Microorganisms In Shellfish

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Shellfish is composed of crustaceans (shrimp, crabs, lobster, crawfish, etc) and mollusks (bivalves, squids, snails, etc). Shellfish normally contains more moisture, greater amount of free amino acids, and more extractable nitrogenous compounds than finfish. These biochemical characteristics facilitate bacterial growth and deteriorative reactions resulting in the rapid spoilage of shellfish (27). Many shellfish grow in estuarine, coastal waters, and aquacultural ponds near residential areas and are hence susceptible to contamination by potential pathogenic organisms. Deterioration of shellfish quality results from enzymatic action from both the tissue and the contaminating organisms. Microorganisms that spoil shellfish are similar to those responsible for finfish spoilage. However, such organisms as Moraxella-Acinetobacter and Lactobacillus are more active in shellfish than in finfish.


Shrimp. Among shellfish, shrimp ranks first in value and second in quantity next to crabs (28). Immediately after shrimp death, the tissue enzymes phenolases become active oxidizing tyrosine to bluish black zones or spots at the edges of the shell segments. The dark color is produced by melanin pigments that form on the internal shell surfaces on the underlying shrimp meat. At the same time a variety of bacteria start to proliferate and the growth can be accelerated if the storage temperature is not kept low enough. Removing the heads can reduce 75% of the bacteria. Gulf of Mexico shrimp contain mainly. Moraxella-Acinetobacter, Bacillus, Micrococcus, and Pseudomonas (29). Most of these bacteria produce hydrolytic enzymes: 62% proteolytic, 35% lipolytic, 18% TMA-0 reductive, and 12% indole positive (29-31). Shrimp unloaded from the trawlers have an average bacterial load of 6.0 x 105/g and market shrimp, 3.2 x 106/g. Bacterial counts used for indicating shrimp quality are 1.3 x 106/g, acceptable; 4.5 x 106/g, good; 1.1 x 107/g, fair; and 1.9 x 107/g, poor (32). During iced storage for 16 days, Moraxella-Acinetobacter increase from 27 to 82% of the total bacterial count while Flavobacterium decrease from 18 to 1.5%; Micrococcus, from 34 to 0%; and Pseudomonas, from 19 to 17% (33).

There are two putrefactive types of spoilage in shrimp. One is the production of indole, presumably from tryptophan by bacterial action before icing when exposed at a temperature favorable for bacterial growth. After commencement, the decomposition proceeds fairly rapidly even under ice. Indole is heat resistant and is a reliable spoilage indicator of raw material prior to processing. The other type of ammoniacal decomposition is slow and is characterized by an odor of free ammonia (34-36). The reaction is attributed to both microbial and tissue enzymatic activities depending on storage temperature and bacterial composition.

Crabs. The dominant crabs harvested in the United States are blue crab, king crab, Dungeness crab, and tanner-snow crabs. The bacterial flora of freshly caught crabs reflect that of the growing water, season, and geographic location. The hemolymph of healthy blue crabs from Chincoteague Bay, Va., is about one-fifth sterile according to 290 freshly caught crabs tested (37). The organisms found in the hemolymph of blue crabs are Acineto-bacter, Aeromonas, Pseudomonas, Flavobacterium, Vibrio, Bacillus, coliforms, and Clostridium (38).

Greater numbers of bacterial species are found in Dungeness crabs from Kodiak Island and the Columbia River, waters close to human habitation, whereas the least number of species are found in the tanner crab from the Bering Sea, an area far from human habitation (39). The highest levels of bacteria occur in the gills, 103 to 107/g, as compared to 1 x 101 to 4 x 102/g in muscle tissue. Gills of Dungeness crabs from the Bering Sea contain Moraxella, Acinetobacter, Alcaligenes, Micrococcus, and Staphylococ-

cus, whereas the muscle carries Micrococcus and Staphylococcus (39)

Crawfish. Spoilage of crawfish is caused by the potential spoilers similar to those detected in other crustaceans. In a total of 280 isolates found from spoiled crawfish, 22.1% were shown to be rapid spoilers; 16.4%, low spoilers; and 61.5%, nonspoilers (40). In the group of rapid spoilers over half were pseudomonads and less than half were Moraxella-Acinetobacter. Slow spoilers include Pseudomonas, Moraxella-Acinetobacter, Alcaligenes, Flavobacterium, Aerobacter, Lactobacillus, Micrococcus, and Staphylococcus. Organisms considered as nonrapid spoilers are Aerobacter, Bacillus, Flavobacterium, Micrococcus, Sar-cina, and Staphylococcus (40,41). It is clear that organisms belonging to the same genus have different activities in spoilage.


