This article was featured in Eurofish Magazine 5 2025.
Food is primarily for nourishment, but it can also be a source of pleasure when we perceive its smell and taste as pleasant. The -disappointment is all the greater when our sensory perceptions do not meet our expectations. Off-flavours can also occur in fish and seafood, and they are more than just a minor annoyance—they cause -enormous economic damage.
Corked wine, sour beer, or rancid-tasting coffee—flavour defects or off-flavours can occur in almost all foods and beverages, even though they may not be spoiled or unfit for consumption. It is not easy to distinguish between normal taste variability and a genuine off-flavour. The causes of off-flavours are diverse: incorrect processing temperatures, improper storage, early microbial spoilage, or oxidation caused by light or oxygen in the air. Often, it is enough for the levels of key flavour compounds to deviate from the norm or for certain components to be missing to noticeably alter the natural flavour profile. Foreign odours from the environment can also enter a food product and perceptibly change its original smell and taste. The range of sensory assessments extends from unpleasant to inedible, sometimes more finely distinguished into descriptions such as bitter, sour, rancid, rotten, unappetising, repulsive, spoiled, or -disgusting.
The sediment contains countless microorganisms that release geosmin and 2-methylisoborneol.
This is why the sludge should be regularly removed.
Aside from severe microbial contamination, off-flavours are rarely harmful to health. The damage they cause to producers and retailers lies mainly in the rejection of off-flavour products and complaints from customers. In addition to financial losses, reputational damage can occur, as consumers often avoid such products for an extended period following negative experiences.
Multiple causes of – off-flavours
In the fish and seafood sector, off-flavour is a truly global problem. It primarily affects aquaculture products from freshwater ponds and recirculating aquaculture systems (RAS), which often have a musty-earthy, mouldy, or damp smell and taste. These sensory defects can occasionally occur in marine fish species too, though far less frequently.
The nose is a sensitive sensory organ that can detect lack of freshness and off-flavours through smell.
There are various possible causes of off-flavour in freshwater species. Many ponds, natural waters, and recirculation aquaculture systems (RAS) are heavily loaded with micronutrients and organic substances, which lead to excessive growth of phytoplankton and microorganisms. These aquaculture systems provide ideal conditions for the proliferation of aquatic fungi, myxobacteria, and other proteobacteria. When certain filament-forming actinomycetes (bacteria) and cyanobacteria (blue-green algae) multiply excessively, their secondary metabolites accumulate in the water and produce a musty-earthy, muddy-rotten smell and taste.
The two substances most commonly associated with off-flavour in aquaculture are geosmin (GSM) and 2-methylisoborneol (MIB). In ponds and nutrient-rich lakes, both free-floating planktonic cyanobacteria and actinomycetes—particularly Streptomyces species that live in muddy sediments—contribute to the presence of GSM and MIB in the water. In RAS, odour-producing microorganisms mainly settle in the biofilter but also develop in thin biofilms on tank walls and floors.
Identifying off-flavours does not require special expertise or sensory training, as the human nose is highly sensitive to musty smells reminiscent of a potato cellar. However, detailed sensory training can help with precise diagnosis and verbal description. Some companies that specialise in this field offer “flavour kits” to help users train with and recognise standard flavours. The sensory detection thresholds for GSM and MIB in fish flesh vary by species but are relatively low. GSM is -detectable by the human nose at concentrations as low as 0.3 to 10 µg/kg, and MIB at 0.035 to 0.1 µg/kg.
Subjectively perceptible, but hard to quantify
To precisely identify and quantify off-flavour components call for instruments and analytical equipment typically available only in well-equipped laboratories. Accurate results require significant time, as complex extraction and concentration steps are necessary before analysis using gas chromatography–mass spectrometry (GC-MS). The difficulty is compounded by the fact that only a few laboratories specialise in GSM and MIB analysis and possess sufficient aquaculture expertise. Modern sensors and analytical instruments, such as “electronic noses” developed specifically for off-flavour detection, may offer a solution.
GSM and MIB are lipophilic substances that tend to accumulate in the fat tissue of fish species such as carp.
