Geosmin depuration from European eel (Anguilla anguilla) is not affected by the water renewal rate of depuration tanks

This study established that geosmin depuration from European eel is not affected by the water renewal rate of depuration tanks. A general fish bioaccumulation model extended with terms that account for effects of tank water renewal rate and system losses of chemicals, predicted strong effects of the water renewal rate of depuration tanks on geosmin depuration from European eel. Model predictions were validated in a depuration experiment with geosmin‐loaded European eel (n = 95) with a mean (SD) individual weight of 134.4 (5.0) g and a mean (SD) lipid content of 33.7 (2.8) % (w/w). Fish were depurated for 24, 48 or 72 h at three different tank water renewal rates (0.3, 3.3 and 33 day^?1). Treatments were installed by three different mean (SD) water flow rates (13.8 (1.3), 143.5 (9.2) and 1511 (80) L kg fish^?1 day^?1) over 30‐L tanks. Eels eliminated geosmin from their bodies, but unlike the model predicted, this was independent of the water renewal rate of the depuration tanks. Although being eliminated from the fish, geosmin hardly appeared and certainly did not accumulate in the water of the depuration tanks as the model predicted. This observation may be explained by geosmin being eliminated from eel as metabolite rather than the parent compound. Geosmin elimination from eel seems not to occur according to the generally accepted passive diffusion mechanism for excretion of lipophilic chemicals, and geosmin biotransformation by the eel seems indicated. Clearly geosmin depuration from European eel cannot be enhanced by increasing water renewal rates of depuration tanks.     

Geosmin causes off‐flavour in arctic charr in recirculating aquaculture systems

The ‘earthy’ and ‘muddy’ off‐flavours in pond‐reared fish are due to the presence of geosmin or 2‐methylisoborneol in the flesh of the fish. Similar off‐flavours have been reported in fish raised in recirculating aquaculture systems (RAS); however, little information is available regarding the cause of these off‐flavours. Our hypothesis was that earthy and muddy off‐flavour compounds, found previously in pond‐raised fish, are also responsible for off‐flavours in fish raised in RAS. In this preliminary study, we examined water, biofilms in RAS and fillets from cultured arctic charr known to have off‐flavours and requiring depuration using instrumental [solid‐phase microextraction procedure and gas chromatograph‐mass spectrometry (GC‐MS)] and human sensory analyses. Geosmin was present in the samples taken from the biofilter and on the side walls of the tanks. Two‐methylisoborneol was only found in low levels in the samples. The GC‐MS results indicated the presence of geosmin in the fillets (705 ng kg^?1), but lower levels were found in the water (30.5 ng L^?1). Sensory analyses also detected an earthy flavour (i.e., geosmin presence) in the fillets, and, therefore, it appears that geosmin is the main compound responsible for the off‐flavour in RAS. Further studies are being performed to identify the microorganisms responsible for geosmin production in RAS.     

Off-flavors in pond-grown ictalurid catfish: Causes and management options

Ictalurid catfish grown in ponds often acquire undesirable off-flavors prior to harvest. Off-flavors develop when odorous, lipophilic substances in food or water are absorbed across gut or gill epithelium and concentrated in edible tissues. The most common causes of catfish off-flavors are two nontoxic secondary metabolites of planktonic cyanobacteria: geosmin (causing an earthy off-flavor) and 2-methylisoborneol (causing a musty off-flavor). Off-flavored fish are unacceptable for processing, and harvest must be postponed until the source of the odorous compound disappears and the compound purges from edible fish tissue. Harvest delays caused by episodes of off-flavor increase production time, interrupt cash flow, and increase the risk of fish loss. Catfish farmers consider off-flavor to be one of their most important production-related problems. This paper reviews the causes of off-flavor in catfish, pharmacokinetics of uptake and loss of odorous compounds in catfish, seasonality and prevalence of off-flavors, farm- and industry-level impacts, the ecology of cyanobacteria in catfish ponds, and various strategies for preventing or treating cyanobacterial (and other) off-flavors. A decision-making system based on knowledge gained from research is presented as a guide to effective use of the limited tools available to manage off-flavors.     

Detection of catfish off-flavour compounds by trained dogs

Six mixed‐breed dogs were trained to detect 2‐methylisoborneol and geosmin in laboratory‐prepared water. The three dogs most responsive to training were selected for further testing to measure their ability to detect these compounds at levels typical of the early stages of an ‘off‐flavour’ event in catfish pond culture. The dogs were given one out of five choices of off‐flavour and ‘on‐flavour’ water samples and were trained to sit at boxes containing off‐flavour sample tubes. At the lowest concentration tested, 10 ng L^?1 of the target compound, the mean correct responses for the three dogs were 37%, 43% and 67%. Additional testing was conducted using off‐flavour pond water samples containing known amounts of these two compounds that occurred naturally. Correct responses for off‐flavour samples varied from 30% to 95%, depending on the sample and the dog. On‐flavour samples were correctly identified with 96% accuracy for all dogs and all samples. Dogs may provide practical early detection of off‐flavour problems in catfish ponds.     

