...From the pages of Agricultural Research magazine
Keeping Catfish on Consumers' Menus
Chemist Casey Grimm examines
distilled steam concentrates
from catfish fillets before
analyzing the off-flavor
and geosmin by an automated
gas chromatography mass
Freshwater aquaculture is one of the fastest growing sectors of the
U.S. agricultural economy, and channel catfish production accounts for
70 percent of all freshwater aquaculture earnings. The National Agricultural
Statistics Service estimates that the 2001 farmgate revenue for channel
catfish was about $470 million.
Unfortunately, as much as 60 percent of farm-raised catfish is considered off-flavor at certain times, according to a recent American Fisheries Society study. These off-flavors can lead to delays in harvesting and result in economic losses of up to 20 percent of total farmgate value. Scientists with the Agricultural Research Service are looking for ways to detect and prevent such problems.
Microbiologist Paul Zimba
examines filamentous algae
from a catfish production
pond. Pond conditions often
support blooms of these
undesirable algae, resulting
in increased production costs.
Channel catfish are raised in densely populated ponds, where algae
and bacteria thrive on the large amounts of nutrients that are available.
While most of these algal species are either beneficial or nonharmful,
a few produce off-flavor compounds.
"Fish can be rejected for flavors that are described as woody,
sewage, rotten, and diesel," says Casey Grimm, an analytical chemist
in ARS' Food Processing and Sensory Quality Unit (FPSQU) at the Southern
Regional Research Center in New Orleans, Louisiana. "But the basis
for about 80 percent of all rejections is the muddy/musty aroma produced
by geosmin and 2-methylisoborneol (2-MIB)."
Human taste buds are incredibly sensitive to these two compounds. The average consumer can detect 0.7 parts per billion (ppb), and a skilled, professional catfish taster can detect 0.1 ppb in catfish. But these pros are susceptible to colds, allergies, and other problems that impair their smelling and tasting abilities and desensitize them to off-flavors.
Technician Marilyn Grayer
(foreground) samples water to
be processed by student assistant
Bridget Ester-Fields(center) and
Paul Zimba. This study assesses
how clay concentrations affect
reflectance characteristics of algal
populations when monitored remotely
To make detection of catfish flavors more efficient, Grimm began developing
an instrument method in 1997 for analyzing off-flavor compounds. He
and his collaborators, including support scientist Steve Lloyd, created
a technique that uses steam distillation/solid-phase microextraction
to collect flavor compounds and gas chromatography/mass spectrometry
to separate and analyze them.
The instrument is more objective than human flavor checkers and could
help standardize the catfish industry. It can run an analysis every
15 minutes, and it detects geosmin and 2-MIB at levels less than 0.05
"At a cost of $100,000 per instrument, it's probably too expensive for small producers," says Grimm. "But the state of Mississippi is looking into buying one or more machines and setting up laboratories that could be used by producers from around the region."
Student assistant Susan Towery
(right) measures oxygen levels
in experimental catfish
enclosures treated with
herbicides to control
while Paul Zimba records the
data. Changes in oxygen
provide an early indication
of the herbicides' effect on
Getting to the Source
Detecting off-flavors before they cross the human taste threshold solves
only part of the problem. Ultimately, ARS researchers would like to
control off-flavor algae or prevent them from forming these compounds.
They have begun looking more closely at the biology of the cyanobacteriacommonly
called blue-green algaeand other species that are the sources
of geosmin, 2-MIB, and other off-flavor compounds.
Paul Zimba, a microbiologist at ARS' Catfish Genetics Research Unit in Stoneville, Mississippi, estimates that between 100 and 200 species of cyanobacteria live in catfish production ponds. But, he says, only four or five of these species are known to produce off-flavor compounds. The rest are beneficial because they oxygenate the water.
Zimba says that the most common management system used in the catfish industrythe multiple-batch systemmay contribute significantly to the growth of off-flavors. In this arrangement, hatched fish eggs, or sac fry, are reared in nursery ponds. Once they reach the adolescent, or fingerling, stage, they are moved to production ponds, where they grow to market size.
Farmers harvest the full-grown fish and restock the ponds with more
fingerlings, meaning that several different ages of catfish are present
in a pond at any one time. The restocking process can continue for several
years without the ponds being drained. There is no industry standard
designating how often ponds should be drained.
Recently, Zimba and colleagues studied how the age of a catfish production
pond related to its water quality, phytoplankton and zooplankton populations,
and incidence of off-flavor. They examined 71 catfish production ponds
in Mississippi during the hottest part of the summer, when algal and
cyanobacterial levels are at their highest. The ponds included in the
study were 1, 2, 3, 4, 9, 10, and 15 years of age.
