AQUACULTURE ENVIRONMENT INTERACTIONS
Aquacult Environ Interact
Vol. 5: 107–116, 2014
doi: 10.3354/aei00100
Published online May 8
INTRODUCTION
Taste-and-odour compounds (TOCs) can reduce
the palatability of freshwater fish produced in aqua-
culture systems, especially when water recirculation
is applied (Robertson & Lawton 2003, Smith et al.
2008). The most commonly identified TOCs are
geosmin (earthy flavour) and 2-methylisoborneol
(MIB; mildewed flavour), and both of these off-
flavours have been shown to concentrate 200- to 400-
fold in fish flesh, relative to the ambient concentra-
tion (Howgate 2004). TOCs are produced by several
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*Corresponding author: nogj@plen.ku.dk
Geosmin off-flavour in pond-raised fish in southern
Bangladesh and occurrence of potential
off-flavour producing organisms
Mikael A. Petersen
1
, Md. Ariful Alam
2
, Md. Mizanur Rahman
2
, Md. Lokman Ali
2
,
Sultan Mahmud
2
, Louise Schlüter
3
, Niels O. G. Jørgensen
4,
*
1
Department of Food Science, University of Copenhagen, Thorvaldsensvej 40, 1871 Frederiksberg, Denmark
2
Faculty of Fisheries, Patuakhali Science and Technology University, Dumki, Patuakhali 8602, Bangladesh
3
Environment and Toxicology, DHI Group, Agern Allé 5, 2970 Hørsholm, Denmark
4
Genetics and Microbiology, Department of Plant and Environmental Sciences, University of Copenhagen,
Thorvaldsensvej 40, 1871 Frederiksberg, Denmark
ABSTRACT: Pangas Pangasianodon hypophthalmus and tilapia Oreochromis niloticus were culti-
vated for 6 mo in earthen ponds in Bangladesh to examine occurrence of the off-flavour geosmin
in water and fish and to test procedures for reduction of off-flavour. In the ponds (~1 m depth and
area of 400 m
2
), the average geosmin concentration was 3.9 ng l
−1
(range 0.2 to 20 ng l
−1
). No
effects of season or water treatment (sand filtration or probiotic microbes) were found. The content
of geosmin in the fish was 21 ng kg
−1
(range: 0.0 to 91 ng kg
−1
) for pangas and 17 ng kg
−1
(range:
0.0 to 68 ng kg
−1
) for tilapia. Water treatment reduced the geosmin content by 56 to 74% in pan-
gas, but no effect was found in tilapia. Likewise, depuration for
≥12 h in groundwater lowered the
geosmin content in pangas (by 65 to 90%) but not in tilapia. Sensory analysis indicated a positive
effect of both water treatment and depuration, and the fish were graded ‘no or mild flavour’ after
such treatment, compared to ‘strong off-flavour’ in controls. Abundance analyses of known off-
flavour producing microorganisms (streptomycete bacteria and cyanobacteria) showed a high
density of streptomycetes (0.5 to 13% of the bacterial population), while cyanobacteria made up a
maximum of 9.3% of the phytoplankton biomass or were absent. This first study on off-flavours in
pangas and tilapia in Bangladeshi ponds indicates that geosmin was not a major off-flavour in the
fish, but improvement of sensory quality by water treatment and depuration suggests that other,
unidentified off-flavours were present in the fish.
KEY WORDS: Off-flavour · Aquaculture · Pangasianodon hypophthalmus · Oreochromis niloticus ·
Streptomycetes · Cyanobacteria · Geosmin · GC-MS · Sensory analysis
O
PEN
PEN
A
CCESS
CCESS
Aquacult Environ Interact 5: 107–116, 2014
species of cyanobacteria (also known as blue-green
algae) and by non-photosynthetic Streptomyces bac-
teria (Gram-positive, filamentous bacteria), but recent
research indicates that other bacteria, e.g. species
within the order Myxococcales, may also produce
off-flavours in some aquaculture systems (Zaitlin &
Watson 2006, Auffret et al. 2013).
TOCs impact taste and flavour of fish in both
warm- and cold-water aquaculture systems. In warm
water, tainting by geosmin and MIB was observed in
barramundi Lates calcarifer from freshwater lakes in
northern Australia (Percival et al. 2008), in catfish
Ictalurus punctatus from ponds in Mississippi and
Alabama, USA (Schrader & Dennis 2005), and in
tilapia Oreochromis niloticus reared in pond cages in
Thailand (Gutierrez et al. 2013). In cold-water sys-
tems, tainting of rainbow trout Oncorhynchus mykiss
by geosmin and MIB was detected in outdoor tanks
with recirculation in Denmark (Petersen et al. 2011)
and in arctic charr Salvelinus alpinus from an indoor
system with water recirculation in West Virginia,
USA (Houle et al. 2011). TOCs reduce market value
of fish from aquaculture due to consumer reluctance
to buy food with an earthy or mildewed taste, but the
actual reduction in market value of fish due to taint-
ing is difficult to determine. For American catfish,
TOCs have been estimated to reduce the sale by
30% (Engle et al. 1995).
