Fundamental Food Microbiology, Third Edition - Fuad Fathir

294 FUNDAMENTAL FOOD MICROBIOLOGY
IV. INCIDENCE OF SPOILAGE OF VACUUM-PACKAGED
REFRIGERATED FOOD
A. Spoilage of Unprocessed (Fresh) Beef by Clostridium spp.
Large-scale spoilage of vacuum-packaged refrigerated, unprocessed beef has been
recognized recently. 5–9 The spoilage is characterized by accumulation of large quantities
of H 2S-smelling gas and purge (liquid) in the bag within 2 weeks at 4�C, and
loss of texture (soft) and color (cherry red) of meat, which in 10 to 12 weeks changes
to greenish (Figure 20.1A). Enumeration by plating revealed ca. 10 8 /ml leuconostocs
colony-forming units (CFUs) in the purge. However, examination of a little purge
under a phase-contrast microscope (see cover page photograph) revealed a large
number of medium to large, thick, motile rods, some cells with large terminal spores
(drumstick-shaped), as well as some leuconostoc-like cells (lenticular, small chains).
The rods and the lenticular cells were Gram-positive. The rod-shaped bacterium was
suspected to be a Clostridium species, but could not be cultured in agar and broth
media recommended for Clostridium spp. Later, it was purified by heating the purge
(from the package of a 12-week-old spoiled meat containing spores) at 80�C for 10
min and culturing the material at 15�C in a broth under anaerobic condition. Biochemical
studies revealed it to be a new species and it was designated Clo. laramie
NK1 (ATCC 51254). The cells are extremely sensitive to oxygen and grow optimally
under a good anaerobic environment at 12 to 15�C, with a growth temperature range
of –2 to 22�C. The species can sporulate and germinate at 2�C.
Inoculation of raw beef with these spores followed by vacuum packaging and
refrigeration storage caused the samples to develop similar characteristics and the
purge had the Clostridium cells. A limited in-plant sampling was done from the
slaughtering area to the conveyor systems carrying the primal cuts for vacuum packaging
(in a plant having the specific problem). Three of the 25 samples were positive
for Clo. laramie, and all three were from the conveyor system at the vacuumpackaging
area. The conveyor system used by this plant consisted of small links. In
general, the microbiological quality of the meat was very good (ª10 3 /g aerobic plate
counts, APCs). It was suspected that the sanitation specifically designed to keep the
microbial load low was good, but was unable to kill the spores, especially in the
inaccessible joints (dead spot) and similar places in the links of the conveyor system.
Thus, the fabricated meats were constantly inoculated with the spores (and cells),
which, in the absence of competition, because of low associated bacteria, germinated
or grew (or both) to cause large-scale spoilage (along with psychrotrophic Leuconostoc
spp.). Subsequent studies with spoiled meat samples from several beef processors
having similar problem in the U.S. have revealed that this incidence is not isolated
and more than one strain of Clo. laramie is involved in spoilage. 5–8
A similar spoilage of vacuum-packaged refrigerated beef in Europe was also
reported to be caused by a similar Clostridium sp. and named Clo. estertheticum.
This species also could not be enumerated by agar plating methods and was first
detected microscopically. 7 In recent years, spoilage of vacuum-packaged refrigerated
raw beef by Clostridium spp. has been reported from Canada and New Zealand.