Not surprisingly, the researchers found that fish from younger ponds
had lower incidences and intensities of off-flavors than those from
older ponds. Zimba says that draining production ponds every 4 to 5
years could decrease occurrence of off-flavors.
Zimba and his collaborators also outlined a basic succession pattern
of zooplankton and phytoplankton that grow in catfish production ponds.
Among other things, they determined that by year 4, blue-green algae
are dominant and continue to increase through year 15. Older ponds also
contain higher densities of zooplankton.
Because higher levels of blue-green algae correlate with higher levels
of off-flavors, Zimba and the other researchers say that older ponds
may be used for holding sac fry and fingerlings. The zooplankton in
these ponds could serve as food sources for the fry. Younger ponds are
better suited to growing out the fish and purging them of off-flavors.
Normally, farmers must keep off-flavor fish in the production pond
or in another holding facility for weeks or months until the flavors
dissipate. This holding period can come at a great cost to producers.
Zimba is also working on a way to speed up the time it takes fish to
eliminate off-flavor compounds. He used freshwater raceway systems to
measure how much time was required for fish to eliminate off-flavor.
"Fresh water is pumped through the raceways to flush out the off-flavors acquired by the fish from the pond system," Zimba explains. "Off-flavor concentrations in the fish gradually lessen as the water flows over them. In our studies, fish purged 50 percent of off-flavors within 24 hours."
Charting Microbes' Succession
Though Zimba and his collaborators have made great strides in charting
the year-to-year biological succession of various microorganisms in
a channel catfish pond, the details are still a little unclear. As a
result, the incidence of off-flavor compounds such as 2-MIB and geosmin
According to FPSQU chemist Barry Hurlburt, this unpredictability may
not be a problem much longer. During the past 2 years, he and his collaborators
have developed a molecular assay that can scan a catfish pond and, in
less than a day, determine the relative populations of all the microorganisms
it contains. Over the next year, they will use this assay to test 30
production ponds once a week and note which species each contains. They
will use this data to map the biological succession of the algae and
Hurlburt says, "If a certain species always blooms at a certain
time before an off-flavor-causing species appears, the assay could be
used as a predictive tool by farmers. If their ponds tested positive
for a 3-week predictor species, for instance, they would
know exactly how much time they have to salvage their fish before off-flavor
compounds begin ruining their crop."
In a similar preventive effort, Zimba and Steven J. Thomson, an agricultural
engineer at ARS' Application and Production Technology Research Unit
in Stoneville, are studying a remote-sensing technique that may detect
unwanted algal species in production ponds before the problem gets out
The researchers can now identify specific types of algae by their unique
color profiles, which they obtain from digital video shot during low-altitude
flights. Zimba says that unwanted algae are distinguishable by their
unique chlorophyll and carotenoid compounds. With enough data, the scientists
can predict when certain types of algae will grow in the cycle.
The Stoneville researchers and colleagues at Mississippi State University's Remote Sensing Technologies Center and the National Oceanic and Atmospheric Administration are currently refining their equipment and technique.
A Solution Could Be in the Genes
Besides looking at biological succession in production ponds, ARS scientists
are also studying the genes that cause geosmin and 2-MIB to form. Hurlburt
explains, "A researcher in England has isolated the gene responsible
for the first step in the biosynthesis of geosmin from the soil-dwelling
bacterium Streptomyces coelicor. I plan on using this cloned
gene in an attempt to isolate the equivalent gene in cyanobacteria."
Hurlburt and his collaborators may eventually be able to use this information
to block the biosynthetic pathway that allows geosmin to form in ponds.
Hurlburt is also planning on using a genetic approach to "knock
out" genes in cyanobacterial species involved in off-flavor compound
production. If any of them stop the synthesis of geosmin or 2-MIB, he
will isolate the modified species and introduce them into research production
ponds. He will then determine whether the improved species can compete
with and eventually displace the off-flavor-causing species.
This research is part of Aquaculture, an ARS National Program (#106)
described on the World Wide Web at www.nps.ars.gov.
Casey C. Grimm and Barry K. Hurlburt are with the USDA-ARS Food Processing and Sensory Quality Research Unit, Southern Regional Research Center, 1100 Robert E. Lee Blvd., New Orleans, LA 70179-0687; phone (504) 286-4293 [Grimm], (504) 286-4462 [Hurlburt], fax (504) 286-4430.
"Keeping Catfish on Consumers' Menus" was published in the March 2003 issue of Agricultural Research magazine.