Food Protection with Modified Atmospheres
365
B. thermosphacta grows on beef at pH 5.4 when incubated aerobically but does not grow anaerobically at pH <5.8.
10 Under the latter conditions, the apparent minimum growth pH is 6.0. S. putrefaciens
is also pH sensitive and does not grow on beef of normal pH but grows on DFD meats.
The effect of MAP on the growth of molds on bakery products was assessed by the use of a sponge
cake analog. No fungal growth occurred up to 28 days at 25
◦ C with 100% CO 2 regardless of a w .
37 At
a w values of 0.80–0.90, there was no significant influence on fungal growth. Lag phase doubled when
a w was 0.85 and CO 2 increased to 70% in the headspace.
The spoilage of fresh and thawed MAP-stored salmon was studied at 2
◦ C and the dominant organism
was found to be Photobacterium phosphoreum, which reduced self-life to ca. 14 days for fresh and
21 days for thawed products.
21 This organism was eliminated by freezing and storage at 2
◦ C and
thus product self-life was extended by 1–2 weeks. The spoilage biota of thawed MAP salmon was
dominated by Carnobacterium piscicola, which was apparently responsible for the tyramine found at
the end of the storage period. Numbers of P. phosphoreum exceeded 10
6 cfu/g and it was considered
to be the specific spoilage organism.
21
Volatile Components of Vacuum-Packaged Meats and Poultry
The off-odors and off-flavors produced in vacuum-packaged meat products by the spoilage biota
are summarized in Table 14–6. In general, short-chain fatty acids are produced by both lactobacilli and
B. thermosphacta, and spoiled products may be expected to contain these compounds, which confer
sharp off-odors. In vacuum-packaged luncheon meats, acetoin and diacetyl have been found to be the
most significant relative to spoiled meat odors.
87 Using a culture medium (all purpose Tween—APT)
containing glucose and other simple carbohydrates, the formation by B. thermosphacta of isobutyric
and isovaleric acids was favored by low glucose and near-neutral pH, whereas acetoin, acetic acid,
2,3-butanediol, 3-methylbutanol, and 3-methyl-propanol production were favored by high glucose
and low pH.
13,14 According to these investigators, acetoin is the major volatile compound produced
on raw and cooked meats in O 2 -containing atmospheres. This suggests that the volatile compounds
produced by B. thermosphacta may be expected to vary between products with high and low glucose
concentrations. The addition of 2% glucose to raw ground beef has been shown to decrease pH and
delay off-odor and slime development without affecting the general spoilage flora,
82 and although
the studies noted were not conducted with vacuum-packaged meats, it would seem to be a way to
shift the volatile components from short-chain fatty acids to acetoin and other compounds that derive
from glucose. Because vacuum-packaged, high-pH meats have a much shorter self-life, the addition
of glucose could be of benefit in this regard.
In a study of spoiled vacuum-packaged steaks, a sulfide odor was evident with numbers of 10
7 –
10
8 /cm
2 .
41 The predominant organisms isolated were H. alvei, lactobacilli, and Pseudomonas. H. alvei
was the likely cause of the sulfide odor. Within 1 week after processing, vacuum-packaged refrigerated raw beef underwent spoilage, which was characterized by large amounts of H 2 -smelling gas
along with extensive proteolysis.
56 The causative agent was Clostridium laramie, a species that is
psychrotrophic
55 Another psychrotrophic Clostridium recovered from vacuum-packaged refrigerated
pork is C. algidicarnis. From spoiled refrigerated vacuum-packaged meat was isolated a bacterium
that produced large amounts of H 2 , CO 2 , butanol, and butanoic acid, along with esters and volatile
sulfur-containing compounds.
15 The isolate was a psychrophile that grows between 1
◦ C and 15
◦ C but
not at 22
◦ C, and it has been classified as Clostridium estertheticum.
