Evaluation of depuration procedures to mitigate the off-flavor compounds geosmin and 2-methylisoborneol from Atlantic salmon Salmo salar raised to market-size in recirculating aquaculture systems

Fish cultured within water recirculating aquaculture systems (RAS) can acquire “earthy” or “musty” off-flavors due to bioaccumulation of the compounds geosmin and 2-methylisoborneol (MIB), respectively, which are produced by certain bacterial species present in RAS biosolids and microbial biofilms. Fish cultured in RAS are generally transferred to separate depuration systems that are flushed with water in a single pass or operated with limited water recirculation (with no biofilter), in order to purge these unpalatable flavors. Technologies and standard operating practices that optimize purging kinetics for Atlantic salmon Salmo salar and other species cultured in RAS are needed to improve the consistency and efficacy of depuration. A 2×2 factorial trial was conducted to evaluate techniques to mitigate off-flavor from Atlantic salmon cultured to 3–5kg in a semi-commercial scale freshwater RAS. Twelve replicated depuration systems (0.5m3) were used to evaluate four combinations (n=3) of the following standard operating procedure and system design parameters: (1) disinfection of depuration systems as a 1h static treatment using 250mg/L hydrogen peroxide (H2O2) prior to stocking fish, (2) no disinfection prior to stocking fish, (3) presence of water aeration media within gas transfer columns of depuration systems, and (4) absence of water aeration media within gas transfer columns of depuration systems. Food-size Atlantic salmon were stocked within the depuration systems and kept off feed for 10 days. Six salmon were harvested from the original RAS on Day 0 and fileted for baseline assessment of off-flavor concentrations. Thereafter, filet samples (n=3–4) were taken on Days 3, 6, and 10 to evaluate off-flavor kinetics. Hydrogen peroxide disinfection of depuration systems resulted in significantly reduced off-flavor in salmon filets during the depuration period. Results also indicated that the presence of high-surface-area water aeration media shielded biofilms from complete disinfection, resulting in less and slower off-flavor removal from Atlantic salmon filets; while depuration systems void of media resulted in greater and more rapid off-flavor reduction. Thus, water aeration media should not be used in depuration systems because of the challenges posed for effective cleaning, disinfection, and inactivation of off-flavor producing bacteria that may be present, and unit processes and locations that are difficult-to-clean should be excluded. In addition, a wide range of off-flavor concentrations were measured within individual salmon, indicating that one salmon is not a representative sample size to determine market suitability.