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.
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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.
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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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