Fundamental Food Microbiology, Third Edition - Fuad Fathir

276 FUNDAMENTAL FOOD MICROBIOLOGY
B. Crustaceans
Microbial spoilage of shrimps is more prevalent than that of crabs and lobsters.
Whereas crabs and lobsters remain alive until they are processed, shrimps die during
harvest. The flesh of crustaceans is rich in NPN compounds (amino acids, especially
arginine, trimethylamine oxide), contains ca. 0.5% glycogen, and has a pH above
6.0. The predominant microflora are Pseudomonas and several Gram-negative rods.
If other necessary factors are present, the nature of spoilage is quite similar to that
in fresh fish. Microbial spoilage of shrimp is dominated by odor changes due to
production of volatile metabolites of NPN compounds (from decay and putrefaction),
slime production, and loss of texture (soft) and color. If the shrimps are processed
and frozen rapidly, the spoilage can be minimized. Lobsters are frozen following
processing or sold live and thus are not generally exposed to spoilage conditions.
Crabs, lobsters, and shrimp are also cooked to extend their shelf life. However, they
are subsequently exposed to conditions that cause postheat contamination and then
stored at low temperature (refrigerated and frozen). Blue crabs are steamed under
pressure, and the meat is picked and marketed as fresh crabmeat. To extend shelf
life (and safety), the meat is also heat processed (85�C for 1 min) and stored at
refrigerated temperature. Under refrigerated conditions, they have a limited shelf
life because of growth of surviving bacteria and postheat contaminants.
C. Mollusks
As compared with fish and crustaceans, oyster, clam, and scallop meats are lower in
NPN compounds but higher in carbohydrates (glycogen, 3.5 to 5.5%), with pH
normally above 6.0. The mollusks are kept alive until processed (shucked); thus,
microbiological spoilage occurs only after processing. The resident microflora are
predominantly Pseudomonas and several other Gram-negative rods. During refrigerated
storage, microorganisms metabolize both NPN compounds and carbohydrates.
Carbohydrates can be metabolized to produce organic acids by lactic acid bacteria
(Lactobacillus spp.), enterococci, and coliforms, thereby lowering the pH. Breakdown
of nitrogenous compounds primarily by Pseudomonas and Vibrio, especially at refrigerated
temperature, results in production of NH 3, amines, and volatile fatty acids.
A. Raw Milk
V. MILK AND MILK PRODUCTS
Raw milk contains many types of microorganisms coming from different sources.
The average composition of cow’s milk is protein 3.2%, carbohydrates 4.8%, lipids
3.9%, and minerals 0.9%. Besides casein and lactalbumin, it has free amino acids,
which can provide a good N source (and some C source, if necessary). As the main
carbohydrate is lactose, those microorganisms with lactose-hydrolyzing enzymes
(phospho-b-galactosidase or b-galactosidase) have an advantage over those unable
to metabolize lactose. Milk fat can be hydrolyzed by microbial lipases, with the
release of small-molecular volatile fatty acids (butyric, capric, and caproic acids).