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SOUTHERN
REGIONAL
AQUACULTURE
CENTER

Managing Off-Flavor Problems in Pond-Raised Catfish

by Craig S. Tucker and Martine van der Ploeg*

Problems with inconsistent flavor quality occur in all food industries, but are especially important in aquaculture because these foods are often more expensive than other sources of animal protein. To capture and maintain market share, aquaculture products must be of consistent, superior quality. Flavor quality is all the more important in farm-raised catfish because catfish are not a customary part of the diet for consumers outside the southeastern United States. If catfish with off flavors are marketed, first-time buyers may be reluctant to make future purchases in favor of more familiar foods, such as poultry, pork, beef or seafood from capture fisheries. 

Types of off-flavor 

Most of the seafood off-flavors noticed by consumers are the result of improper post-harvest handling of the product. Post-harvest flavor problems occur in aquaculture products, as well as in seafood from capture fisheries, and are caused by bacterial spoilage or by oxidation of fats (rancidity) during prolonged or improper storage. Post-harvest off-flavors can be prevented by using sound processing, packaging and storage methods. Off-flavors may also develop in fish before harvest, although most consumers are not aware of pre-harvest off-flavors in catfish because processors screen fish for flavor quality before harvest. If a sample of fish from a particular pond is found to be off-flavor, the fish in that pond are not harvested until flavor quality improves. Although pre-harvest flavor screening reduces the impact of inconsistent flavor quality at the market level, the inability to harvest and sell off-flavored fish is a serious economic burden for farmers. Some pre-harvest off-flavors are caused by substances in the diet that are absorbed across the gastrointestinal tract and deposited in the flesh. Diet-related off-flavors are rare in fish fed high quality commercial feeds. However, pond-raised catfish occasionally eat other foods, and some of those may cause flavor problems. For example, “decay” or “rotten” off flavors are occasionally noted in pond-raised catfish during winter when many catfish farmers do not routinely feed their fish. These flavors probably develop when fish eat decaying organic matter as they forage for natural foods. Most pre-harvest off-flavors develop when odorous compounds in the water are absorbed by fish and accumulate in the edible tissue. Some environment related off-flavors are caused by accidental pollution of the water, although such problems are uncommon in aquaculture because it is easy to locate facilities so that routine exposure to odorous water pollutants is avoided. However, petroleum off-flavors occasionally develop in pond-raised fish when waters are contaminated by accidental spills of diesel fuel or gasoline from boats, well-pump engines, or farm equipment. Most pre-harvest flavor problems are caused by odorous compounds produced by naturally occurring aquatic microorganisms. The compounds are synthesized by algae or bacteria, released into the water, and then absorbed through the gills, skin or gastrointestinal tract of fish. The most common off-flavors are described as “earthy,” “muddy,” “moldy,” or “musty.” These off-flavors have been described in writing at least since 1550 and, as evidence of long-standing negative consumer sentiment, a 1909 German newspaper article reported that a customer sued a restaurant owner for serving muddy flavored fish.

 

Figure 1. Filament of Oscillatoria perornata, a species of blue-green algae that produces 2-methylisoborneol (MIB). The filament is about 10 µm in diameter. The lighter-colored filament running vertically is Aulacoseira, a filamentous diatom.

 Common off-flavors in catfish 

The common off-flavors in pondraised catfish are grouped below on the basis of suspected origin. Grouping flavor problems by origin, rather than by similarity of flavor, is useful when considering options for managing the problem.

