Fundamental Food Microbiology, Third Edition - Fuad Fathir

SPOILAGE OF SPECIFIC FOOD GROUPS 271
microorganisms present in a given environment. Postrigor meats are rich in nonprotein
nitrogenous (NPN) compounds (ca. 13 mg/g; amino acids and creatine), peptides,
and proteins, but contain low concentrations of carbohydrates (ca. 1.3 mg/g;
glycogen, glucose, glucose-6-phosphate), with a pH ca. 5.5 and A w more than 0.97.
Dark, firm, dry (DFD) meats have almost no carbohydrates and a pH 6.0 or above. 1–5
To delay microbial spoilage, fresh meats are stored at refrigerated temperature
(£5�C), unless the facilities are not available. Thus, normally psychrotrophic bacteria
are the most predominant types in raw meat spoilage. Under aerobic storage at low
temperature, growth of psychrotrophic aerobes and facultative anaerobes is favored.
In retail-cut meats, because of a shorter generation time, Pseudomonas spp. grows
rapidly, using glucose first and then amino acids; the metabolism of amino acids is
accompanied by the production of malodorous methyl sulfides, esters, and acids. In
meats with high pH or low glucose content, or both, Acinetobacter and Morexella,
which preferentially metabolize amino acids instead of glucose, can grow rapidly
and produce undesirable odors. Spoilage by these strict aerobes in the form of offodor
is detected at a population of ca. 10 7–8 cells/cm 2 and slime at ca. 10 8–9 cells/cm 2 .
The oxygenated red color of myoglobin undergoes oxidation to produce gray or
brown metmyoglobin. DFD meats are spoiled more rapidly, inasmuch as the bacteria
utilize amino acids immediately because of the absence of carbohydrates.
Refrigerated meat in a modified atmosphere, such as in a mixture of CO 2 and
O 2, favors growth of facultative anaerobic Brochothrix thermosphacta, especially in
meat with pH 6.0 or higher (DFD meat). It metabolizes glucose to acetic acid and
acetoin, and leucine and valine to isovaleric and isobutyric acids, to produce offodor
(cheesy odor). Under anaerobic conditions, it metabolizes glucose to produce
small amounts of lactic acid (which is not considered a cause of spoilage).
Psychrotrophic facultative anaerobes and anaerobes can grow in vacuum-packaged
meat to produce different types of spoilage. Lactobacillus curvatus and Lab.
sake metabolize glucose to produce lactic acid and the amino acids leucine and
valine to isovaleric and isobutyric acids. These volatile fatty acids impart a cheesy
odor in meat at a population level of more than 10 7–8 cells/cm 2 . This spoilage is not
considered to be highly undesirable, because after opening the package, the odor
disappears. However, when they metabolize cysteine and produce H 2S gas, the
products have undesirable odor and color. Heterofermentative Leuconostoc carnosum
and Leu. gelidum produce CO 2 and lactic acid, causing accumulation of gas
and liquid in the package. Shewanella putrefacience, which can grow under both
aerobic and anaerobic conditions, metabolizes amino acids (particularly cysteine) to
produce methylsulfides and H 2S in large quantities. Along with offensive odors, they
adversely affect the normal color of meats. H 2S oxidizes myoglobin to a form of
metmyoglobin, causing a green discoloration. Facultative anaerobic Enterobacter,
Serratia, Proteus, and Hafnia species metabolize amino acids while growing in meat
to produce amines, ammonia, methylsulfides, and mercaptans, and cause putrefaction.
Some strains also produce H 2S in small amounts to cause greening of the meat.
Because amines and ammonia are produced, the pH of the meat usually changes to
alkaline range, and meat can have a pinkish to red color. Psychrotrophic Clostridium
spp., such as Clostridium laramie, have been found to cause spoilage associated
with proteolysis and loss of texture of meat, accumulation of liquid in the bag, and
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