The ratio between digestible protein and digestible energy affects accumulation and depuration of geosmin and 2-methylisoborneol (2-MIB) in Japanese seabass (Lateolabrax japonicus) raised in a recirculated aquaculture system

Actinobacteria and cyanobacteria accounted for less than 1% of total of bacteria in water in a recirculated aquaculture system (RAS) during a 15-week feeding trial with 0.11-g Japanese seabass. Resulting concentration of geosmin and 2-methylisoborneol (2-MIB) in RAS water was 169 and 45 ng L^?1, sufficient to produce strong off-flavour. The seabass were fed diets with 42%, 45% and 49% protein, and each protein level was supplemented with 15% or 18% lipid. Accumulation of off-flavours was independent of diet in fatty ventral tissue. Dietary protein significantly reduced off-flavours in lean, dorsal tissue. This was mainly rationalized by linear reduction in 2-MIB in response to increasing DP/DE and a strong, 2nd degree polynomial response in geosmin. The ratio between geosmin and 2-MIB was slightly higher at the beginning of a 10-day period with clean water and fasting, than what was observed throughout depuration. 2-MIB remained between 0.2 and 1 µg kg^?1 in dorsal tissue throughout depuration. Geosmin in ventral tissue ranged from 10 to more than 30 µg kg^?1at the termination of the feeding period and was reduced to a range from 6 to 20 µg kg^?1 by depuration.     

Experimental validation of geosmin uptake in rainbow trout,Oncorhynchus mykiss (Waldbaum) suggests biotransformation

The bioconcentration of waterborne geosmin in rainbow trout, Oncorhynchus mykiss (Waldbaum) was assessed. Fifty rainbow trout with a mean (SD) weight of 226.6 (29.0) g and lipid content of 6.2 (0.6) % (w/w) were exposed to geosmin in static water for 0, 2, 4, 6, 8, 12, 24, 36, 48 and 120 hr, with one tank containing five fish for each exposure period. Geosmin concentrations were measured in fish tissue and water samples collected over time. With time the geosmin concentration in the fish increased and decreased in the water. However, the total absolute amount of geosmin in the system declined over time which could be explained by induction of biotransformation. This is in accordance with the decreasing lipid normalized geosmin levels in the liver compared with the liver‐free carcass. Geosmin distribution within rainbow trout clearly is not exclusively governed by the lipid content of tissues. In vivo geosmin bioconcentration in rainbow trout is slower and the body burden reached is lower than the generally accepted theoretical model predicts.     

Controlling of geosmin and 2‐methylisoborneol induced off‐flavours in recirculating aquaculture system farmed fish—A review

Recirculating aquaculture system (RAS) is an increasingly popular alternative to open aquaculture production systems. However, off‐flavours and odours can accumulate in the fish flesh from the circulating water and decrease the fish meat quality. Off‐flavours are typically caused by geosmin (GSM) and 2‐methylisoborneol (MIB) that are lipophilic compounds formed as secondary by‐products of bacterial metabolism. Even though GSM and MIB are not toxic, they often are disliked by consumers, and both have very low human sensory detection limits. Multiple methods have been suggested to remove or decrease GSM and MIB in fish, including ozonation, advanced oxidation processes (AOP)s and adsorption removal from water using activated carbon and/or zeolites. So far, purging with fresh water is the only efficient method available to remove the off‐flavours. There are multiple analytical methods available for the extraction and separation of GSM and MIB from fish flesh and water. This review discusses the current knowledge of GSM and MIB formation, the challenges faced by RAS farms due to these compounds and process solutions available for their removal.     

致土腥味链霉菌的拮抗菌筛选、鉴定及抑制作用研究

为筛选可抑制水产养殖中产土臭素（GSM）和2-甲基异莰醇（2-MIB）的链霉菌生长能力的益生菌,以嗜热一氧化碳链霉菌（Streptomyces thermocarboxydus）、蓝微褐链霉菌（S. cyaneofuscatus）和玫瑰黄链霉菌（S. roseoflavus）为指示菌株,进行抑菌活性筛选,并对具有抑菌活性菌株进行鉴定。本研究共筛选获得8株拮抗菌,结果表明其中2株为地衣芽孢杆菌属（Bacillus licheniformis）,6株为枯草芽孢杆菌属（B. subtilis）。所筛菌株对供试链霉菌的生长均有抑制作用,其中2株可抑制嗜热一氧化碳链霉菌和玫瑰黄链霉菌的生长,另外6株对嗜热一氧化碳链霉的生长有抑制作用。筛选出的8株细菌可作为防控土腥味的益生菌,为生物控制和去除水产养殖中的土腥味提供了理论参考和实践依据。