Abstract
Tainting of outdoor pond-reared barramundi Lates calcarifer by muddy-earthy off-flavours 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 dose-effect 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 (similar to 1.00 mu g l(-1)) to extreme levels (similar to 14.36 mu g 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 similar to 4 mu g 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 mu g kg(-1), similar to estimates for GSM detection in rainbow trout Oncorhynchus mykiss (similar to 0.9 mu g kg(-1)) and for 2-MIB in channel catfish Ictalurus punctatus (0.7 mu g 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.
Highlights
Muddy-earthy-musty-type flavours are generally regarded as a natural characteristic of wild caught freshwater fish (Tucker 2000, Howgate 2004), the occurrence of such flavours has been reported for a diverse range of freshwater aquaculture species (Lovell 1983, Yamprayoon & Noonhorm 2000, Robertson et al 2005, Petersen et al 2011)
GSM levels differed between rearing ponds and were highly variable within individual ponds, with mean GSM levels showing coefficients of variation (CV) ranging from 74.9 to 125.2%
In the present study we identified the compound GSM as the primary contributor to off-flavour tainting of tropical pond-reared barramundi
Summary
Muddy-earthy-musty-type flavours are generally regarded as a natural characteristic of wild caught freshwater fish (Tucker 2000, Howgate 2004), the occurrence of such flavours has been reported for a diverse range of freshwater aquaculture species (Lovell 1983, Yamprayoon & Noonhorm 2000, Robertson et al 2005, Petersen et al 2011). Fish presenting with these flavour characteristics are often referred to as being ‘off-flavour’ or ‘tainted’ and are commonly considered to be spoiled or of low quality. GSM and 2-MIB are metabolites of certain groups of algae, actinomycetes and cyanobacteria (Tucker 2000) and are found in various water sources such as lakes, reservoirs and running waters (Jüttner & Watson 2007)
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