Bivalves are mollusks, including oysters, clams, and mussels. They are soft-bodied animals that are enclosed by two rigid, bilaterally symmetrical shells. Bivalves are filter feeders and pass a large volume of water through their gills to obtain oxygen and food. Particulate matter, including microorganisms, from the water is trapped in mucus on the gills, then conveyed to the mouth, and finally to the digestive system. Bivalves, particularly oysters, ingest many microorganisms that can survive the digestive process and accumulate in the animals (42-44). The concentration of microorganisms in bivalves can be tens to hundreds of times as high as that in their growing water (42,45). Consumers and public health regulatory agencies are concerned about the pathogenic organisms found in bivalves that are affected by sewage pollution.

Fecal coliforms are generally used as indicators for bivalve quality and for domestic pollution in shellfish-growing waters (46,47). Following harvest, two microbiological guidelines are applied to determine the acceptability of shellfish meats. Bivalves at wholesale market level should have a 35°C standard plate count (SPC) of <500,000 g and a most probable numbers (MPN) fecal coliforms of <230/100 g (46,47).

The microflora of bivalves at harvest is composed of both organisms that are symbiotic with the bivalves and organisms that are filtered from the water and ingested as food. These microorganisms vary qualitatively and quantitatively, depending on the nutrient level, salinity, temperature, and water quality. The commensal microflora include Cristispira pectineus, which colonizes the crystalline style of oysters, and spirochetes such as Saprospira, found in the crystalline style, stomach, and intestine of eastern oysters. These commensal organisms are difficult to culture and have no pathological significance to humans (48). Bivalves at harvest normally carry a total plate count of 103 to 105/g. Soft-shell clams harvested from different growing areas in the Chesapeake Bay contain a geometric mean of SPC, 2.0 x 104-7.2 x 104/g: total coliforms, 1.5 x 10s-6.3 x 103 100 g: fecal coliforms, 29-62/100 g: and E. coli, 14-27 100 g (49).

The common microflora of bivalves at harvest consists primarily of gram-negative rods including Pseudomonas and Vibrio species (50,51). The Flavobacterium-Cytophaga group occasionally exists in a certain level in oysters. Other organisms in oysters include Acinetobacter, Coryne-bacterium, Moraxella, Alcaligenes, Micrococcus, and Bacillus. The microflora of Gulf of Mexico's oysters is dominated by Vibrio, Aeromonas, Moraxella, and Pseudomonas (52). Low levels of yeasts such as Rhodotorula rubra, Tricho-sporon, Candida, and Torulopsis are also frequently encountered in eastern oysters. Several potential pathogenic strains of Vibrionaceae are naturally occurring in non-polluted estuarine waters and may be encountered in bivalves. Coliforms, fecal coliforms, and E. coli are the most common contaminating organisms in bivalves at harvest (45,46,49).

Reducing a high microbiological load in bivalves can be accomplished by placing the animals in clean water that is free of undesirable microorganisms and under conditions in which the bivalves will actively feed. This process is called relaying and usually requires about 15 days to reach the satisfactory microbiological quality (53,54). The relaying process is often applied by transferring bivalves harvested from moderately polluted water into approved shellfish-growing water until the animals clean themselves. A second approach is called depuration in which the shellfish are maintained in tanks of clean water with controlled salinity and temperature (55). The water is often recycled through a biofilter to control water quality. The bivalves in this condition can reach the satisfactory microbiological quality within two to three days (53,55). Nevertheless, removal of viruses often does not correlate with fecal coliform elimination even if fecal indicators have a similar reduction rate as do enteric bacterial pathogens (56,57). Ultraviolet irradiation can facilitate the reduction of contaminating bacterial flora in oysters (56). Because hepatitis A may not be eliminated as readily as other enteroviruses during dupuration, hepatitis may still occur by consuming raw depurated shellfish (58).


Fish carries a variety of organisms and should be handled adequately and processed as soon as possible. Refrigeration, freezing, canning, pasteurization, salting, drying, fermentation, curing, and a combination of these methods are commonly used to process seafood into relatively stable, marketable products. Other methods such as irradiation and modified atmospheric preservation (59) have been extensively studied and proved to have potential application.

Refrigerated and Frozen Fish. Refrigeration at 5°C ceases the growth of the mesophiles and as the temperature is further lowered, psychrophiles are eliminated. During refrigeration storage a gradual killing off of the microorganisms occurs. Gram-negative asporogenous pseudomonads are cold sensitive, whereas gram-positives such as micrococci, lactobacilli, and streptococci are more resistant (60,61).