In Bangladesh, occurrence of off-flavour in pan-
gasiid catfish has been reported for fish reared in
ponds (Khan et al. 2009), but the extent of off-flavour
problems in pond-raised fish in Bangladesh has not
been scientifically documented. Various treatments
of fish meat to reduce off-flavours are practised in
Bangladesh and Indian households, e.g. dipping and
washing in tamarind pulp, lemon juice, lemon grass,
salt solution, or a suspension of banana leaf ash, and
some of these treatments appear to improve the sen-
sory quality of the fish (Mohsin et al. 1999).
In large and industrial aquaculture systems, off-
flavours in fish may be reduced by depuration for few
to several days in clean water and without feed
before slaughtering (Robertson et al. 2005, Percival
et al. 2008). Depuration is an efficient procedure for
off-flavour reduction, but access to clean and off-
flavour-free water may be limited, and the fish lose
weight (and hereby value) during the treatment
(Burr et al. 2012).
In this study, the presence of geosmin and proce-
dures for its removal from water and fish were exam-
ined in tilapia Oreochromis niloticus and pangas
Pangasianodon hypophthalmus (also known as pan-
gasius), reared in local earthen ponds in southern
Bangladesh (Dumki Upazila). The 2 fish species con-
stitute a major source of animal protein to the local
population, and both species are of large economic
importance (Ahmed et al. 2012). When considering
the importance of the 2 fish species, the lack of stud-
ies on sources of the reported off-flavour problems
(see above) is surprising. In southeast Asia, the con-
tent of geosmin has been examined in tilapia in Thai
fish ponds (Gutierrez et al. 2013), but the content of
off-flavours in wild or farmed pangas has not previ-
ously been measured. Here, we determined the con-
tent of geosmin in water and fish (tilapia and pangas)
by a dynamic headspace extraction method and GC-
MS detection. MIB was targeted for inclusion in the
study, but the compound could not be quantified
with an acceptable accuracy. In addition, the taste
and flavour of the fish were characterized by sensory
analysis. Different procedures were applied to re
-
duce potential off-flavours in water and fish, includ-
ing sand filtration and probiotic treatment of the
pond water during the production phase as well as
depuration of the fish in groundwater before slaugh-
tering. The presence of potential off-flavour produc-
ing organisms (cyanobacteria and streptomycetes)
in the water was examined by a chromatographic
method (for cyanobacteria) and a molecular ap
-
proach (for streptomycetes).
MATERIALS AND METHODS
Set-up and sampling
Details on ponds, experimental set-up, growth
rates of fish and water quality are reported sepa-
rately (S. Mahmud et al. unpubl.). Briefly, 8 earthen
ponds of 309 to 497 m
2
area and with depth of 1.1 to
1.6 m were selected in Dumki Upazila, Patuakhali,
in southern Bangladesh (22° 21
′ 15″ N, 90° 23′21″ W).
Each pond received 50 tilapia Oreochromis niloticus
fry (5.2 to 5.3 g each) and 30 pangas Pangasianodon
hypophthalmus fry (62 to 75 g each) in late January
2012. The fish were fed artificial feed (Mega Fish
sinking feed pellets; www.spectragroup.com.bd) at
the rate of 30% of their body weight for first month,
20% for the next month and 3 to 10% for the last
4 mo. Planktonic matter in the ponds is another
important food source to the fish. After 6 mo, the fish
had grown to 182−199 g (tilapia) and 689−870 g (pan-
gas). At termination of the study (early August 2012),
selected fish were taken for depuration in cement
tanks with ~2 m
3
groundwater for up to 48 h. The
water was not exchanged and no agitation was
108
Petersen et al.: Geosmin in pond-raised fish
applied during the depuration. After slaughtering,
each of the fish were divided into 2 fillets. One fillet
was used for chemical analysis of content of geosmin
and MIB in the flesh, while the other fillet was used
for sensory analyses (see ‘Sensory profiling of fish’).
The fillets were kept frozen in individual plastic bags
at −20°C until analysis.
To test the effect of water treatment on the content
of off-flavour sources in the water, the following
treatments were performed in duplicates (2 ponds for
each treatment): continuous filtration of the water
through a sand filter, addition of probiotic material,
either PondPlus® by Novozymes (www.novozymes.
com) or AquaPhoto by ACI (www.aci-bd.com) at
weekly intervals according to the producer’s instruc-
tions, or no treatment (controls). The probiotic prod-
ucts are designed to improve remineralisation of
inorganic nutrients to promote growth of fish-benefi-
cial phytoplankton.