Blue-green off-flavors 

Three common off-flavors are placed in this group based on their known or suspected origin as metabolites produced by bluegreen algae, which are plant-like bacteria common in the blooms of many fish ponds. Two off-flavors in this category—2-methylisoborneol and geosmin—are known to be caused by blue-green algae. The third off-flavor, called “woody,” is commonly included in this category although there is only weak evidence that it is caused by blue-green algae. The most common cause of flavor problems in catfish raised in northwest Mississippi, southeast Arkansas, and northeast Louisiana is caused by 2-methylisoborneol (MIB). The chemical causes a unique musty-medicinal off-flavor that can be quite intense and disagreeable. In catfish ponds, MIB is nearly always produced by the microscopic blue-green alga Oscillatoria perornata, although in other environments MIB can be produced by several other species of blue-green algae, as well as by actinomycete bacteria.  Actinomycetes are routinely found in freshwater environments,
but they are primarily soil microorganisms. In fact, the characteristic earthy smell of
soil is caused by the production of MIB, geosmin and other odorous metabolites by actinomycetes.
2The blue-green algae that cause off-flavors cannot be seen with the unaided eye.  Blooms of O. perornata consist of free-floating, straight filaments that are slightly bent and gradually tapering at one end (Fig. 1). The filaments are 7 to 12 µm wide and 500 µm long or longer.  Metric dimensions are provided as an identification aid for readers with access to a microscope equipped with an ocular micrometer.  Cells of O. perornata contain many gasfilled vesicles that make the filament look dark and grainy when viewed under a microscope. The organism grows slowly when water temperatures are below about 70 degrees F (20 degrees C), so O. perornata is usually present only during the warmer months. Populations may develop in any nutrient-enriched freshwater pond but the organism appears to be most common in waters of high total alkalinity and hardness. O. perornata is most likely to develop in ponds where populations have grown in previous summers. The organism probably overwinters in the pond bottom muds and then begins to grow when water temperatures rise above 60 to 70 degrees F (15 to 20 degrees C) in the spring. Off-flavors caused by MIB can develop rapidly, but they dissipate slowly. Fish exposed to MIB become noticeably off-flavor within minutes or hours. Fish purge the chemical naturally when exposure ceases, but days or weeks may be needed for the off-flavor to completely dissipate. The rate at which the off-flavor disappears is related primarily to water temperature and the size and fat content of the fish. Small, lean fish held in warm, odor-free water purge MIB off-flavors within 2 to 4 days. Large, fatty fish held in cold water may not purge the flavor for weeks or months following exposure. Another common “blue-green” off-flavor is caused by geosmin. Geosmin gives fish a distinctive earthy or muddy flavor that is somewhat reminiscent of the odor of a damp basement. In catfishproducing areas outside the Mississippi River floodplain, geosmin off-flavors are often more common than those caused by MIB. Many species of blue-green algae and actinomycete bacteria can produce geosmin, but in catfish ponds the main geosminproducers are species of the bluegreen algae Anabaena or, less commonly, Aphanizomenon or Lyngbya.  Members of the genus Anabaena are easy to recognize, although it is difficult to differentiate geosmin-producing species from those that do not produce geosmin. The microscopic filaments of Anabaena (Fig. 2) are free-floating, straight or coiled, and consist of a series of spherical or barrel-shaped cells that look like a string of beads. Individual filaments of Aphanizomenon (Fig. 3) somewhat resemble straight filaments of Anabaena, but unlike the individual or tangled filaments of Anabaena, filaments of Aphanizomenon usually lie parallel in free-floating bundles or flakes. Although geosmin-producing blooms of Anabaena and Aphanizomenon may occur at any time when water temperature is warm, they are most common in late spring when water temperatures are rapidly increasing. Unlike Anabaena and Aphanizomenon, which float free in the water, the geosmin-producing species of Lyngbya found in southeastern fish ponds usually grow in tangled clumps or mats on the pond bottom or among shoreline vegetation. The individual filaments of Lyngbya have a firm sheath that usually extends beyond the end of the filament (Fig. 4). Like the other odor-producing blue-green algae, Lyngbya is found most frequently during the warmer months of the year. The rates at which fish acquire and purge geosmin off-flavors are similar to those for MIB. The third off-flavor in the bluegreen category is called “woody.” The flavor is usually not very intense, and some people find it difficult to detect. The flavor is most frequently described as reminiscent of wood chips, although some people find the flavor to be somewhat like that caused by low levels of MIB. Woody off-flavor is often accompanied by an astringent or bitter aftertaste that is not experienced with other off-flavors. The chemical cause of the woody off-flavor is not known. Woody off-flavors in pond-raised catfish are most common in late autumn and winter. There is weak evidence (that may well prove to be wrong) that the woody off-flavor is related to prior exposure of fish to MIB. This is the only basis for including woody flavors in the blue-green category. Woody off-flavor purges from fish much more slowly than off-flavors caused by MIB or geosmin. In one study, channel catfish with woody off-flavor were held in clean, flowing well water at 75 degrees F (24 degrees C) for 21 days with only modest improvement in flavor quality. Intense MIB off-flavor was completely purged from fish in 4 days under the same conditions. Decay/rotten off-flavors A wide variety of offensive off-flavors are grouped into this category. Descriptions of flavors in this group include “egg-sulfury,” “sewage,” “decaying vegetation,” and “rotten.” All have an apparent common origin in the decay of plant or animal matter in a pond. In one study of off-flavors in pond-raised catfish, “rotten” offflavors were most common in the winter months and in ponds that contained large numbers of dead Actinomycetes are a group of branching, filamentous bacteria that are somewhat similar in appearance to filamentous fungi. 

Figure 2. A filament of Anabaena, a geosmin-producing species of blue-green algae. The sample was photographed in a solution of India ink to show the mucilaginous envelope (the clear halo) surrounding the filament. The cells are
about 8 µm in diameter.

 

*Mississippi State University; Delft University of Technology, The Netherlands.

 

 

 


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