486 ◾ Fundamental Food Microbiology
meat along with some O 2 , are used in a mixture. N 2 is used as inert filler, and O 2 is used to give the
red oxymyoglobin color to the meat. CO 2 is used for its antimicrobial effect. As low as 20% CO 2
has been found to control the growth of aerobes, such as Pseudomonas, Acinetobacter, and Moraxella;
however, in general, CO 2 at 40%–60% gives better results. In some cases, 100% CO 2 is used.
Nature of Foods
The amounts of oxygen (dissolved and trapped), metabolizable carbohydrates, and other nutrients and
the reducing components present influence the growth of microbial types in a MAP food. In the presence of O 2 , aerobes, such as Pseudomonas, utilize glucose and lactate. Growth of facultative anaerobes,
however, discourages the growth of aerobes but can stimulate the growth of anaerobes. Reducing components also encourage the growth of anaerobes. The presence of specific inhibitors, either present in a
food or produced by associated bacteria (such as bacteriocins by some lactic acid bacteria), low A W , and
low pH also influence the microbial ability to grow or not grow under specific packaging conditions.
Nature of Microorganisms
Aerobes can have limited initial growth, depending on the oxygen present in a vacuum- packaged
or gas-flushed packaged product. However, their growth will stop as soon as the facultative
anaerobes start growing. These include lactic acid bacteria, Brochothrix thermosphacta, some
Enterobacteriaceae, and Corynebacteriaceae. In low-pH products, a favorable growth of lactic acid
bacteria can reduce or prevent growth of other bacteria because the former can produce acids
and other antibacterial substances (e.g., bacteriocins). Also, the composition of the gas mixture
influences the predominant types; with 100% CO 2 , lactic acid bacteria, especially Leuconostoc
and Lactobacillus spp., predominate. CO 2 at approximately 20% concentration controls growth
of Pseudomonas spp., but a CO 2 concentration of more than 60% is required to control growth of
Enterobacteriaceae. As the Eh of a product starts reducing, the anaerobes, especially Clostridium
spp., start growing. The presence of reducing agents in the food also favors the growth of anaerobes.
The possible growth of Clo. botulinum in MAP refrigerated foods, especially Type E and nonproteolytic Type B, is a major concern. Non-spore-forming psychrotrophic pathogens, especially the
facultative anaerobes (Listeria monocytogenes and Yersinia enterocolitica), can also multiply in MAP
foods. In addition, some mesophilic facultative anaerobic pathogens with growth capabilities at
10°C–12°C (some Salmonella and Escherichia coli O157:H7 strains and Staphylococcus aureus) can
also grow if the MAP foods are temperature abused during storage. Suitable preservatives, low pH,
or low A W , or their combination, should be used as additional hurdles for their control. 4,6
Methods
Vacuum Packaging
Vacuum packaging is predominantly used as retail packages in many fresh and ready-to-eat meat
products, including beef, pork, lamb, chicken, and turkey. Packaged this way, red meat, because of
a change in color to purple (reduced myoglobin), is not very popular with consumers. The refrigerated storage life of these products varies greatly: approximately three to four weeks in fresh meat
and as long as eight weeks or more in processed meats. If the products have low A W , or low pH, or
both, they are produced under sanitary conditions.
meat along with some O 2 , are used in a mixture. N 2 is used as inert filler, and O 2 is used to give the
red oxymyoglobin color to the meat. CO 2 is used for its antimicrobial effect. As low as 20% CO 2
has been found to control the growth of aerobes, such as Pseudomonas, Acinetobacter, and Moraxella;
however, in general, CO 2 at 40%–60% gives better results. In some cases, 100% CO 2 is used.
Nature of Foods
The amounts of oxygen (dissolved and trapped), metabolizable carbohydrates, and other nutrients and
the reducing components present influence the growth of microbial types in a MAP food. In the presence of O 2 , aerobes, such as Pseudomonas, utilize glucose and lactate. Growth of facultative anaerobes,
however, discourages the growth of aerobes but can stimulate the growth of anaerobes. Reducing components also encourage the growth of anaerobes. The presence of specific inhibitors, either present in a
food or produced by associated bacteria (such as bacteriocins by some lactic acid bacteria), low A W , and
low pH also influence the microbial ability to grow or not grow under specific packaging conditions.
Nature of Microorganisms
Aerobes can have limited initial growth, depending on the oxygen present in a vacuum- packaged
or gas-flushed packaged product. However, their growth will stop as soon as the facultative
anaerobes start growing. These include lactic acid bacteria, Brochothrix thermosphacta, some
Enterobacteriaceae, and Corynebacteriaceae. In low-pH products, a favorable growth of lactic acid
bacteria can reduce or prevent growth of other bacteria because the former can produce acids
and other antibacterial substances (e.g., bacteriocins). Also, the composition of the gas mixture
influences the predominant types; with 100% CO 2 , lactic acid bacteria, especially Leuconostoc
and Lactobacillus spp., predominate. CO 2 at approximately 20% concentration controls growth
of Pseudomonas spp., but a CO 2 concentration of more than 60% is required to control growth of
Enterobacteriaceae. As the Eh of a product starts reducing, the anaerobes, especially Clostridium
spp., start growing. The presence of reducing agents in the food also favors the growth of anaerobes.
The possible growth of Clo. botulinum in MAP refrigerated foods, especially Type E and nonproteolytic Type B, is a major concern. Non-spore-forming psychrotrophic pathogens, especially the
facultative anaerobes (Listeria monocytogenes and Yersinia enterocolitica), can also multiply in MAP
foods. In addition, some mesophilic facultative anaerobic pathogens with growth capabilities at
10°C–12°C (some Salmonella and Escherichia coli O157:H7 strains and Staphylococcus aureus) can
also grow if the MAP foods are temperature abused during storage. Suitable preservatives, low pH,
or low A W , or their combination, should be used as additional hurdles for their control. 4,6
Methods
Vacuum Packaging
Vacuum packaging is predominantly used as retail packages in many fresh and ready-to-eat meat
products, including beef, pork, lamb, chicken, and turkey. Packaged this way, red meat, because of
a change in color to purple (reduced myoglobin), is not very popular with consumers. The refrigerated storage life of these products varies greatly: approximately three to four weeks in fresh meat
and as long as eight weeks or more in processed meats. If the products have low A W , or low pH, or
both, they are produced under sanitary conditions.