Samples for analysis of off-flavours in the water
were collected every second month. Volumes of
10 ml water were transferred in duplicates to 20 ml
glass vials and treated with 3 g NaCl before capping
with silicone-PTFE seals and were kept at 4°C until
analysis. To detect potential off-flavour producing
organisms in the water, volumes of 200 and 500 ml
water were filtered through 47 mm diameter 0.2 µm
pore size cellulose nitrate membrane filters (for
Streptomyces bacteria) or 47 mm diameter GA 100
GF/F filters (for cyanobacteria; www.advantecmfs.
com) (see below). The filters were folded and
wrapped individually in aluminium foil and kept at
−20°C until analysis.
Dynamic headspace sampling of volatiles
(including geosmin) in water and fish
Volatiles were extracted from water samples and
fish flesh by dynamic headspace sampling using pro-
cedures similar to those applied by Petersen et al.
(2011). Briefly, water samples were transferred to a
gas-washing bottle, which was immediately closed
with a purge head. The bottles were purged with
100 ml of N
2
min
−1
for 60 min in a water bath at 37°C,
simultaneous with collection of volatiles on Tenax TA
traps. To extract volatiles from the fish fillets, 10 g of
fish flesh was transferred to a gas-washing bottle
with 20 ml of water and 100 µl of internal standard
solution (5 mg 4-methyl-1-pentanol l
−1
). The flesh
was homogenized for 45 s at 13 500 rpm using an
Ultra Turrax homogenizer (www.ika.com) followed
by rinsing with 10 ml of water before immediate clos-
ing with a purge head. The bottles were purged with
100 ml of N
2
min
−1
for 60 min in a water bath at 50°C
with simultaneous collection of the volatiles on Tenax
TA traps. After headspace sampling, the Tenax traps
were dried at 100 ml min
−1
flow of dry N
2
for 10 min.
Analysis of volatiles by GC-MS
Volatiles were stripped from the traps in an auto-
matic 2-step thermal desorption unit (ATD 400;
Perkin Elmer). In the first step, the volatiles were des-
orbed by heating to 250°C at 60 ml He min
−1
for
15 min. The volatiles were focussed on a cold (5°C)
Tenax TA trap that subsequently was heated to
300°C for 4 min (second step). The 2-step procedure
allowed for a rapid transfer of volatiles to a GC-MS
(7890A GC system interfaced with a 5975C VL MSD
with a Triple-Axis detector; www.agilent.com) by a
heated (225°C) transfer line.
The volatiles were separated on a DB-Wax capil-
lary column (30 m
× 0.25 mm internal diameter and
0.50 µm film thickness) at 1 ml He min
−1
and with a
temperature program of 40°C (10 min) and 8°C min
−1
until 240°C (5 min). The mass spectrometer was oper-
ated in electron ionization mode at 70 eV, and mass-
to-charge ratios between 15 and 300 were scanned.
Simultaneously, data were collected in selected ion
monitoring mode, monitoring mass 95 for MIB and
mass 112 for geosmin. Peak identity was confirmed
by probability-based matching of mass spectra
with those of a commercial database (Wiley275.L,
HP
product no. G1035A) and by comparison
with retention times of authentic standards. For
analysis of the acquired data, MSD Chemstation soft-
ware (v. E.02.00, Agilent Technologies) was used. For
absolute quantification of geosmin and MIB in fish
meat, amounts of 0.25, 1, and 4 µg kg
−1
were added
to meat from 4 selected fish during homogenization.
Correspondingly, absolute quantification of water
samples was carried out by adding 1, 5, 10, 50, and
100 ng l
−1
to samples of 10 ml water with 3 g NaCl.
Serial dilutions of the geosmin and MIB stocks (in
methanol; products G5908 and M3933, www. sigma-
aldrich.com) were performed in ethanol, but water
was used for the most dilute concentrations. An
acceptable linearity between peak area and concen-
tration was observed (R
2
= 0.91 to 0.93), and the
obtained graphs were used to generate peak area-to-
concentration calibration curves. MIB was identified
in most of the chromatograms but co-eluted with an
unknown compound. Therefore, no results for MIB in
water or fish are presented.
109
Aquacult Environ Interact 5: 107–116, 2014
Sensory profiling of fish
Sensory evaluation of the fish was performed at
Patuakhali Science and Technology University by a
panel of 10 people. The panel members were not
trained in off-flavour characterization but consisted
of typical local consumers of freshwater fish: 2 mem-
bers of the Fish Breeder Community Based Organi-
zation (CBO), 2 teachers and 2 students at Patuakhali
Science and Technology University, 2 business per-
sons, and 2 local fish farmers. The sensory partici-
pants did not receive training in identification of
geosmin or MIB before the tests but were asked to
categorize the fish flavour (Table 1). Fillets from the
fish were prepared in boiling water for 10 min,
divided in 10 pieces, and served randomly and with-
out identification to the panel members.
