Food Protection with Modified Atmospheres
363
CO 2 + 85% N 2 , 80% CO 2 + 20% N 2 , under vacuum, and in pure nitrogen at 4
◦ C for up to 2 weeks.
In another study of C. jejuni in beef, two of three strains decreased slightly when stored at 20
◦ C
and 4
◦ C for 48 hours but increased when stored at 37
◦ C under all gas mixtures used.
44 The storage
conditions consisted of vacuum packaging, 20% CO 2 + 80% N 2 , 5% O 2 + 10% CO 2 + 85% N 2 ,
and the background biota apparently had no effect on C. jejuni. When heat-injured and uninjured
cells of Aeromonas hydrophila were stored at 5
◦ C for 22 days under 100% N 2 or 100% CO 2 , both
types steadily declined in numbers, but growth of both was enhanced when incubated under N 2 .
33 The
heat-injured cells apparently were not particularly disadvantaged under the conditions noted.
In a study of raw crayfish stored at 4–10
◦ C for up to 30 days, Aeromonas hydrophila, S. aureus, C.
perfringens, and C. botulinum type E were not found.
63 When cooked crawfish was inoculated with
10
3 cfu of C. botulinum type E spores/g of crawfish tail, vacuum packaged with both high-barrier
film and air-impermeable bags followed by storage at 4 and 10
◦ C for 30 days, type E toxin was not
detected in any packages.
63
SPOILAGE OF MAP AND VACUUM-PACKAGED MEATS
From the research of many groups, it is clear that when vacuum-packaged meats undergo long-term
refrigerator spoilage, often the predominant organisms are lactobacilli, other lactics, B. thermosphacta,
or all of these. Other organisms can be found and, indeed, others may predominate. Among the
determining factors are the following:
1. whether the product is raw or cooked;
2. concentration of nitrites present;
3. relative load of psychrotrophic bacteria;
4. the degree to which the vacuum-package film excludes O 2 ;
5. product pH.
Cooked or partially cooked meats, along with dark, firm, and dry (DFD) and dark-cutting meats,
have a higher pH than raw and light-cutting meats, and the organisms that dominate these products
during vacuum storage are generally different from those found in vacuum-packaged normal meats.
In vacuum-packaged DFD meats held at 2
◦ C for 6 weeks, the dominant biota consisted of Yersinia
enterocolitica, Serratia liquefaciens, Shewanella putrefaciens, and a Lactobacillus sp.
32 S. putrefaciens
caused greening of product, but a pH <6.0 was inhibitory to its growth. When dark-cutting beef of
pH 6.6 was vacuum packaged and stored at 0–2
◦ C, lactobacilli were dominant after 6 weeks, but
after 8 weeks psychrotrophic Enterobacteriaceae became dominant.
75 Most of the Enterobacteriaceae
resembled S. liquefaciens and the remainder resembled Hafnia alvei. In vacuum-packaged beef with
pH 6.0, Y. enterocolitica-like organisms were found at levels of 10
7 /g after 6 weeks at 0–2
◦ C, but on
meats with pH <6.0, their numbers did not exceed 10
5 /g even after 10 weeks.
80 The high-pH meat
also yielded S. putrefaciens with counts as high as log 6.58/g after 10 weeks.
When normal raw beef with an ultimate pH of about 5.6 is vacuum packaged, lactobacilli and
other lactic acid bacteria predominate. When the beef was allowed to spoil aerobically, acidic/sour
odors were noted when the APC was about 10
7 –10
8 /cm
2 with approximately 15% of the biota being
Pseudomonas spp.; but when vacuum-packaged samples spoiled, the product was accompanied by
a slight increase in pH with a general increase in extract-release volume (ERV).
42 After a 9-week
storage at 0–1
◦ C, Hitchener et al.
