266 ◾ Fundamental Food Microbiology
Liquid Sweeteners and Confectioneries
Liquid sweeteners include honey, sugar syrups, maple syrups, corn syrups, and molasses.
Confectionery products include soft-centered fondant, cream, jellies, chocolate, and Turkish
delight. Most of these products have an A W of 0.8 or below and are normally not susceptible to
bacterial spoilage. Under aerobic conditions, some xerophilic molds can produce visible spoilage. However, osmophilic yeasts from genera Zygosaccharomyces (Zygosaccharomyces rouxii),
Saccharomyces (Saccharomyces cerevisiae), Torulopsis (Torulopsis holmii), and Candida (Candida
valida) can ferment these products. To prevent growth of yeasts in some of these products with
slightly higher A W (such as in maple syrup), chemical preservatives are added.
Mayonnaise, Salad Dressings, and Condiments
These products normally contain some molds, yeasts, spores of Bacillus and Clostridium, and aciduric bacteria, such as Lactobacillus and related species. Because of low pH, acid-sensitive bacteria
may not survive long. Mayonnaise, with 65% or more edible oil and approximately 0.5% acetic
acid, has an A W of approximately 0.92 and a pH of 3.6–4.0. Salad dressings generally contain
30% or more edible oil, 0.9%–1.2% acetic acid, an A W of 0.92, and a pH of 3.2–3.9. The major
factors controlling microbial growth are undissociated acetic acid, low pH, and relatively low A W .
However, some aciduric microorganisms can grow to cause spoilage. Molds can grow only on the
surface exposed to air. Some microaerophilic and facultative anaerobic yeasts and heterolactic
Lactobacillus spp. (especially those that can grow at an A W of 0.92) can also multiply to produce
CO 2 . Lab. fructivorans hydrolyzes sucrose present in the products and produces gas (CO 2 ), especially from rapid metabolism of fructose that is released from sucrose. Strains of Saccharomyces bailii have also been implicated in gas spoilage (CO 2 and alcohol) of these products. Lab. fructivorans
cells usually die rapidly after multiplication and are difficult to isolate unless specific methods are
used. This can lead to a wrong assumption regarding the microorganisms that cause spoilage of
these products.
Low-calorie salad dressing, in which oil and acetic acid are added in much lower concentrations, has high pH and A W values. Many microorganisms can grow in these products. To enhance
their shelf life (and safety), refrigerated storage is recommended.
Yeasts, Lactobacillus spp., and Leuconostoc spp. can also cause gassy spoilage of ketchup, salsa,
sauces, and prepared mustard. Some Bacillus spp. have also been implicated in gassy spoilage
of mustard preparations. To control growth of spoilage microorganisms, additional preservation
methods, especially chemical preservatives, are used. 12,13
Fermented Foods
Desirable microorganisms are used directly or indirectly to produce many types of fermented
foods and beverages from meat, fish, milk, vegetables, fruits, cereal grains, and others (Chapter 15).
The desirable microorganisms are present in very high numbers and the products contain either
high levels of organic acids or alcohol. In addition, the products have a low pH, and some have a
low A W (e.g., dry salami). Generally, these products have a longer shelf life. However, under certain
conditions, they are susceptible to microbial spoilage. This aspect is discussed briefly. 1
Liquid Sweeteners and Confectioneries
Liquid sweeteners include honey, sugar syrups, maple syrups, corn syrups, and molasses.
Confectionery products include soft-centered fondant, cream, jellies, chocolate, and Turkish
delight. Most of these products have an A W of 0.8 or below and are normally not susceptible to
bacterial spoilage. Under aerobic conditions, some xerophilic molds can produce visible spoilage. However, osmophilic yeasts from genera Zygosaccharomyces (Zygosaccharomyces rouxii),
Saccharomyces (Saccharomyces cerevisiae), Torulopsis (Torulopsis holmii), and Candida (Candida
valida) can ferment these products. To prevent growth of yeasts in some of these products with
slightly higher A W (such as in maple syrup), chemical preservatives are added.
Mayonnaise, Salad Dressings, and Condiments
These products normally contain some molds, yeasts, spores of Bacillus and Clostridium, and aciduric bacteria, such as Lactobacillus and related species. Because of low pH, acid-sensitive bacteria
may not survive long. Mayonnaise, with 65% or more edible oil and approximately 0.5% acetic
acid, has an A W of approximately 0.92 and a pH of 3.6–4.0. Salad dressings generally contain
30% or more edible oil, 0.9%–1.2% acetic acid, an A W of 0.92, and a pH of 3.2–3.9. The major
factors controlling microbial growth are undissociated acetic acid, low pH, and relatively low A W .
However, some aciduric microorganisms can grow to cause spoilage. Molds can grow only on the
surface exposed to air. Some microaerophilic and facultative anaerobic yeasts and heterolactic
Lactobacillus spp. (especially those that can grow at an A W of 0.92) can also multiply to produce
CO 2 . Lab. fructivorans hydrolyzes sucrose present in the products and produces gas (CO 2 ), especially from rapid metabolism of fructose that is released from sucrose. Strains of Saccharomyces bailii have also been implicated in gas spoilage (CO 2 and alcohol) of these products. Lab. fructivorans
cells usually die rapidly after multiplication and are difficult to isolate unless specific methods are
used. This can lead to a wrong assumption regarding the microorganisms that cause spoilage of
these products.
Low-calorie salad dressing, in which oil and acetic acid are added in much lower concentrations, has high pH and A W values. Many microorganisms can grow in these products. To enhance
their shelf life (and safety), refrigerated storage is recommended.
Yeasts, Lactobacillus spp., and Leuconostoc spp. can also cause gassy spoilage of ketchup, salsa,
sauces, and prepared mustard. Some Bacillus spp. have also been implicated in gassy spoilage
of mustard preparations. To control growth of spoilage microorganisms, additional preservation
methods, especially chemical preservatives, are used. 12,13
Fermented Foods
Desirable microorganisms are used directly or indirectly to produce many types of fermented
foods and beverages from meat, fish, milk, vegetables, fruits, cereal grains, and others (Chapter 15).
The desirable microorganisms are present in very high numbers and the products contain either
high levels of organic acids or alcohol. In addition, the products have a low pH, and some have a
low A W (e.g., dry salami). Generally, these products have a longer shelf life. However, under certain
conditions, they are susceptible to microbial spoilage. This aspect is discussed briefly. 1