260 ◾ Fundamental Food Microbiology
putrid. Bacterial growth is also associated with slime production, discoloration of gills and eyes (in
whole fish), and loss of muscle texture (soft because of proteolysis).
In fish stored by vacuum or by CO 2 packaging, growth of aerobic spoilage bacteria is prevented. However, anaerobic and facultative anaerobic bacteria can grow, including lactic acid bacteria. Under refrigeration, products have relatively long shelf life as a result of slower growth of
spoilage bacteria. Salted fish, especially lightly salted fish, are susceptible to spoilage by halophilic
bacteria, such as Vibrio (at lower temperatures) and Micrococcus (at higher temperatures). Smoked
fish, especially with lower A W , inhibit the growth of most bacteria. However, molds can grow on
the surface. Minced fish flesh, surimi, and seafood analogs prepared from fish tissues generally
have high initial bacterial levels as a result of extensive processing (ca. 10 5–6 /g). The types include
those present in fish and those that get in during processing. These products, such as fresh fish,
can be spoiled rapidly by Gram-negative rods unless frozen quickly or used soon after thawing.
Canned fish (tuna, salmon, and sardines) are given heat treatment to produce commercially sterile
products. They can be spoiled by thermophilic spore formers unless proper preservation and storage conditions are used. 1,4
Crustaceans
Microbial spoilage of shrimp 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 approximately 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 resulting from production of volatile metabolites of NPN compounds (from decay and
putrefaction), slime production, and loss of texture (soft) and color. If the shrimp 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
post-heat contamination and then stored at low temperatures (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
temperatures. Under refrigerated conditions, they have a limited shelf life because of growth of
surviving bacteria and post-heat contaminants.
Mollusks
As compared with fish and crustaceans, oyster, clam, and scallop meats are lower in NPN compounds but higher in carbohydrates (glycogen, 3.5%–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 Gramnegative 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 temperatures, results in production of NH 3 , amines, and volatile fatty acids.
putrid. Bacterial growth is also associated with slime production, discoloration of gills and eyes (in
whole fish), and loss of muscle texture (soft because of proteolysis).
In fish stored by vacuum or by CO 2 packaging, growth of aerobic spoilage bacteria is prevented. However, anaerobic and facultative anaerobic bacteria can grow, including lactic acid bacteria. Under refrigeration, products have relatively long shelf life as a result of slower growth of
spoilage bacteria. Salted fish, especially lightly salted fish, are susceptible to spoilage by halophilic
bacteria, such as Vibrio (at lower temperatures) and Micrococcus (at higher temperatures). Smoked
fish, especially with lower A W , inhibit the growth of most bacteria. However, molds can grow on
the surface. Minced fish flesh, surimi, and seafood analogs prepared from fish tissues generally
have high initial bacterial levels as a result of extensive processing (ca. 10 5–6 /g). The types include
those present in fish and those that get in during processing. These products, such as fresh fish,
can be spoiled rapidly by Gram-negative rods unless frozen quickly or used soon after thawing.
Canned fish (tuna, salmon, and sardines) are given heat treatment to produce commercially sterile
products. They can be spoiled by thermophilic spore formers unless proper preservation and storage conditions are used. 1,4
Crustaceans
Microbial spoilage of shrimp 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 approximately 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 resulting from production of volatile metabolites of NPN compounds (from decay and
putrefaction), slime production, and loss of texture (soft) and color. If the shrimp 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
post-heat contamination and then stored at low temperatures (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
temperatures. Under refrigerated conditions, they have a limited shelf life because of growth of
surviving bacteria and post-heat contaminants.
Mollusks
As compared with fish and crustaceans, oyster, clam, and scallop meats are lower in NPN compounds but higher in carbohydrates (glycogen, 3.5%–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 Gramnegative 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 temperatures, results in production of NH 3 , amines, and volatile fatty acids.