Abstract
The effect of feeding during off-flavour depuration on the elimination of geosmin from muscle tissue (fillet) and ovaries as a model for caviar was assessed in Nile tilapia (Oreochromis niloticus) (mean ± SD weight of 185 ± 15.0 g). The experiment had a 2 × 4 factorial design with feeding level (starved or fed) and depuration time (24, 48, 72 and 96 h) as factors with duplicates for each of the 8 treatment combinations. Fish were normally loaded with geosmin prior to the experiment.During off-flavour depuration geosmin levels in fillet and ovary declined over time in both fed and starved tilapia. In fed tilapia geosmin declined faster from the ovaries compared to starved fish (p = 0.018). The same trend of a faster decline was observed for the muscle tissue (fillets) of fed tilapia, though only numerically (p = 0.11). Because faster geosmin elimination paralleled with high blood lipids, we do not rule out that blood lipids are involved in geosmin transport via the circulatory system and that low blood lipid levels are limiting geosmin elimination in starved fish. No difference in geosmin elimination rate was detected between ovary and muscle tissue in Nile tilapia. Off-flavour depuration time is strongly reduced when farmers adopt a practice of feeding Nile tilapia during off-flavour depuration.
Highlights
Geosmin and 2-methylisoborneol (2-MIB) are secondary metabolites produced by a wide range of microbiota common to land-based aquaculture systems
Feeding Nile tilapia during off-flavour depuration resulted in a significant higher elimination rate of geosmin from the ovary
We attribute this to the relatively higher analytical variation in the data caused by the low geosmin levels in fillets rather than absence of an enhancing effect of feeding during off-flavour depuration on geosmin elimination from the fillets
Summary
Geosmin and 2-methylisoborneol (2-MIB) are secondary metabolites produced by a wide range of microbiota common to land-based aquaculture systems Upon their release to the water these chemicals are quickly bioconcentrated by fish due to their lipophilicity (geosmin log Kow: 3.57 and 2-MIB log Kow: 3.31, Howgate, 2004). Bioconcentration of geosmin and 2-MIB is dynamic and reversible; the chemicals can freely diffuse in and out of the fish via the gills depending on the current fugacity gradient between water and fish (Howgate, 2004). Fish are depurated until excretion has resulted in a reduction of the geosmin and 2-MIB concentrations in the fish to levels below their sensory detection limits. During offflavour depuration fish are generally not fed to allow fish to empty their intestinal tracts prior to slaughter and to reduce the water flow over the depuration system required to maintain good water quality
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