276 ◾ Fundamental Food Microbiology
MA packaging. Some of these can be incorporated into the product formulation to reduce the pH
and A W . In addition, acceptable and suitable preservatives (e.g., biopreservatives; see Chapter 17)
can be included to combat microbial survival and growth. Some packaged products, such as processed meats, can also be given a hydrostatic pressure processing treatment (Chapter 40).
Many of the food processors are following one or more of these recommendations, especially
good sanitation. This helps reduce the initial microbial population of the products considerably.
Yet incidences of spoilage of (and foodborne diseases from) vacuum- or MA-packaged refrigerated products are not infrequent and, at times with some processors, spoilage occurs at epidemic
proportions both in raw and low-heat-processed products. Many of the bacteria frequently isolated from such spoiled products are being recognized either as new species or species that were
not of major concern before. Some of these are Clo. laramie, Clo. estertheticum, Clo. algidicarnis,
Carnobacterium spp., Leuconostoc carnosum, Leu. gelidum, Lactobacillus sake, Lab. curvatus, atypical or unidentifiable lactobacilli and leuconostocs, Brochothrix thermosphacta, Enterococcus spp.,
Serratia liquefaciens, Hafnia spp., Proteus spp., and some other Enterobacteriaceae. Except for
spores of Clostridium spp., and some thermodurics (some lactobacilli and enterococci), cells of the
others are sensitive to heat treatment (pasteurization) given to the processed products. Thus, they
are getting in heated products as post-heat contaminants on the product surfaces (so are some
pathogens). There is a growing speculation and belief among scientists that although these bacterial species have become major causes of spoilage of vacuum- or MA-packaged refrigerated foods,
most are neither new nor variants of the existing species. They are present in the environment but
probably as minor flora. Three reasons could have helped them become major spoilage organisms: the changes in the environment and food, such as efficient vacuum (or compositions used
in MA) packaging and oxygen barrier systems; higher pH in some meat products (low fat, high
phosphate); and long storage times at low temperatures—these might have controlled the growth
of traditional bacteria associated with food spoilage before but are not efficient for these nontraditional bacteria. A second reason could be that in order to produce a safer food with a long shelf
life, food processors are using “super sanitation,” which has efficiently eliminated the traditional
microorganisms associated with spoilage before but, at the same time, has enabled the minor flora
to establish in the environment and contaminate the products. Finally, processors might be introducing equipment that are highly efficient in producing a large volume of products but may also be
the microbiologist’s nightmare for efficient cleaning and sanitation. As a result, the equipment harboring the microorganisms in dead spots remains unaffected by sanitation and serves as a source
of inoculation to the products. Some of the examples used here may justify these assumptions. 5
incidence of Spoilage of Vacuum-Packaged Refrigerated Food
Spoilage of Unprocessed (Fresh) Beef by Clostridium Species
Large-scale spoilage of vacuum-packaged, refrigerated, unprocessed beef has been recognized
recently. 5–9 The spoilage is characterized by accumulation of large quantities of H 2 S-smelling gas
and purge (liquid) in the bag within two weeks at 4°C (39.2°F) and loss of texture (soft) and color
(cherry red) of meat, which, in 10–12 weeks, changes to greenish (Figure 21.1a). Enumeration by
plating revealed approximately 10 8 /mL leuconostocs colony-forming units (CFUs) in the purge.
However, examination of a little purge under a phase-contrast microscope revealed a large number
of medium-to-large, thick, motile rods and some cells with large terminal spores (drumstickshaped) as well as some leuconostoc-like cells (lenticular, small chains). The rods and the lenticular
MA packaging. Some of these can be incorporated into the product formulation to reduce the pH
and A W . In addition, acceptable and suitable preservatives (e.g., biopreservatives; see Chapter 17)
can be included to combat microbial survival and growth. Some packaged products, such as processed meats, can also be given a hydrostatic pressure processing treatment (Chapter 40).
Many of the food processors are following one or more of these recommendations, especially
good sanitation. This helps reduce the initial microbial population of the products considerably.
Yet incidences of spoilage of (and foodborne diseases from) vacuum- or MA-packaged refrigerated products are not infrequent and, at times with some processors, spoilage occurs at epidemic
proportions both in raw and low-heat-processed products. Many of the bacteria frequently isolated from such spoiled products are being recognized either as new species or species that were
not of major concern before. Some of these are Clo. laramie, Clo. estertheticum, Clo. algidicarnis,
Carnobacterium spp., Leuconostoc carnosum, Leu. gelidum, Lactobacillus sake, Lab. curvatus, atypical or unidentifiable lactobacilli and leuconostocs, Brochothrix thermosphacta, Enterococcus spp.,
Serratia liquefaciens, Hafnia spp., Proteus spp., and some other Enterobacteriaceae. Except for
spores of Clostridium spp., and some thermodurics (some lactobacilli and enterococci), cells of the
others are sensitive to heat treatment (pasteurization) given to the processed products. Thus, they
are getting in heated products as post-heat contaminants on the product surfaces (so are some
pathogens). There is a growing speculation and belief among scientists that although these bacterial species have become major causes of spoilage of vacuum- or MA-packaged refrigerated foods,
most are neither new nor variants of the existing species. They are present in the environment but
probably as minor flora. Three reasons could have helped them become major spoilage organisms: the changes in the environment and food, such as efficient vacuum (or compositions used
in MA) packaging and oxygen barrier systems; higher pH in some meat products (low fat, high
phosphate); and long storage times at low temperatures—these might have controlled the growth
of traditional bacteria associated with food spoilage before but are not efficient for these nontraditional bacteria. A second reason could be that in order to produce a safer food with a long shelf
life, food processors are using “super sanitation,” which has efficiently eliminated the traditional
microorganisms associated with spoilage before but, at the same time, has enabled the minor flora
to establish in the environment and contaminate the products. Finally, processors might be introducing equipment that are highly efficient in producing a large volume of products but may also be
the microbiologist’s nightmare for efficient cleaning and sanitation. As a result, the equipment harboring the microorganisms in dead spots remains unaffected by sanitation and serves as a source
of inoculation to the products. Some of the examples used here may justify these assumptions. 5
incidence of Spoilage of Vacuum-Packaged Refrigerated Food
Spoilage of Unprocessed (Fresh) Beef by Clostridium Species
Large-scale spoilage of vacuum-packaged, refrigerated, unprocessed beef has been recognized
recently. 5–9 The spoilage is characterized by accumulation of large quantities of H 2 S-smelling gas
and purge (liquid) in the bag within two weeks at 4°C (39.2°F) and loss of texture (soft) and color
(cherry red) of meat, which, in 10–12 weeks, changes to greenish (Figure 21.1a). Enumeration by
plating revealed approximately 10 8 /mL leuconostocs colony-forming units (CFUs) in the purge.
However, examination of a little purge under a phase-contrast microscope revealed a large number
of medium-to-large, thick, motile rods and some cells with large terminal spores (drumstickshaped) as well as some leuconostoc-like cells (lenticular, small chains). The rods and the lenticular