The most common off-flavour compounds found in cultivated fish are 2-Methylisoborneol (2-MIB) and geosmin (GSM), which are secondary metabolites released by microorganisms such as cyanobacteria or actinomycetes that exist in most aquaculture systems. Due to their high bioaccumulation rates in lipid-rich tissue, even very low concentrations of 2-MIB and GSM in the water can cause an undesirable earthy-musty smell and taste in fish flesh. This leads to a loss of product quality and reduces marketability.
Purging off-flavours by moving the fish to clean, odour-free water for a certain time prior to harvest is still the only reliable, albeit costly and time-intensive way to remove these off-flavours. Proven alternative strategies and processes for efficient prevention and removal of off-flavours are still lacking.
At the Second Workshop on Recirculating Aquaculture Systems (RAS) held in Aalborg, Denmark last October, several papers specifically addressed this subject, two are reported on here.
The role of sludge digestion
In their study, two GSM- and 2-MIB-producing bacterial strains were isolated from an aerobic and organic-rich nitrification filter and a drum filter in an RAS used for tilapia culture. Bacterial isolates were found to be closely related to Streptomyces roseoflavus and S. thermocarboxidus. Both grew in aerobic- as well as in oxygen-limited conditions, but with higher production rates in the aerobic conditions.
It was found that a decrease in GSM and 2-MIB levels in the digestion basin of the RAS was due to the compounds being absorbed by the sludge in the basin. From parallel runs with non-sterilized sludge it was concluded that in addition to physical/chemical removal processes, GSM and 2-MIB were also biologically degraded within the sludge.
Continuous enrichments of crude sludge with GSM and 2-MIB resulted in the isolation of three bacterial strains capable of growth with GSM or 2-MIB as their sole carbon and energy source. These bacterial strains were found to be closely related to Variovorax paradoxus, Rhodococcus sp. and Comamonas sp.
All isolates showed the highest removal rates of GSM and 2-MIB when these compounds were present as the sole carbon- and energy source in the growth medium. The addition of more readily available carbon sources resulted in higher growth rates of the bacterial isolates but slower removal rates of GSM and 2-MIB. Growth rates of the V. paradoxus-like isolate were similar in both aerobic and non-aerated conditions, whereas the other two bacterial strains grew fastest under aerobic conditions. Fluorescent in situ hybridization (FISH), showed the prevalence of bacteria belonging to the Rhodococcus and Comamonas genera in sludge from the digestion basin.
Can ultrasonics play a role?
Samples of clean freshwater and saltwater were spiked with 2-MIB and GSM standards and treated with a lab-scale ultrasound transducer at a frequency of 850 kHz in pulsed mode. Additionally, samples from commercial recirculating aquaculture systems containing natural 2-MIB and GSM were also treated. The off-flavour compounds were extracted with a liquid-liquid-extraction, concentrated, and analyzed using a GC-MS. Furthermore, the effect of salinity on the removal of 2-MIB and GSM via ultrasonic treatment was also investigated.
Their results showed that ultrasonic water treatment significantly reduced the concentration of off-flavour compounds in all sample types, though the reduction of GSM was slightly higher than that of 2-MIB. The addition of salt to freshwater samples significantly improved the reduction of both off-flavour compounds by ultrasonic treatment. Clearly, ultrasonic water treatment may have the potential to remove off-flavour compounds effectively from RASs, and the addition of salt to freshwater systems in which euryhaline species are grown may further enhance removal efficiency.
- Dave Scarratt