46 found that 75% of the biota of vacuum-packaged raw beef consisted of catalase-negative organisms. Upon further characterization of 177 isolates, 18 were found
363
CO 2 + 85% N 2 , 80% CO 2 + 20% N 2 , under vacuum, and in pure nitrogen at 4
◦ C for up to 2 weeks.
In another study of C. jejuni in beef, two of three strains decreased slightly when stored at 20
◦ C
and 4
◦ C for 48 hours but increased when stored at 37
◦ C under all gas mixtures used.
44 The storage
conditions consisted of vacuum packaging, 20% CO 2 + 80% N 2 , 5% O 2 + 10% CO 2 + 85% N 2 ,
and the background biota apparently had no effect on C. jejuni. When heat-injured and uninjured
cells of Aeromonas hydrophila were stored at 5
◦ C for 22 days under 100% N 2 or 100% CO 2 , both
types steadily declined in numbers, but growth of both was enhanced when incubated under N 2 .
33 The
heat-injured cells apparently were not particularly disadvantaged under the conditions noted.
In a study of raw crayfish stored at 4–10
◦ C for up to 30 days, Aeromonas hydrophila, S. aureus, C.
perfringens, and C. botulinum type E were not found.
63 When cooked crawfish was inoculated with
10
3 cfu of C. botulinum type E spores/g of crawfish tail, vacuum packaged with both high-barrier
film and air-impermeable bags followed by storage at 4 and 10
◦ C for 30 days, type E toxin was not
detected in any packages.
63
SPOILAGE OF MAP AND VACUUM-PACKAGED MEATS
From the research of many groups, it is clear that when vacuum-packaged meats undergo long-term
refrigerator spoilage, often the predominant organisms are lactobacilli, other lactics, B. thermosphacta,
or all of these. Other organisms can be found and, indeed, others may predominate. Among the
determining factors are the following:
1. whether the product is raw or cooked;
2. concentration of nitrites present;
3. relative load of psychrotrophic bacteria;
4. the degree to which the vacuum-package film excludes O 2 ;
5. product pH.
Cooked or partially cooked meats, along with dark, firm, and dry (DFD) and dark-cutting meats,
have a higher pH than raw and light-cutting meats, and the organisms that dominate these products
during vacuum storage are generally different from those found in vacuum-packaged normal meats.
In vacuum-packaged DFD meats held at 2
◦ C for 6 weeks, the dominant biota consisted of Yersinia
enterocolitica, Serratia liquefaciens, Shewanella putrefaciens, and a Lactobacillus sp.
32 S. putrefaciens
caused greening of product, but a pH <6.0 was inhibitory to its growth. When dark-cutting beef of
pH 6.6 was vacuum packaged and stored at 0–2
◦ C, lactobacilli were dominant after 6 weeks, but
after 8 weeks psychrotrophic Enterobacteriaceae became dominant.
75 Most of the Enterobacteriaceae
resembled S. liquefaciens and the remainder resembled Hafnia alvei. In vacuum-packaged beef with
pH 6.0, Y. enterocolitica-like organisms were found at levels of 10
7 /g after 6 weeks at 0–2
◦ C, but on
meats with pH <6.0, their numbers did not exceed 10
5 /g even after 10 weeks.
80 The high-pH meat
also yielded S. putrefaciens with counts as high as log 6.58/g after 10 weeks.
When normal raw beef with an ultimate pH of about 5.6 is vacuum packaged, lactobacilli and
other lactic acid bacteria predominate. When the beef was allowed to spoil aerobically, acidic/sour
odors were noted when the APC was about 10
7 –10
8 /cm
2 with approximately 15% of the biota being
Pseudomonas spp.; but when vacuum-packaged samples spoiled, the product was accompanied by
a slight increase in pH with a general increase in extract-release volume (ERV).
42 After a 9-week
storage at 0–1
◦ C, Hitchener et al.
46 found that 75% of the biota of vacuum-packaged raw beef consisted of catalase-negative organisms. Upon further characterization of 177 isolates, 18 were found