Cooling rate affects the survival of microorganisms. The maximum survival of E. coli is at a cooling rate of about

6°C/min, and the minimum at about 100°C/min (62). A similar minimum survival rate has been found for Streptococcus faecalis, Salmonella typhimurium, Klebsiella aerogenes, Pseudomonas aeruginosa, and Azotobacter chroococcum, but these organisms have an optimum survival rate varying from 7°C/min for A. chroococcum to ll°C/min for P. aeruginosa (63).

Frozen fish should be stored at or below — 20°C and preferably at — 30°C. During freezing the number of cells that are inactive ranges from 50 to 90% of the initial bacterial population. Gram-positive bacteria are more resistant to freeze injury than gram-negative ones. Some pathogenic organisms can also have a certain survivability at freezing temperatures (65). Spores are the most resistant microbial entity to freeze damage. Poliovirus inoculated into oysters showed a gradual decline in plaque-forming units during frozen storage at — 36°C (64).

Reasons for the cryoinjury of cells are thermal shock, concentration of extracellular solutes, toxic action of concentrated intracellular solutes, dehydration, internal ice formation, and attainment of a minimum cell volume (62). Although cryoinjury results in the cell death, survival of microorganisms occurs and is greater in a supercooled environment than in a frozen one. Some V. parahaemolyticus cells, inoculated into oysters, sole fillets, and crabmeat, can persist at —15 or — 30°C with a greater survival at — 30°C although there is a sharp reduction in viability during freezing (66). Some pathogens such as Listeria can survive in freezing temperatures (60).

Cryoprotective agents are substances that can protect bacterial cells during freezing and thawing. Glycerol, dimethyl sulfoxide, egg white, carbohydrates, peptides, serum albumin, meat extract, milk, glutamic acid, malic acid, diethylene glycol, dextran, Tween 80, glucose, polyethylene glycol, and erythritol have cryoprotective functions probably due to reduction of damage to the cell wall and membrane (60).

Canned Fish. Most canned fish are fully processed products such as canned tuna, salmon, sardines, mackerel, fish balls, and other fish. These canned fish are commercially sterile with 12D process to destroy all pathogenic and other organisms, allowing a satisfactory shelf life at room temperature. However, some problems may arise due to the presence of heat-resistant spore formers in the under-processed products or can leakage from improper seam closure and cross-contamination through cooling water. In oil pack, the oil may protect bacterial spores against heat resulting in nonsterile canned products (67). Flat sour spoilage may occur due to thermophiles, such as B. stearotker-mophilus, which survive processing and multiply during slow cooling and storage at high temperatures. Swollen cans are occasionally encountered when clostria such as C. sporogenes survive inadequate processing.

Dried, Salted, and Smoked Fish. Dehydration is an old process that reduces the water activity (Aw) of the fish products below that required for the growth of microorganisms. The process involves drying with or without other preservatives to form dried, salted, or smoked fish.

Dried salted fish, fish bits (fried shredded fish), katsuo-bushi (dried and smoked skipjack stick), dried shark fins, and dried mullet roe are popular in Asia (68). Smoked salmon, herring, dogfish, and other fish are common fishery products in Europe. Growth of halophilic bacteria or molds may occur, resulting in the spoilage of salted fish. Aspergillus and Penicillium are major species associated with the color deterioration of salted round herring (68). Halophilic bacteria such as Halobacterium and Halococcus commonly present in solar salts are most troublesome during the salting and drying process. These bacteria cause red and pink discoloration and induce softness in salted fish.

Surimi-Based Products. Surimi is a mechanically de-boned, water-washed frozen fish paste containing cryopro-tectants. This high-protein, gel-forming material can be chopped and then mixed with salt, starch, and flavor compounds. The surimi mix is colored, textured, and cooked in two stages to set the gel to process into seafood analogues such as imitation crab, shrimp, lobster, or scallop. Freshly processed crab leg and flaked crab leg analogues contain only 102-103/g bacteria. During storage bacteria grow and SPC reaches 2 x 108/g at 10°C in 25 days and 104-106/g at 5°C in six weeks (69). SPC in flaked crab leg increases rapidly to 109/g after two and four weeks at 5 and 0°C, respectively. Spoilage and quality deterioration of crab analogues are indicated by number of bacteria (107/g), visible slime, odor, and appearance. Slime formation, softened texture, sour odor, and discoloration are consequences of spoilage. Bacillus is predominant initially but Pseudomonas gradually grows and finally outnumbers other genera at two weeks of storage at 0-5°C. Bacillus, which is possibly derived from the ingredient starch, is the major organism throughout the six-day storage at 15°C (69). The spoilage of other fish cake products such as kamaboko can be attributed to Streptococcus, Leuconostoc, and Micrococcus (70,71).

