Fundamental Food Microbiology, Third Edition - Fuad Fathir

296 FUNDAMENTAL FOOD MICROBIOLOGY
with typical terminal spores. Colonies of this bacterium were not obtained by
isolation in specific agar media, but large numbers of leuconostoc colonies (@10 8 /ml
of purge) were obtained. The spore-containing purge was heated and cultured to
purify the Clostridium spp. Inoculation of the pure culture to rare roasted beef,
followed by vacuum packaging and refrigeration, showed the formation of H 2S gas,
change of meat color from light brown to pink, and accumulation of pink to red
purge within 2 to 3 weeks. Examination of the purge under a phase-contrast microscope
revealed the presence of Clostridium cells and spores. Some of these isolates
seem to differ in some biochemical and physiological patterns when compared with
Clo. laramie isolated from the spoiled raw beef (see Section IV-A). 5,7
C. Spoilage of Pork Chops by Clostridium algidicarnis
This new species is associated with spoilage of cooked, vacuum-packaged pork meat
products during storage at 4�C. 9 It produces an offensive spoilage odor.
D. Spoilage of Tofu by Clostridium spp.
A vacuum-packaged soybean milk curd product (Tofu) during refrigerated storage
revealed gas and liquid accumulation in the bag and off-flavor (Figure 20.1B). 8
Microscopic examination of the liquid showed the presence of large numbers of
motile rods, many with terminal oval spores. The strain was isolated in pure form
and found to have some biochemical and physiological characteristics different from
those of Clo. laramie NK 1 because it could be grown in agar medium under
anaerobic condition. However, it is psychrophilic and does not grow at 25�C or
above.
E. Spoilage of Unripened Soft Cheese by Leuconostoc spp.
Spanish-style soft cheese, made by coagulating pasteurized milk with lactic acid
and rennet and collecting the curd and vacuum packaging, showed gas formation
and liquid accumulation in the package within 30 d at refrigerated temperature. (The
expected shelf life is 50+ d.) 8 The curd texture was soft and gassy. Examination of
the liquid under a phase-contrast microscope revealed short chains of leuconostoclike
cells. Enumeration in pH 5.0 plating media revealed 10 8–9 CFUs/g product.
Purified cells from the colonies were Gram-positive, heterofermentative, and biochemically
identified as Leuconostoc spp. The bacteria possibly get in the product
during extensive handling of the product following pasteurization of milk. Even
when they are initially present at a low level, during long storage they can multiply,
form gas, and produce the defect. Any temperature abuse accelerates the spoilage.
F. Spoilage of Low-Heat-Processed Meat Products by Leuconostoc
spp.
Many types of commercially processed, vacuum-packaged refrigerated meat products,
following spoilage, were collected from local supermarkets. The products were