Earthy, musty off-flavor caused by the microbially-derived compounds geosmin (GSM) and 2-methylisoborneol (MIB) is a priority concern for producing lipid-rich fish like salmonids in RAS. Currently, depuration is the only proven method that consistently eliminates objectionable off-flavor from RAS-produced fish before harvest. However, this procedure requires fish transfer to separate systems where substantial water volumes are exchanged, typically while withholding feed for five to ten days. Solutions that limit off-flavor in the primary RAS could reduce or eliminate the need for depuration, thereby lowering water use, energy requirements, fish handling, and weight loss, which could ultimately improve the economics of RAS operations.
Two studies were conducted. Study 1 aimed to improve our understanding of conditions that favor off-flavor production by assessing GSM and MIB levels in water and Atlantic salmon fillets in pre-disinfected- vs. microbially mature freshwater RAS (9.5 m3 total volume) operated for more than two years without shutdown (n=3 RAS). GSM and MIB levels spiked several weeks after stocking fish in the pre-disinfected RAS. In contrast, levels remained relatively low in the mature RAS (Fig. 1). Similar trends were observed in Atlantic salmon flesh. Off-flavor levels correlated with nitrite-nitrogen concentrations and the establishment of complete nitrification. Microbial analysis indicated that the relative abundance of off-flavor-producing organisms was statistically similar between treatments, suggesting GSM and MIB production was tied to aspects of the fish culture environment such as nutrient concentrations or ratios.
Pre-disinfection of RAS was deemed a procedure that could be repeated for testing off-flavor mitigation strategies due to the consistent spike in GSM and MIB. Thus, all six RAS were disinfected before Study 2. Nitrification was re-established, and the same number of Atlantic salmon were stocked in six identical RAS. Low-dose ozone, which provides significant water quality improvements, was then applied in three RAS along with the periodic addition of peracetic acid (PAA), which has been shown to have biocidal effects against harmful microorganisms. Geosmin and MIB levels were compared in RAS where this advanced oxidation strategy was applied vs. RAS without oxidant use. Off-flavor levels measured during Study 2 will be reported at the meeting.