11
From the summary of volatiles in Table 14–6, it is evident that all organisms produced either dimethyl
di- or trisulfide, or methyl mercaptan, except B. thermosphacta. Dimethyl disulfide was produced in
365
B. thermosphacta grows on beef at pH 5.4 when incubated aerobically but does not grow anaerobically at pH <5.8.
10 Under the latter conditions, the apparent minimum growth pH is 6.0. S. putrefaciens
is also pH sensitive and does not grow on beef of normal pH but grows on DFD meats.
The effect of MAP on the growth of molds on bakery products was assessed by the use of a sponge
cake analog. No fungal growth occurred up to 28 days at 25
◦ C with 100% CO 2 regardless of a w .
37 At
a w values of 0.80–0.90, there was no significant influence on fungal growth. Lag phase doubled when
a w was 0.85 and CO 2 increased to 70% in the headspace.
The spoilage of fresh and thawed MAP-stored salmon was studied at 2
◦ C and the dominant organism
was found to be Photobacterium phosphoreum, which reduced self-life to ca. 14 days for fresh and
21 days for thawed products.
21 This organism was eliminated by freezing and storage at 2
◦ C and
thus product self-life was extended by 1–2 weeks. The spoilage biota of thawed MAP salmon was
dominated by Carnobacterium piscicola, which was apparently responsible for the tyramine found at
the end of the storage period. Numbers of P. phosphoreum exceeded 10
6 cfu/g and it was considered
to be the specific spoilage organism.
21
Volatile Components of Vacuum-Packaged Meats and Poultry
The off-odors and off-flavors produced in vacuum-packaged meat products by the spoilage biota
are summarized in Table 14–6. In general, short-chain fatty acids are produced by both lactobacilli and
B. thermosphacta, and spoiled products may be expected to contain these compounds, which confer
sharp off-odors. In vacuum-packaged luncheon meats, acetoin and diacetyl have been found to be the
most significant relative to spoiled meat odors.
87 Using a culture medium (all purpose Tween—APT)
containing glucose and other simple carbohydrates, the formation by B. thermosphacta of isobutyric
and isovaleric acids was favored by low glucose and near-neutral pH, whereas acetoin, acetic acid,
2,3-butanediol, 3-methylbutanol, and 3-methyl-propanol production were favored by high glucose
and low pH.
13,14 According to these investigators, acetoin is the major volatile compound produced
on raw and cooked meats in O 2 -containing atmospheres. This suggests that the volatile compounds
produced by B. thermosphacta may be expected to vary between products with high and low glucose
concentrations. The addition of 2% glucose to raw ground beef has been shown to decrease pH and
delay off-odor and slime development without affecting the general spoilage flora,
82 and although
the studies noted were not conducted with vacuum-packaged meats, it would seem to be a way to
shift the volatile components from short-chain fatty acids to acetoin and other compounds that derive
from glucose. Because vacuum-packaged, high-pH meats have a much shorter self-life, the addition
of glucose could be of benefit in this regard.
In a study of spoiled vacuum-packaged steaks, a sulfide odor was evident with numbers of 10
7 –
10
8 /cm
2 .
41 The predominant organisms isolated were H. alvei, lactobacilli, and Pseudomonas. H. alvei
was the likely cause of the sulfide odor. Within 1 week after processing, vacuum-packaged refrigerated raw beef underwent spoilage, which was characterized by large amounts of H 2 -smelling gas
along with extensive proteolysis.
56 The causative agent was Clostridium laramie, a species that is
psychrotrophic
55 Another psychrotrophic Clostridium recovered from vacuum-packaged refrigerated
pork is C. algidicarnis. From spoiled refrigerated vacuum-packaged meat was isolated a bacterium
that produced large amounts of H 2 , CO 2 , butanol, and butanoic acid, along with esters and volatile
sulfur-containing compounds.
15 The isolate was a psychrophile that grows between 1
◦ C and 15
◦ C but
not at 22
◦ C, and it has been classified as Clostridium estertheticum.
11
From the summary of volatiles in Table 14–6, it is evident that all organisms produced either dimethyl
di- or trisulfide, or methyl mercaptan, except B. thermosphacta. Dimethyl disulfide was produced in