Shellfish Products

After being harvested, shellfish should be kept refrigerated or at low temperature and processed as soon as possible. Shrimp should be beheaded, peeled, or left unshelled and frozen.

Crabmeat. Blue crabs rank first in U.S. crab landings, and their major products are fresh and pasteurized meat. Other crabs caught are king, Dungeness, and tanner-snow crabs from which frozen section, claws and meat, and canned meat are commonly made (72,73).

To process blue crabmeat, live crabs are steam cooked at 121°C for 10 min. Cooked crabs are refrigerated overnight and the meat is removed by hand or machine. The meat is packed in plastic cups for fresh crabmeat or sealed in tin cans to process for pasteurized crabmeat (74). Three kinds of meat are available: lump meat taken from the back fins, claw meat extracted from claws, and regular meat collected from main body (72). In good plant sanitation, fresh crabmeat usually has a geometric mean SPC of 1.5 x 104-4.5 x 104/g, which increases to 1.4 x 10®-3.2

x 106/g under poor plant sanitation (75). Cooked crabs should be stored in refrigeration (<2°C), separated from the live crabs. Refrigerated cooked crabs before picking usually contain bacteria of <104/g while cooked sponge crabs (gravid females carrying an egg mass) taken from the picking table contain bacterial levels as high as 106/g of whole crabs (76,77). Cooked sponge crabs have consistently been found to harbor greater numbers of bacteria than crabs without a sponge (77). Similarly, cooked green crabs, blue crabs that have recently molted and contain a higher level of water than fat crabs, carry higher levels of bacteria than normal crabs into the picking room. They contain higher moisture, which encourages bacterial growth during overnight refrigerated storage (76).

The commercial machine Quik Pik, which mechanically removes the body meat of blue crabs was started on a trial basis in 1978 and now operates successfully in Maryland under proper procedures (76,78). One quick Pik machine can pick 150 lb meat per hour, a rate equal to the work capacity of 30 hand pickers. The cooked crabs are placed in a round, rotating slotted cage to remove legs, fins, and claws, which are dropped through the slots. The crabs then pass through the debacking machine and cleaning device. The cores are loaded on racks for steam heating and then placed in the quik Pik shaker, which vigorously vibrates at 70 oscillations/s for 4 s. All the meat from cores will fall on the collecting belt for further inspection for broken shells and packaging (76).

Machine picking requires constant attention to cleanliness and sanitation to produce a meat product with a satisfactory bacterial quality. The machine should be disassembled and thoroughly cleaned and sanitized at the end of the day. Liquid on the machine during operation creates an aerosol that can greatly contaminate the meat (79). The meat conveyor belt needs continual washing with a tap water spray and sanitation in a chlorine (>200 ppm) bath. A comparison of the bacterial levels indicates that both the SPC and coagulase positive S. aureus for machine-picked meat are lower than for the hand product. However, a higher E. coli count is found in machine-picked meat than in hand-picked meat. This is not surprising because machine picking is processed under wet and warm environment whereas hand picking is operated in cool and dry conditions.

The normal shelf life of fresh crabmeat is 7-10 days and may last up to 14 days if meat with a low initial bacterial count (80) is stored under optimum refrigeration temperature. To extend the shelf life of crabmeat, the pasteurization process of holding crabmeat for 1 min at 171°-210°F, depending on the desired shelf life from 1 to 12 months, was patented (81). A process of 185°F for 1 min in the center of a 1-lb can (401 x 301) was found to sufficiently reduce an inoculated 108/100 g of C. botulinum type E spores to <6/100 g and to keep the meat nontoxic for 6 months at 40°F (82,83). A recommendation has been made to increase the time at 185°F to 3 min to provide for 12D cook based on the thermal death time studies of type E C. botulinum (84-86). Table 2 shows the thermal resistance characteristics of C. botulinum type E. For a complete process, an F-value of 31 based on an F 16/185 value and a cooling

Table 2. Decimal Reduction Time (D10) for Heating C. botulinum Type E Spores


Heating medium

Temperature range (°C)

D10-value minimum at 82.2°C

Z-value (°C)



Blue crab meat

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  • joel
    Why is shellfish prone to rapid spoilage?
    3 years ago
  • susanne
    What is the spoilage organism for shellfish?
    12 months ago

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