Identification of potential off-flavour producing
organisms
Densities of Streptomyces bacteria in the water
were determined by quantitative PCR (TaqMan pro-
cedure) targeting the 23S rRNA gene according to
Rintala & Nevalainen (2006) with the modifications
given by Lylloff et al. (2012). Genomic DNA was
extracted from the 47 mm membrane filters after cut-
ting each filter in 3 portions, using the PowerWater
®
DNA isolation kit by Mobio (www.mobio.com)
according to instructions by the producer. The PCR
reactions were performed in an Mx3000P Stratagene
system (www.genomics.agilent.com) using TaqMan
Universal PCR Master Mix (www.appliedbiosys-
tems.com). Densities of Streptomyces on the filters
were determined from calibration curves produced
by amplification of DNA from known densities of
spores in 2 species of Streptomyces. Possible interfer-
ence on the PCR reaction by compounds in the sam-
ples was checked by measuring the amplification
efficiency of a plasmid construct with and without
addition of DNA extracted from the water samples
(Lylloff et al. 2012).
Densities of cyanobacteria were estimated from the
composition and concentration of specific pigments
in the phytoplankton according to Schlüter et al.
(2004). The GF/F filters were transferred to vials with
95% acetone with internal standard (vitamin E). The
samples were vortexed, sonicated on ice, extracted at
4°C for 20 h, and mixed again. The extracts were fil-
tered through 0.2 µm Teflon syringe filters into HPLC
vials and placed in the cooling rack (4°C) of the
HPLC system together with a parallel set of vials with
injection buffer (28 mM aqueous tetrabutyl ammo-
nium acetate [TBA] at pH 6.5 and methanol, in a
90:10 ratio). The extracts were injected by program-
ming the auto-injector to make a sandwich injection
of buffer and sample in the ratio 5:2. The total injec-
tion volume was 500 µl. For detection of the pig-
ments, a Shimadzu LC-10A HPLC system was used
following the method by Van Heukelem & Thomas
(2001) (with slight modifications as given below). The
HPLC system consisted of a LC-10ADVP pump, an
SPD-M10A VP photodiode array detector, an SCL-
10ADVP System controller with Lab Solution soft-
ware, an auto sampler (set at 4°C), a column oven
(CTO-10ASVP), and a degasser. The column was an
Eclipse XDB C8, 4.6 mm × 150 mm (Agilent Technol -
ogies) operated at 60°C. The solvents were (A) meth -
anol and 28 mM aqueous TBA at pH 6.4 in a 70:30
ratio and (B) 100% methanol. The following time
programming was used: 0 min: 95% A, 5% B; 27 min:
5% A, 95% B; 34 min: 5% A, 95% B; 35 min: 0% A,
100% B; 38 min: 0% A, 100% B; 39.5 min: 95% A,
5% B; 50 min: stop. The flow rate was 1.1 ml min
−1
.
The analysis was calibrated with pigment standards
from DHI Lab Products. The internal standard was
detected at 222 nm, while the phytoplankton pig-
ments were detected at 450 nm. Peak identities were
routinely confirmed by on-line PDA analysis.
Pigment concentrations were subsequently loaded
into the CHEMTAX 1.95 program to calculate chloro-
phyll a biomass of individual phytoplankton groups
according to Mackey et al. (1996) and Higgins et al.
(2011). Pigment ratios used as input values for
CHEMTAX calculations were from Schlüter et al.
(2006).
RESULTS AND DISCUSSION
Geosmin in water
Occurrence of geosmin in the water from the 4
types of ponds (sand filtration, PondPlus or Aqua
-
Photo enrichment, and untreated controls) were
110
Off-flavour level
Grade
Numeric score
Strong
A
1.0 to 2.5
Medium
B
2.5 to 5.0
Mild
C
5.0 to 7.5
No off-flavour
D
7.5 to 10.0
Table 1. Categories for sensory analysis of fish
Petersen et al.: Geosmin in pond-raised fish
rather similar during the 6 mo period with bimonthly
sampling of duplicate ponds. Mean concentrations of
geosmin (in ng l
−
) in the pond water were 4.5 (range:
1.0 to 14.6) with sand filtration, 3.4 (range: 0.3 to 20.0)
for enrichment with PondPlus, 3.7 (range: 0.2 to 16.1)
for enrichment with AquaPhoto, and 4.1 (range 0.6 to
8.1) for the controls. Mean concentration of geosmin
in all water samples was 3.9 ng l
−1
, and only 3 of 56
water samples had geosmin concentrations > 10 ng
l
−1
. No significant effects of water treatment or sea-
sonality were observed (p > 0.05; t-test).
Occurrence of geosmin in the pond water is low
relative to concentrations measured in other fish
ponds in warm water, e.g. catfish ponds in Louisiana,
USA (4 to 246 ng l
−1
; Hurlburt et al. 2009), and barra-
mundi ponds in Queensland, Australia (range <1 to
14 370 ng l
−1
, typical level of 1000 ng l
−1
; Jones et al.
