Off-Flavor (Geosmin/MIB)

Off-flavor is the musty, earthy taint in farmed fish flesh caused by geosmin and 2-methylisoborneol (MIB), semi-volatile compounds produced mainly by benthic cyanobacteria such as Oscillatoria and by actinomycetes in pond sediments, absorbed across the gills and skin and concentrated in lipid tissue. Human detection thresholds are extremely low: consumers detect geosmin at roughly 0.7–1.0 micrograms per kg of flesh, while water concentrations above about 1 ng/L geosmin or 2 ng/L MIB are enough to taint fish over a grow-out period. The defect is purely sensory, not a safety issue, but it is economically severe in pond-raised catfish and tilapia, where industry surveys have found a substantial share of fish rejected at processing. Management combines pre-harvest flavor testing (a 3–5 fish cooked-panel check at least two weeks before scheduled harvest) and purging: holding fish in clean, low-algae flowing water for 1–3 weeks, the time needed for the compounds to clear at roughly 50% per week. Prevention focuses on managing algal community composition through water exchange, algaecide spot-treatment of benthic mats, and limiting nutrient buildup that favors off-flavor-producing species.