Fundamental Food Microbiology, Third Edition - Fuad Fathir

298 FUNDAMENTAL FOOD MICROBIOLOGY
I. Gray Discoloration of Turkey Luncheon Meat
Turkey luncheon meat slices prepared mainly from dark meat developed gray spots
or patches within 2 to 3 d during aerobic storage at refrigeration temperature. 8 The
causative bacteria was isolated and identified to be an H 2O 2-producing strain of
Lactobacillus species. The product formulation contained ca. 1% lactate. It is suspected
that the bacterial strain under aerobic conditions of growth utilized lactate
to produce H 2O 2 (lactate + O 2 Æ pyruvate + H 2O 2, reaction catalyzed by L-lactate
oxidase). H 2O 2 then oxidized the myoglobin to produce a white-gray color. The
H 2O 2-producing strain could come as postheat contaminants or survive low-heat
treatment, as some lactobacilli are thermoduric. Under vacuum storage, the strain
does not produce H 2O 2 to cause discoloration.
J. Pink Discoloration of Sliced, Chopped, and Formed Roast Beef
Vacuum-packaged chopped and formed low-fat roast beef slices during refrigeration
storage for 1 to 2 weeks developed pink to red patches over the normal brown color. 8
Initially, the fresh product had a pH of 5.8 to 6.0 and a slight off-odor. Within 4 to
5 weeks at refrigerated temperature, the slices developed a strong fishy to putrid odor;
no H 2S was detected, and the pH remained ca. 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 ca. 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 leuconostocs- 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 temperature, 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 2S. Analysis of absorption spectra of the extract revealed that the color
change occurs because of the partial reduction of metmyoglobin.
The lactic acid bacteria as well as the Gram-negative species isolated from these
samples were heat-sensitive psychotrophic and facultative anaerobic. They definitely
entered the product as postheat contaminants, indicating poor sanitation in the
production facilities. The absence of large amounts of gas in this product, as opposed
to some products described before, could be due to the presence of very little
metabolizable carbohydrate. The reason for the pH to remain unchanged (@ pH 6.0)
during deamination of amino acids could be the use of high amounts of phosphate
in the formulation to bind water.
K. Gas Distension and Pink Discoloration of Sliced Turkey Rolls
Sliced low-fat turkey rolls (initial pH 6.5) were packaged by modified atmosphere
(drawing vacuum and then flushing a mixture of 30% CO 2 and 70% N 2 in small