202 Modern Food Microbiology
microorganisms. Once inside the yolk, bacteria apparently grow in this nutritious medium, producing
byproducts of protein and amino acid metabolism such as H 2 S and other foul-smelling compounds.
The effect of significant growth is to cause the yolk to become “runny” and discolored. Molds generally
multiply first in the region of the air sac, where oxygen favors growth of these forms. Under conditions
of high humidity, molds may be seen growing over the outer surface of eggs. Under conditions of low
humidity and low temperatures, surface growth is not favored, but eggs lose water at a faster rate and
thereby become undesirable as products of commerce.
The antimicrobial systems of eggs are noted in Chapter 3. In addition, hen egg albumen contains
ovotransferrin, which chelates metal ions, particularly Fe
3+ , and ovoflavoprotein, which binds riboflavin. At its normal pH of 9.0–10.0, egg albumen is cidal to Gram-positive bacteria and yeasts at
both 30
◦ C and 39.5
◦ C.
61 The addition of iron reduces the antimicrobial properties of egg albumen.
In regards to the destruction of salmonellae in boiled shell eggs, it is generally recognized that
cooking until the entire yolk is solidified is sufficient to destroy S. enterica serotype Enteritidis. In one
study, boiling for 7 minutes destroyed 10
8 cfu of artifically inoculated S. Typhimurium.
1 In another
study, the American Egg Board method of heating to 100
◦ C, removing from heat, and holding for 15
minutes was found to be better than two other methods tested.
5 The recommended handling of fresh
eggs to prevent salmonellosis is presented in Chapter 26.
MAYONNAISE AND SALAD DRESSING
Mayonnaise can be defined as a semisolid emulsion of edible vegetable oil, egg yolk or whole egg,
vinegar, and/or lemon juice, and other ingredients such as salt and other seasonings and glucose, in a
finished product containing not less than 50% edible oil. The pH of this product ranges from 3.6 to 4.0,
with acetic acid as the predominant acid, representing 0.29–0.5% of total product (aqueous phase up
to 2%) with a water activity (a w ) of 0.925. The aqueous phase contains 9–11% salt and 7–10% sugar.
53
Salad dressings are quite similar in composition to mayonnaise, but the finished product contains at
least 30% edible vegetable oil and has an a w of 0.929, a pH of 3.2 to 3.9, with acetic acid usually
the predominant acid accounting for 0.9 to 1.2% of total product. The aqueous phase contains 3.0 to
4.0% salt and 20 to 30% sugar.
53 Although the nutrient content of these products is suitable as food
sources for many spoilage organisms, the pH, organic acids, and low a w restrict spoilers to yeasts, a
few bacteria, and molds. The yeast Zygosaccharomyces bailii is known to cause the spoilage of salad
dressings, tomato catsup, carbonated beverages, and some wines. Yeasts of the genus Saccharomyces
have been implicated in the spoilage of mayonnaise, salad dressing, and French dressing. The two main
spoilers for these products are Lactobacillus fructivorans and Z. bailii. In the spoilage of mayonnaise,
Z. bailii produces product separation and a “yeasty” odor. In one study, the shelf-life of mayonnaise
was extended by the addition of encapsulated cells of Bifidobacterium bifidum and B. infantis.
27 With
the addition of nearly 10
7 cfu/g of the bifidobacteria, yeasts and molds were delayed for about 12
weeks compared to uninoculated controls; and the sensory quality of the preparation was improved
by the bifidobacteria. Another spoilage organism is Lactobacillus brevis subsp. lindneri. Growth of
the latter in buttermilk ranch dressing at pH 3.8–4.2 was inhibited by 200-ppm nisin over a 90-day
incubation period.
41
Bacillus vulgatus has been recovered from spoiled Thousand Island dressing, where it caused
darkening and separation of the emulsion. In one study of the spoilage of Thousand Island dressing,
pepper and paprika were shown to be the sources of B. vulgatus.
