280 ◾ Fundamental Food Microbiology
contaminated the products after heat treatment and before vacuum packaging. Leuconostoc sp.
produced gas (CO 2 ) and acid, and Lab. sake also produced acid but not gas because it is homofermentative. But the pH did not drop owing to the high phosphate content used in this low-fat
product. Subsequently, Serratia sp. grew and caused deamination of amino acids, thus releasing
ammonia. The products also had a slight pink color, which could be a result of partial reduction
of metmyoglobin (see the section on pink discoloration of sliced, chopped, and formed roast beef
and the section on gas distension and pink discoloration of sliced turkey rolls).
Yellow Discoloration of Luncheon Meat
Vacuum-packaged cooked luncheon meat, prepared from chopped ham, developed yellow-colored
spots within 3–4 weeks of storage at 40°F (4.4°C). 12 Microbiological analysis indicated the presence of high numbers of Enterococcus faecium ssp. casseliflavus, which was able to survive at 71.1°C
for 20 min. Thus, the species probably survived cooking. The yellow discoloration was suspected
to be a result of the production of a carotenoid substance by the bacterial species.
Gray Discoloration of Turkey Luncheon Meat
Turkey luncheon meat slices prepared mainly from dark meat developed gray spots or patches
within 2–3 days during aerobic storage at refrigeration temperatures. 8 The causative bacteria was
isolated and identified to be an H 2 O 2 -producing strain of Lactobacillus species. The product formulation contained approximately 1% lactate. It is suspected that the bacterial strain under aerobic
conditions of growth utilized lactate to produce H 2 O 2 (lactate + O 2 → pyruvate + H 2 O 2 , reaction
catalyzed by l-lactate oxidase). H 2 O 2 then oxidized the myoglobin to produce a white-gray color.
The H 2 O 2 -producing strain could come as a post-heat contaminant or survive low-heat treatment
as some lactobacilli are thermoduric. Under vacuum storage, the strain does not produce H 2 O 2 to
cause discoloration.
Pink Discoloration of Sliced, Chopped, and Formed Roast Beef
Vacuum-packaged chopped and formed low-fat roast beef slices during refrigerated storage for
1–2 weeks developed pink to red patches over the normal brown color. 8 Initially, the fresh product had a pH of 5.8–6.0 and a slight off-odor. Within 4–5 weeks at refrigerated temperatures,
the slices developed a strong fishy to putrid odor; no H 2 S was detected, and the pH remained at
approximately 6.0. The meat color changed to deep pink to red with a dark-red colored purge,
but there was a very small amount of gas accumulation. Microbiological enumeration revealed
approximately 9 × 10 9 /mL purge of APC and 1 × 10 9 /mL purge of lactic acid bacteria (leuconostocs or lactobacilli). Examination of the purge under a phase-contrast microscope revealed
leuconostoc- and lactobacilli-like cells as well as motile rods. Streaking on xylose-lysine iron agar
plates helped isolate three types of colonies of Gram-negative rods, which were biochemically
identified as Hafnia alvei, Proteus vulgaris, and Ser. liquifaciens. Inoculation of the three isolates
in roast beef, followed by vacuum packaging and storage at refrigerated temperatures, revealed
that all three Gram-negative isolates could change the brown color of roasts to pink or red (like
rare roast beef ) with a red-colored purge, but Pro. vulgaris changed the meat color to cherry red.
The packages inoculated with Ser. liquifaciens also had slight gas. All three Gram-negative isolates
produced small amounts of H 2 S. Analysis of absorption spectra of the extract revealed that the
color change occurs because of the partial reduction of metmyoglobin.
contaminated the products after heat treatment and before vacuum packaging. Leuconostoc sp.
produced gas (CO 2 ) and acid, and Lab. sake also produced acid but not gas because it is homofermentative. But the pH did not drop owing to the high phosphate content used in this low-fat
product. Subsequently, Serratia sp. grew and caused deamination of amino acids, thus releasing
ammonia. The products also had a slight pink color, which could be a result of partial reduction
of metmyoglobin (see the section on pink discoloration of sliced, chopped, and formed roast beef
and the section on gas distension and pink discoloration of sliced turkey rolls).
Yellow Discoloration of Luncheon Meat
Vacuum-packaged cooked luncheon meat, prepared from chopped ham, developed yellow-colored
spots within 3–4 weeks of storage at 40°F (4.4°C). 12 Microbiological analysis indicated the presence of high numbers of Enterococcus faecium ssp. casseliflavus, which was able to survive at 71.1°C
for 20 min. Thus, the species probably survived cooking. The yellow discoloration was suspected
to be a result of the production of a carotenoid substance by the bacterial species.
Gray Discoloration of Turkey Luncheon Meat
Turkey luncheon meat slices prepared mainly from dark meat developed gray spots or patches
within 2–3 days during aerobic storage at refrigeration temperatures. 8 The causative bacteria was
isolated and identified to be an H 2 O 2 -producing strain of Lactobacillus species. The product formulation contained approximately 1% lactate. It is suspected that the bacterial strain under aerobic
conditions of growth utilized lactate to produce H 2 O 2 (lactate + O 2 → pyruvate + H 2 O 2 , reaction
catalyzed by l-lactate oxidase). H 2 O 2 then oxidized the myoglobin to produce a white-gray color.
The H 2 O 2 -producing strain could come as a post-heat contaminant or survive low-heat treatment
as some lactobacilli are thermoduric. Under vacuum storage, the strain does not produce H 2 O 2 to
cause discoloration.
Pink Discoloration of Sliced, Chopped, and Formed Roast Beef
Vacuum-packaged chopped and formed low-fat roast beef slices during refrigerated storage for
1–2 weeks developed pink to red patches over the normal brown color. 8 Initially, the fresh product had a pH of 5.8–6.0 and a slight off-odor. Within 4–5 weeks at refrigerated temperatures,
the slices developed a strong fishy to putrid odor; no H 2 S was detected, and the pH remained at
approximately 6.0. The meat color changed to deep pink to red with a dark-red colored purge,
but there was a very small amount of gas accumulation. Microbiological enumeration revealed
approximately 9 × 10 9 /mL purge of APC and 1 × 10 9 /mL purge of lactic acid bacteria (leuconostocs or lactobacilli). Examination of the purge under a phase-contrast microscope revealed
leuconostoc- and lactobacilli-like cells as well as motile rods. Streaking on xylose-lysine iron agar
plates helped isolate three types of colonies of Gram-negative rods, which were biochemically
identified as Hafnia alvei, Proteus vulgaris, and Ser. liquifaciens. Inoculation of the three isolates
in roast beef, followed by vacuum packaging and storage at refrigerated temperatures, revealed
that all three Gram-negative isolates could change the brown color of roasts to pink or red (like
rare roast beef ) with a red-colored purge, but Pro. vulgaris changed the meat color to cherry red.
The packages inoculated with Ser. liquifaciens also had slight gas. All three Gram-negative isolates
produced small amounts of H 2 S. Analysis of absorption spectra of the extract revealed that the
color change occurs because of the partial reduction of metmyoglobin.