2013). Geosmin concentrations in the Patuakhali
ponds more resembled levels measured in cold
stream water (< 20°C), e.g. in Denmark, where levels
of geosmin in inlet water of traditional fishponds
were <10 ng l
−1
(Klausen et al. 2005). The low level of
geosmin, as well as the lack of effects of the different
treatments, suggests that geosmin-producing organ-
isms were not abundant in the water, as discussed
below.
Geosmin in fish
Mean concentration of geosmin in meat from all
the analysed fish was 21 ng kg
−1
for pangas Pan-
gasianodon hypophthalmus (range 0 to 91 ng kg
−1
)
and 17 ng kg
−1
for tilapia Oreochromis niloticus
(range: 0 to 68 ng kg
−1
). Statistical analyses showed
that there was a systematic effect of water treatment
and depuration on the content of geosmin in the fish.
An initial ANOVA test demonstrated significant
interaction between both fish species and effect of
water treatment and between fish species and the
duration of depuration. Therefore, data from pangas
and tilapia are treated separately. For pangas, treat-
ment of the pond water reduced the level of geosmin
by 56 to 74% relative to the content in fish from the
control ponds (Table 2). The effect of the 3 treatment
types (PondPlus, AquaPhoto, and sand filter) was not
significantly different. In contrast, for tilapia, no sig-
nificant effect of any of the water treatments on the
content of geosmin was observed. In fish from the
control ponds, a lower content of geosmin was found
in tilapia (11 ng kg
−1
) than in pangas (54 ng kg
−1
), but
this difference was not observed in the other water
treatments (Table 2).
Depuration of the fish for 12 to 48 h had a signifi-
cant effect in pangas, in which the geosmin content
was reduced to 65 to 90% of the level in the control
fish (Table 3). No effects of duration of the depura-
tion time (12 to 48 h) were found. For tilapia, the
depuration did not affect the content of geosmin in
the flesh.
The concentration of geosmin in tilapia and pangas
in the present ponds is low compared to geosmin
content in tilapia in other studies, e.g. fish from cages
in Thailand. Gutierrez et al. (2013) measured 0.49 to
3.51 µg geosmin kg
−1
wet weight in a seasonal study
of tilapia cultured in pond cages in northern Thai-
land. The relatively large content (10- to 70-fold
above the level in tilapia in the Patuakhali ponds)
may reflect the higher concentrations of geosmin in
the pond water (0.41 to 2.33 µg l
−1
, or 100- to 600-fold
above the level in the Patuakhali ponds). In addition
to geosmin, high levels of MIB were measured in
both fish and water in tilapia in the Thai ponds.
The geosmin content in tilapia and pangas was also
low relative to barramundi from cages in freshwater
in tropical Australia. Jones et al. (2013) reported con-
centrations of 0.74 to 4.47 µg geosmin kg
−1
wet
weight (13- to 82-fold above the levels in Patuakhali
fish). The larger geosmin content in the barramundi
111
Water treatment
Pangas
n
Tilapia
n
Control 54 A
6
11 A
6
(no water treatment)
PondPlus
24 B
5
24 A
6
Sand filter
18 B
6
23 A
6
AquaPhoto
14 B
6
12 A
5
Table 2. Mean concentrations (least squares, from ANOVA)
of geosmin (ng kg
−1
wet weight) in meat from fish from
ponds with different water treatments. For each species,
treatments not marked with the same letter (A or B) are
significantly different (p < 0.05). n: number of analyzed fish
Depuration time
Pangas
n
Tilapia
n
Control (no depuration)
54 A
6
11 A
6
12 h
5 B
6
8 A
6
24 h
12 B
3
39 A
2
36 h
19 B
6
13 A
5
48 h
16 B
5
15 A
2
Table 3. Mean concentrations (least squares, from ANOVA)
of geosmin (ng kg
−1
wet weight) in meat from fish from
ponds with different depuration times. For each species,
depuration times not marked with the same letter are
significantly different (p < 0.05). n: number of analyzed fish
Aquacult Environ Interact 5: 107–116, 2014
may reflect the high geosmin concentration in the
water (<1 to 14.4 µg l
−1
, or up to 3700-fold higher than
in the Bangladeshi ponds). No data for geosmin con-
tent in pangas are available in the literature.
The higher content of geosmin in pangas than in
tilapia (54 vs. 11 ng kg
−1
) from the control ponds was
unexpected since similar concentrations of geosmin
were found in the 2 fish species (12 to 24 ng kg
−1
)
from all ponds with water treatments. We have no
explanation for the greater geosmin content in pan-
gas from the control ponds. The ponds in this study
are individual, earthen, rain-fed ponds, and specula-
tively, environmental conditions in the 2 adjacent
pangas control ponds were similar, e.g. with respect
to general water quality, vegetation, shading, and
influence from livestock and households, but might
have differed from the other ponds. Feeding habits of
the fish may also have influenced the geosmin con-
tent, although the mechanisms are not clear. Tilapia
is a ‘column feeder’ fish, but it can also feed on ben-
thos and ingest mud, while pangas tend to prefer
artificial feed in the water column if feed is available
(data not shown). Sediment in lakes has previously
been found to harbour higher concentrations of geo -
smin than the water above (Nielsen et al. 2006).
However, the occurrence of geosmin in bottom mate-
rial of the ponds is unknown, and more research is
needed to determine possible relations between
feeding habits, feed sources, and levels of off-flavour
compounds in fish.
Effect of depuration on geosmin content in fish
The positive effect in pangas of depuration for 12
to 48 h in groundwater (reduction of the geosmin
content to about one-third of the original value), but
not for tilapia, shows that even a short depuration
period can purge geosmin from pangas. Tilapia had
a low content of geosmin before depuration, and
this might explain the lack of effect from the depu-
ration. Unfortunately, the geosmin content of the
groundwater used for the depuration is unknown,
and the groundwater might have had a variable
geosmin content, affecting the geosmin content of
the fish. For cold-water fish, such as rainbow trout,
depuration periods of ~1 wk are typically practised
(Petersen et al. 2011) and are supported by experi-
mental studies of off-flavour depuration in this spe-
cies (Howgate 2004). Possibly, the high water tem-
perature (~30°C) in this study increased the release
of geosmin in pangas, e.g. due to a higher metabolic
activity.
Sensory analysis of off-flavour
For pangas, grading of the off-flavour level by the
panel members largely reflected the geosmin content
determined by the chemical analysis. Fish from the
control ponds had the highest grading (strong off-
flavour) relative to PondPlus (medium off-flavour),
sand filter (mild off-flavour), and AquaPhoto (no off-
flavour) treatments (Table 4). For tilapia, the grading
pattern was similar, except that the treatments with
sand filter and AquaPhoto were similar (mild off-
flavour) (Table 5). Thus, AquaPhoto treatment and
sand filtration were most efficient in reducing the off-
flavour content of the fish, followed by PondPlus.
Depuration of pangas and tilapia in tanks with
groundwater significantly reduced the perceived off-
flavour relative to control fish. In pangas, all fish
were categorized as flavour-free (grade D) after 12 h
depuration, although a depuration time of
≥24 h gave
a slightly higher numeric score (Table 6). For tilapia,
all fish were given a score of mild off-flavour (grade
C) after 12 to 48 h of depuration. No data on geosmin
concentrations in the groundwater are available, but
the decline in geosmin content in the fish suggests a
lower content than in the pond water.
The coincidence between a lower geosmin content
and ‘less off-flavour’ grading after depuration, as
112
Water treatment
Average grade
Off-flavour level
Control
2.4 ± 0.2
Strong (A)
PondPlus
4.8 ± 0.8
Medium (B)
Sand filter
7.1 ± 1.1
Mild (C)
AquaPhoto
7.8 ± 0.1
No off-flavour (D)
Table 4. Off-flavour profiling of pangas from ponds with dif-
ferent treatment. Numeric scores of grades (see Table 1) are
mean ± SE; n = 48 (12 from each pond treatment). The grad-
ing was statistically different for each water treatment type
(p < 0.05; ANOVA)
Water treatment
Average grade
Off-flavour level
Control
2.7 ± 0.0
Strong (A)
PondPlus
4.9 ± 0.1
Medium (B)
Sand filter
6.2 ± 0.3
Mild (C)
AquaPhoto
6.5 ± 0.2
Mild (C)
Table 5. Off-flavour profiling of tilapia from ponds with dif-
ferent treatment. Numeric scores of grades (see Table 1) are
mean ± SE; n = 48 (12 from each pond treatment). The grad-
ing was statistically different for each water treatment type
(p < 0.05; ANOVA) except between the sand filter and
AquaPhoto treatments
Petersen et al.: Geosmin in pond-raised fish
observed for pangas but not for tilapia (the geosmin
content was unchanged after depuration in tilapia),
might point to geosmin as a dominant component in
the perceived off-flavour. However, in sensory stud-
ies of channel catfish and rainbow trout, the human
threshold for geosmin detection was determined as
~250 ng kg
−1
fish (Grimm et al. 2004, Robertson et al.
2005, Robin et al. 2006), although a slightly lower
threshold of ~100 ng kg
−1
fish was found for rainbow
trout by Petersen et al. (2011). In the Patuakhali
ponds, the maximum geosmin content was 68 and
91 ng kg
−1
for tilapia and pangas, respectively. As -
suming that the detection threshold of geosmin is
identical for catfish, trout, pangas, and tilapia, our
results indicate that compounds other than geosmin
caused the off-flavour in the Patuakhali fish. MIB is
frequently re ported as a major off-flavour compound
in fish (Tucker 2000), but our analyses suggest that
this compound occurred at similar or lower concen-
trations in fish in this study. Unfortunately, MIB co-
eluted with an unknown compound and could not be
properly quantified.
The positive effect of off-flavour reduction after
only 12 h of depuration is surprising, e.g. when com-
pared to depuration of rainbow trout Onchorhynchus
mykiss for several days to obtain a content below
the human threshold level (Robertson et al. 2005).
Although the initial off-flavour (geosmin) content
was higher in the rainbow trouts, the warmer water
in Bangladesh (~30°C, or 15°C above the tempera-
ture used during the depuration of the rainbow trout)
probably increased the off-flavour removal due to a
high metabolic rate. Supporting a temperature effect,
Jones et al. (2013) also observed an efficient reduc-
tion of geosmin in barramundi in water at 26°C after
24 h.
The sensory panel in this study did not receive
training in identification of specific off-flavours or in
description of sensory properties, and thus, compar-
ison of off-flavour grading by this panel and trained
panels in other studies is problematic. Yet, despite
the lack of training, grading by the panel agreed
well with the different levels of geo -
smin in the fish (although geosmin
did not appear to be a dominant off-
flavour). An advantage of the local
panel is that its members can identify
flavour preferences by Bangladeshi
consumers, implying that their grad-
ing is valuable to fish farmers in the
region.
Potential producers of off-flavours
Since streptomycete bacteria and cyanobacteria
are recognized as notorious off-flavour producers in
freshwater, including aquaculture systems (Zaitlin &
Watson 2006), abundances of these 2 groups of
microorganisms were determined in the ponds. The
density of Streptomyces bacteria in the pond water
varied from 0.21 × 10
9
l
−1
to 4.2 × 10
9
l
−1
(Fig. 1). Large
seasonal and pond-to-pond variations, even between
duplicate ponds, occurred. A general trend, however,
was presence of higher densities in March than in
May and July (p < 0.05; t-test). Except for the high
density in AquaPhoto Pond #1 at March 25, water
treatment by sand filtration or probiotics resulted in
lower densities of streptomycetes, relative to the con-
trols, in March, but this trend was not found in May
and July. The density of all bacteria in the pond
113
Fish
No
12 h
24 h
36 h
48 h
species
depuration
Pangas
2.4 ± 0.1 A
7.9 ± 0.2 B
9.2 ± 0.2 C
9.0 ± 0.2 C
8.7 ± 0.2 C
Tilapia
2.4 ± 0.2 A
6.7 ± 0.3 B
6.9 ± 0.2 B
7.1 ± 0.2 B
6.7 ± 0.3 B
Table 6. Off-flavour profiling of fish without and with depuration for varying
time periods. Mean numeric scores of grades (see Table 1) ± SE are shown; n =
40 (4 fish of each species from each treatment). Treatments not marked with
the same letter (A, B and C) are significantly different (p < 0.05; ANOVA)
Pond
Sand filter 1 Sand filter 2 PondPlus 1 PondPlus 2AquaPhoto 1AquaPhoto 2
Control 1 Contro
l 2
D
e
n
s
it
y o
f s
tre
p
to
m
yc
e
te
s
(10 c
e
lls
l
-1
9
)
0
1
2
3
4
5
March 25
May 25
July 25
Fig. 1. Abundance of Streptomyces in the different ponds
during the growth period. Mean values of triplicate samples
(on filter divided into 3 portions) ± 1 SD are shown
Aquacult Environ Interact 5: 107–116, 2014
water was not determined, but the abundance of bac-
teria in 4 similar fish ponds in the area was deter-
mined in December 2011 as 23 × 10
9
to 32 × 10
9
cells
l
−1
(data not shown). Assuming that these densities
are also representative of the 8 studied ponds, strep-
tomycete bacteria made up 0.45 to 13.3% (mean of
3.4%) of all bacteria.
Occurrence of Streptomyces in aquatic environ-
ments is relatively non-documented, but they are as -
sumed potential contributors of off-flavours (Klausen
et al. 2005). Using cultivation methods, densities of
10
2
to 10
5
streptomycetes l
−1
were found in various
freshwater environments (Lanciotti et al. 2003, Lee et
al. 2011), while fluorescence in situ hybridization,
targeting rRNA, indicated up to 2 × 10
9
cells l
−1
in a
water reservoir (Nielsen et al. 2006). In contrast,
using the present PCR approach, Lylloff et al. (2012)
only found up to 46 × 10
3
streptomycetes l
−1
in vari-
ous Australian freshwaters. Relative to these studies,
the present abundance of Streptomyces in the Patu-
akhali ponds appears relatively high. The low con-
centrations of geosmin in the pond water suggest
that streptomycetes, although abundant, were not
significant producers of geosmin in the water during
the studied period.
Cyanobacteria made up a relatively small portion
of the plankton biomass during the study, ranging
from 0.7% (May 25) to 1.9% (July 25) of the phyto-
plankton biomass (based on chlorophyll a) (Fig. 2).
The highest abundance (9.3% of the biomass)
occurred in AquaPhoto Pond #2 on 25 March. Treat-
ment of the water (sand filtration or probiotics)
reduced the abundance of cyanobacteria by 69% rel-
ative to the controls (p < 0.02; Mann-Whitney rank
sum test of all samples). The phytoplankton commu-
nity was dominated by chlorophytes (green algae),
followed by diatoms and cryptophytes, making up
53−76%, 13−23%, and 7.5−22%, respectively, of the
mean biomass from March to July.
The pigment analyses indicate that cyanobacteria
had a low abundance among phytoplankton in the
ponds. Microscopy analysis of phytoplankton in
some fish ponds in the region, e.g. in the main PSTU
campus fish pond, showed that the phytoplankton in
May and June 2013 was dominated by Oscillatoria,
Microcystis, Anabaena, and other cyanobacterial
species (data not shown). This contrasts our obser -
vations of phytoplankton populations determined by
pigment analysis. Here, cyanobacteria were identi-
fied by the pigments myxoxanthophyll, echinenone,
and canthaxanthin. Myxoxanthophyll and echinenone
have been found in selected species of Oscillatoria
and Microcystis (Schlüter et al. 2006), while Ana -
baena spp. may contain all 3 pigments (Schlüter et
al. 2004). These 3 cyano-pigments were only present
in some of the samples and in relatively small con-
centrations, as reflected in Fig. 2. Zeaxanthin is
another cyanobacterial pigment that was detected in
all samples in relatively low concentrations. How-
ever, zeaxanthin is also present in chlorophytes and
has limited value as diagnostic pigment in the pres-
ent study, where chlorophytes dominated.
Pheophytin a and pheophorbide a (degradation
products of chlorophyll a; not included in the CHEM-
TAX calculations) were present in considerable
amounts in some samples and constituted up to 27%
of chlorophyll a (average of 14% of chlorophyll a in
all samples). These degradation products indicate a
considerable breakdown of phytoplankton in the
ponds.
114
May 25
Chlorophyll
a
(µg l )
-1
0
25
50
200
July 25
0
10
20
30
40
100
March 25
0
50
100
150
200
350
400
Dinoflagellates
Diatoms
Chlorophytes
Cryptophytes
Cyanobacteria
Sand filter 1 Sand filter 2 PondPlus 1 PondPlus 2 AquaPhoto 1AquaPhot
o 2
Control 1 Control 2
Fig. 2. Concentrations of chlorophyll a and composition of
major groups of phytoplankton in the ponds determined by
pigment and CHEMTAX analyses. For each sampling, result
from 1 filter is shown
Petersen et al.: Geosmin in pond-raised fish
Cyanobacteria are assumed dominant producers of
off-flavours in open, outdoor aquaculture systems
due to presence of sunlight and inorganic nutrients
(Tucker 2000, Schrader & Dennis 2005). In the Patu-
akhali ponds, the low content of both cyanobacteria
and geosmin suggests that cyanobacteria were not
major producers of off-flavours in the water. How-
ever, more information on species of cyanobacteria
and their potential production of off-flavours in the
ponds is needed to draw conclusions, e.g. on the risk
of off-flavour episodes due to cyanobacteria.
CONCLUSION
The off-flavour geosmin in pangas and tilapia from
the Bangladeshi ponds was below the human detec-
tion threshold based on other studies of off-flavours
in fish, and geosmin probably did not contribute to
tainting of the fish. The chemical analyses demon-
strated that treatment of the pond water (via sand fil-
tration or probiotic microbes) and depuration for at
least 12 h reduced the geosmin content in pangas but
not in tilapia. Sensory studies confirmed a positive
effect of both water treatment and depuration on the
off-flavour content of the fish, especially for pangas.
This improvement of the sensory quality, despite the
fact that the geosmin content appeared to be consis-
tently below the human threshold, suggests that off-
flavour compounds other than geosmin were present
in water and fish. Studies are underway to demon-
strate whether phytoplankton, vegetation, or anthro-
pogenic sources may contribute to the off-flavour
content in water and fish in Bangladeshi ponds.
Acknowledgements. We thank the Regional Fisheries &
Livestock Development Component − Barisal TSU, Danish
International Development Assistance (Danida)/Govern-
ment of Bangladesh, Barisal, Bangladesh, for providing a
grant for this study. Technician Ulla Rasmussen, University
of Copenhagen, is acknowledged for skillful PCR analyses
of streptomycetes.
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Editorial responsibility: Megan La Peyre,
Baton Rouge, Louisiana, USA
Submitted: October 14, 2013; Accepted: March 20, 2014
Proofs received from author(s): April 16, 2014
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