46 Mold spoilage of products of this
type occurs only at the surfaces when sufficient oxygen is available. Separation of the emulsion is
generally one of the first signs of spoilage of these products, although bubbles of gas and the rancid
microorganisms. Once inside the yolk, bacteria apparently grow in this nutritious medium, producing
byproducts of protein and amino acid metabolism such as H 2 S and other foul-smelling compounds.
The effect of significant growth is to cause the yolk to become “runny” and discolored. Molds generally
multiply first in the region of the air sac, where oxygen favors growth of these forms. Under conditions
of high humidity, molds may be seen growing over the outer surface of eggs. Under conditions of low
humidity and low temperatures, surface growth is not favored, but eggs lose water at a faster rate and
thereby become undesirable as products of commerce.
The antimicrobial systems of eggs are noted in Chapter 3. In addition, hen egg albumen contains
ovotransferrin, which chelates metal ions, particularly Fe
3+ , and ovoflavoprotein, which binds riboflavin. At its normal pH of 9.0–10.0, egg albumen is cidal to Gram-positive bacteria and yeasts at
both 30
◦ C and 39.5
◦ C.
61 The addition of iron reduces the antimicrobial properties of egg albumen.
In regards to the destruction of salmonellae in boiled shell eggs, it is generally recognized that
cooking until the entire yolk is solidified is sufficient to destroy S. enterica serotype Enteritidis. In one
study, boiling for 7 minutes destroyed 10
8 cfu of artifically inoculated S. Typhimurium.
1 In another
study, the American Egg Board method of heating to 100
◦ C, removing from heat, and holding for 15
minutes was found to be better than two other methods tested.
5 The recommended handling of fresh
eggs to prevent salmonellosis is presented in Chapter 26.
MAYONNAISE AND SALAD DRESSING
Mayonnaise can be defined as a semisolid emulsion of edible vegetable oil, egg yolk or whole egg,
vinegar, and/or lemon juice, and other ingredients such as salt and other seasonings and glucose, in a
finished product containing not less than 50% edible oil. The pH of this product ranges from 3.6 to 4.0,
with acetic acid as the predominant acid, representing 0.29–0.5% of total product (aqueous phase up
to 2%) with a water activity (a w ) of 0.925. The aqueous phase contains 9–11% salt and 7–10% sugar.
53
Salad dressings are quite similar in composition to mayonnaise, but the finished product contains at
least 30% edible vegetable oil and has an a w of 0.929, a pH of 3.2 to 3.9, with acetic acid usually
the predominant acid accounting for 0.9 to 1.2% of total product. The aqueous phase contains 3.0 to
4.0% salt and 20 to 30% sugar.
53 Although the nutrient content of these products is suitable as food
sources for many spoilage organisms, the pH, organic acids, and low a w restrict spoilers to yeasts, a
few bacteria, and molds. The yeast Zygosaccharomyces bailii is known to cause the spoilage of salad
dressings, tomato catsup, carbonated beverages, and some wines. Yeasts of the genus Saccharomyces
have been implicated in the spoilage of mayonnaise, salad dressing, and French dressing. The two main
spoilers for these products are Lactobacillus fructivorans and Z. bailii. In the spoilage of mayonnaise,
Z. bailii produces product separation and a “yeasty” odor. In one study, the shelf-life of mayonnaise
was extended by the addition of encapsulated cells of Bifidobacterium bifidum and B. infantis.
27 With
the addition of nearly 10
7 cfu/g of the bifidobacteria, yeasts and molds were delayed for about 12
weeks compared to uninoculated controls; and the sensory quality of the preparation was improved
by the bifidobacteria. Another spoilage organism is Lactobacillus brevis subsp. lindneri. Growth of
the latter in buttermilk ranch dressing at pH 3.8–4.2 was inhibited by 200-ppm nisin over a 90-day
incubation period.
41
Bacillus vulgatus has been recovered from spoiled Thousand Island dressing, where it caused
darkening and separation of the emulsion. In one study of the spoilage of Thousand Island dressing,
pepper and paprika were shown to be the sources of B. vulgatus.
46 Mold spoilage of products of this
type occurs only at the surfaces when sufficient oxygen is available. Separation of the emulsion is
generally one of the first signs of spoilage of these products, although bubbles of gas and the rancid