Spoilage of Specific Food Groups ◾ 267
Fermented Meat Products
Fermented meat products normally have a pH between 4.5 and 5.0 and an A W between 0.73 and
0.93 (Chapter 15). During fermentation, if the acid production of homofermentative lactic acid
bacteria is slow, undesirable bacteria can grow. Clostridium, Bacillus, and other mesophilic bacteria
have been reported to cause spoilage in such conditions. Products with a pH lower than 5.0 but an
A W of 0.92 or above and vacuum packaged can be spoiled by heterofermentative Leuconostoc and
Lactobacillus spp., with the accumulation of gas and liquid inside the package and creamy white
growth of the bacterial cells. If they are not vacuum packaged and have a low A W (0.72–0.90),
yeasts and molds can grow on the surface, resulting in slime formation, discoloration, and undesirable flavor of the products.
Fermented Dairy Products
Cultured buttermilk, yogurt, and cheese are a few of the large number of fermented dairy products generally produced by inoculating milk with specific starter-culture bacteria (Chapter 15).
They differ in their acidity, A W , and storage stability. Buttermilk generally has approximately 0.8%
lactic acid and a pH of 4.8. Yeasts can grow and cause spoilage by producing gas. Some strains of
starter cultures can produce exopolysaccharides to give a slimy texture (which is desirable in some
products).
Plain yogurt generally has a pH of 4.5 or lower (with ca. 1% lactic acid) and is not spoiled by
undesirable bacteria. However, the product can develop a bitter flavor because of production of
bitter peptides by some strains of Lab. delbrueckii ssp. bulgaricus used as a starter culture. During
storage, the starter bacteria can continue to produce lactic acid, causing an objectionable sharp
acid taste. Both starters can produce exopolysaccharides, causing sliminess of the product. Yeasts
(especially in fruit yogurts) may grow in the acid environment and produce CO 2 as well as yeasty
and fruity off-flavors. In blended yogurts, many of these changes are not detected because of added
ingredients. Some species of molds can grow on the surface if yogurt is stored for a long time.
Microbial cheese spoilage is greatly influenced by A W and pH. Unripened cottage cheese with
high moisture content and low acidity is susceptible to spoilage by Gram-negative bacteria, predominantly psychrotrophic rods, and yeasts and molds. Alcaligenes and Pseudomonas spp. are frequently involved, causing a slimy texture and an unclean, putrid flavor. Some unripened ethnic
cheeses (such as Mexican-style) are vacuum packaged and stored at refrigerated temperature for
a shelf life of approximately 30 days or more (see Chapter 21). They are occasionally spoiled by
heterofermentative Leuconostoc spp., characterized by gas (CO 2 ) and liquid accumulation in the
bag. Gassiness in some cheeses with high pH, low salt, and relatively high A W (such as Gouda,
Emmentaler, and provolone) can also occur from the growth of some Clostridium (e.g., Clo. tyrobutyricum). Their spores survive pasteurization of milk, germinate, and grow in the anaerobic
environment to produce CO 2 , H 2 , and butyrate from the metabolism of lactate. Hard-ripened
cheese, such as Cheddar, can have a bitter taste resulting from rapid production of bitter peptides
during ripening. Fast acid-producing strains of Lactococcus lactis used as starters are generally associated with this defect. Sharp-ripened cheeses can also have large amounts of biologically active
amines (e.g., histamine and tyramine), produced from the decarboxylation of the respective amino
acids by decarboxylase. Decarboxylase enzymes can be present in some starter strains or in secondary microflora of the cheeses (Enterococcus, some coliforms). Lysis of the cells releases the enzymes
during the ripening process, causing decarboxylation of amino acids and accumulation of these
amines. Hard and semihard cheeses are generally susceptible to spoilage from mold growth on the
Fermented Meat Products
Fermented meat products normally have a pH between 4.5 and 5.0 and an A W between 0.73 and
0.93 (Chapter 15). During fermentation, if the acid production of homofermentative lactic acid
bacteria is slow, undesirable bacteria can grow. Clostridium, Bacillus, and other mesophilic bacteria
have been reported to cause spoilage in such conditions. Products with a pH lower than 5.0 but an
A W of 0.92 or above and vacuum packaged can be spoiled by heterofermentative Leuconostoc and
Lactobacillus spp., with the accumulation of gas and liquid inside the package and creamy white
growth of the bacterial cells. If they are not vacuum packaged and have a low A W (0.72–0.90),
yeasts and molds can grow on the surface, resulting in slime formation, discoloration, and undesirable flavor of the products.
Fermented Dairy Products
Cultured buttermilk, yogurt, and cheese are a few of the large number of fermented dairy products generally produced by inoculating milk with specific starter-culture bacteria (Chapter 15).
They differ in their acidity, A W , and storage stability. Buttermilk generally has approximately 0.8%
lactic acid and a pH of 4.8. Yeasts can grow and cause spoilage by producing gas. Some strains of
starter cultures can produce exopolysaccharides to give a slimy texture (which is desirable in some
products).
Plain yogurt generally has a pH of 4.5 or lower (with ca. 1% lactic acid) and is not spoiled by
undesirable bacteria. However, the product can develop a bitter flavor because of production of
bitter peptides by some strains of Lab. delbrueckii ssp. bulgaricus used as a starter culture. During
storage, the starter bacteria can continue to produce lactic acid, causing an objectionable sharp
acid taste. Both starters can produce exopolysaccharides, causing sliminess of the product. Yeasts
(especially in fruit yogurts) may grow in the acid environment and produce CO 2 as well as yeasty
and fruity off-flavors. In blended yogurts, many of these changes are not detected because of added
ingredients. Some species of molds can grow on the surface if yogurt is stored for a long time.
Microbial cheese spoilage is greatly influenced by A W and pH. Unripened cottage cheese with
high moisture content and low acidity is susceptible to spoilage by Gram-negative bacteria, predominantly psychrotrophic rods, and yeasts and molds. Alcaligenes and Pseudomonas spp. are frequently involved, causing a slimy texture and an unclean, putrid flavor. Some unripened ethnic
cheeses (such as Mexican-style) are vacuum packaged and stored at refrigerated temperature for
a shelf life of approximately 30 days or more (see Chapter 21). They are occasionally spoiled by
heterofermentative Leuconostoc spp., characterized by gas (CO 2 ) and liquid accumulation in the
bag. Gassiness in some cheeses with high pH, low salt, and relatively high A W (such as Gouda,
Emmentaler, and provolone) can also occur from the growth of some Clostridium (e.g., Clo. tyrobutyricum). Their spores survive pasteurization of milk, germinate, and grow in the anaerobic
environment to produce CO 2 , H 2 , and butyrate from the metabolism of lactate. Hard-ripened
cheese, such as Cheddar, can have a bitter taste resulting from rapid production of bitter peptides
during ripening. Fast acid-producing strains of Lactococcus lactis used as starters are generally associated with this defect. Sharp-ripened cheeses can also have large amounts of biologically active
amines (e.g., histamine and tyramine), produced from the decarboxylation of the respective amino
acids by decarboxylase. Decarboxylase enzymes can be present in some starter strains or in secondary microflora of the cheeses (Enterococcus, some coliforms). Lysis of the cells releases the enzymes
during the ripening process, causing decarboxylation of amino acids and accumulation of these
amines. Hard and semihard cheeses are generally susceptible to spoilage from mold growth on the