout the day as well as for the immediate preparation of these items.
Method for Controlling Salmonellae Contamination in Egg-Based Sauces and Dressings
The simplest method for controlling microbial contamination and minimizing the risk of foodborne illness in a kitchen is the application of heat. This occurs when products are pasteurized. However, heating fresh eggs (whole or yolks) to temperatures sufficient to decrease microbial hazards for emulsified salad dressings and sauces has not been thought by cooks to be possible. This is because most cooks know that when eggs (whites and yolks) are heated above 150F (65.6C), the egg proteins solidify and become hard. When egg-thickened sauces are heated excessively, the emulsions break, and the sauces curdle and separate. Therefore, cooks attempt to keep the temperature of the sauces just slightly warm [110 to 120F (43.3 to 48.9C)] when these products are prepared. In the attempt to maintain the stability of the sauces, these sauces are held at temperatures of 110 to 115F (43.3 to 46.1C). These temperatures allow the growth of pathogenic bacteria such as Salmonella spp.
Cooks must understand the importance of acid and temperature in the preparation of egg-based sauces, mayonnaise, and Caesar dressing in order to prepare these items safely. The procedure is accomplished by diluting and acidifying the yolk of eggs and heating the mixture to a pasteurizing temperature [heating for a sufficient period of time necessary for a 100,000-to-1 CFU/g (5D) reduction]. This procedure ensures the destruction of vegetative cells of Salmonella spp. and other pathogenic bacteria such as L. monocytogenes and Escherichia coli O157:H7. The method has been adapted from that described by McGee (1990a, 1990b). Actually many formulated cookbook recipes for mayonnaise, Caesar dressing, Hollandaise and Béarnaise sauce include more than enough acid. However, the amount of acid in these foods is not recognized as a critical hazard control.
The first step is to recognize that if the egg yolk protein is mixed with an equal volume of water and from one-third to an equal volume of lemon juice or vinegar, the temperature at which coagulation of the egg yolk protein occurs is raised. When the egg yolk/acid mixture is heated to 150F (65.6C), vegetative pathogenic bacteria are reduced at least 100,000 to 1 CFU/g, while the egg yolk proteins have not been heated sufficiently to denature and coagulate because of the water dilution and acid. It is critically important that an accurate thermocouple thermometer (such as theAtkins 33040Ô ) be used to measure the temperature of the mixture. Temperature measurement is necessary to ensure that the mixture is heated to 150F (65.6C) to assure safety, but not heated to temperatures above this point [approximately 180 to 190F (82.2 to 87.8C)] which cause the yolk proteins to coagulate.
The emulsifying capability of egg yolk is mainly related to its content of lecithin (about 1.22% of the yolk) (Stadleman 1986). Lecithin is a phospholipid and is not affected by the acid/heat pasteurization process and remains an effective emulsifying agent in the pasteurized egg yolk/acid mixture.
Specific Procedures for Cold Oil Sauces
Mayonnaise and Caesar dressing are examples of cold oil sauces containing egg yolks. See recipe formulations for mayonnaise (Figure 1), as adapted from Bocuse and Metz (1996), and Caesar dressing (Figure 2), as adapted from Rombauer and Becker (1974).
Method for assuring destruction of Salmonella spp. in egg yolk. Place egg yolk(s) in a small, stainless steel bowl. (The container must be large enough so that it can allow the egg yolk/acid mixture to be stirred or whisked as it is heated.) Place the container containing the egg yolk/acid mixture in a pan or bowl of water (such as a small double boiler) that is at a simmering temperature of 180 to 190F (82.2 to 87.8C). Heat the yolk/acid mixture to a temperature of 150F (65.6C). This will take about 1 minute. The mixture must be stirred or whisked constantly and the temperature measured frequently by using a micro-tip thermocouple thermometer (such as the Atkins 33040Ô ). Immediately remove the pan containing the yolk/acid mixture from the hot-water heat source. The yolk/acid mixture is now pasteurized and can be used in the preparation of mayonnaise and Caesar dressing.
Recipes for these products should be checked, or recipes provided in this paper should be used to assure that there is the correct amount of acidity. As a starting point, the standard of identity for vinegar is 5% acetic acid. The amount of citric acid in lemon juice (bottled or freshly squeezed) is 4.7%. A typical mayonnaise should be prepared with 1 raw egg yolk per 8 ounces of oil and the acid concentration should be 1.4% of the aqueous phase as recommended by the FDA (CFR Title 21 Part 101.100).
The calculation for percent of water in the aqueous phase is as follows:
(total amount of acid / total amount of water) x 100 = % acid in water (aqueous) phase.
|Egg yolks (3)||x 48% water*|
|Wine vinegar (2 Tbsp.)|
|Lemon juice (2 Tbsp.)|
|Water (2 Tbsp.)|
The calculation for percent acid in the aqueous phase is:
(2.91 g acid/113 g water) x 100 = 2.6% acid in aqueous phase. The final measured pH of this mixture is 3.5.
|Egg yolk (1)||x 48% water|
|Wine vinegar (2-1/2 Tbsp.)|
- There are 1.875 g acetic acid (based on 5% in vinegar) in 37.5 g. wine vinegar.
The calculation for percent acid in the aqueous phase is: