Geosmin off-flavour in pond-raised fish in southern Bangladesh and occurrence of potential off-flavour producing organisms

Pangas Pangasianodon hypophthalmus and tilapia Oreochromis niloticus were cultivated 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 m2), 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 pangas, 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 offflavour 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.