Earthy-muddy tainting of cultured barramundi linked to geosmin in tropical northern Australia

Tainting of outdoor pond-reared barramundi Lates calcarifer by muddy-earthy offflavours is frequently reported across tropical Australia. To investigate the possible causes and effects of off-flavour tainting, we analysed water samples from outdoor rearing ponds for the presence of geosmin (GSM) and 2-methylisoborneol (2-MIB), 2 microbial metabolites often associated with tainting episodes. We then conducted controlled doseeffect experiments which measured the accumulation of tainting metabolites in the flesh, and the impact tainting had on taste and flavour attributes. GSM was deemed to be the compound most likely responsible for off-flavour tainting, persisting at moderate (~1.00 μg l-1) to extreme levels (~14.36 μg l-1), while 2-MIB was never detected during the study. Controlled experiments revealed that the accumulation of GSM in the flesh of market-sized barramundi was directly related to GSM levels of the holding water (0 to ~4 μg l-1), with higher levels resulting in significant increases in undesirable taste and flavour attributes, particularly muddy-earthy flavour and weedy aftertaste. We identified the sensory detection threshold for GSM in farmed barramundi to be ≤0.74 μg kg-1, similar to estimates for GSM detection in rainbow trout Oncorhynchus mykiss (~0.9 μg kg-1) and for 2-MIB in channel catfish Ictalurus punctatus (0.7 μg kg-1). Quantitative estimation of flesh-bound GSM using gas chromatography-mass spectrometry (GC-MS) agreed well with human sensory assessment scores and highlights the reliability of chemical analysis of GSM in barramundi flesh while also indicating the value of GC-MS analysis in predicting the impact of GSM on the sensory properties of farmed barramundi. © Inter-Research 2013.