Although the main causes of off-flavour are largely understood, reliable prediction—let alone prevention—of its occurrence in farmed fish has not been achieved. Nevertheless, there have been repeated attempts with varying success, depending on the aquaculture system. Any measures to combat off-flavours must be environmentally -friendly, consumer-safe, efficient, and cost-effective. Since eutrophication is a major cause of off-flavour, efforts have been made to reduce nutrient accumulation in the water. One approach is to remove pond sediment to prevent the resuspension of nitrogen and -phosphorus compounds and shifts in phytoplankton communities (e.g., from eukaryotes to cyanobacteria). However, even fish from such treated ponds -often still exhibited an earthy-musty off–flavour, sometimes making them unsellable. The intensity of the off-flavour varied widely, but even small amounts of GSM and MIB were enough—depending on fat content, farming method, and diet—to significantly reduce the palatability of the fish. This is due to the uptake mechanisms of these -substances, which enter fish through the skin and gills directly from the water or via the gastrointestinal mucosa with food. Because GSM and MIB accumulate in the rearing water and sediments, it is nearly impossible to prevent fish—especially bottom-feeding species that consume natural food sources—from rapidly developing varying degrees of off-flavour and off-odour.
Ponds and RAS particularly affected
In aquaculture systems such as ponds and raceways, cyanobacteria are the most common producers of GSM and MIB, while in RAS, chemotrophic -bacteria—especially actinomycetes—are considered the main contributors. Studies have shown that in around 80 percent of water samples from commercial freshwater RAS, geosmin concentrations were higher than those of 2-methylisoborneol. This suggests that GSM plays a more significant role in causing off-flavours than MIB. A further complication is that fish absorb GSM more quickly, accumulate it more strongly, and eliminate it more slowly than MIB. As mentioned earlier, off-flavour in aquaculture products leads to considerable economic costs. Rough estimates suggest that flavour defects in Ictalurus products cause losses of 12 to 15 percent of the annual revenue of the U.S. catfish industry in the -Mississippi–Missouri region. -Additional, harder-to-quantify losses arise from reputational damage and declining market demand.
The growing pressure of the off-flavour problem is motivating researchers and industry professionals around the world to search more intensively for viable solutions. Three main strategies are being pursued: preventive, curative, and alternative methods. The preventive approach seeks to halt the microbial production of GSM and MIB in aquaculture systems—to address the problem at its root. Given the wide variety of microbiota capable of producing GSM and MIB, completely eliminating these organisms from the systems appears impossible. However, limited success has been achieved with the use of algicides to control or eliminate harmful cyanobacteria. Such substances must not harm fish or later pose a risk to consumers. Based on current experience, low-dose copper sulphate appears to be a promising and relatively economical solution in ponds. Co-stocking ponds with plankton-eating fish or mussels may also help mitigate algae problems. In isolated cases, colouring the pond water to reduce light penetration and photosynthesis, thereby slowing algae growth, has been considered.
Purging often remains the only solution
The second approach, the curative strategy, is based on removing the GSM and MIB absorbed into the fish’s body. As previously described, these substances enter the fish mainly through the gills, skin, and intestines, and accumulate primarily in body fat. Their uptake reflects a dynamic equilibrium that continues until approximately equal concentrations exist in the water and the fat of the fish. The absorption and bioaccumulation of geosmin and MIB are reversible. If fish with off-flavour are placed in pure water free from these compounds, the absorbed substances will diffuse back into the water depending on concentration gradients. However, the net efflux—or release—of GSM and MIB from fish tissue into the water occurs significantly more slowly than their uptake. Aquaculture -producers use this principle during purging or depuration to rid fish of off-flavours.
Fish from RAS often show pronounced off-flavours. Chemotrophic bacteria,
especially actinomycetes, are the chief culprits.
This cleaning process typically takes place at the end of the production cycle before delivery. During purging, fish are held without feeding in large volumes of clean, GSM- and MIB-free, ideally flowing freshwater to allow accumulated off-flavours to be eliminated from their bodies. The success of this process depends on multiple factors. Since off-flavours are released more slowly than they are absorbed, the effectiveness of purging depends heavily on the duration of exposure to clean water. If the “cleaning time” is too short, only partial removal of off-flavour occurs. Achieving satisfactory results requires -experience. Moreover, the removal of off-flavours also depends on the fish species, fat content, intensity of the off-flavour, water temperature, swimming activity, feeding frequency prior to purging, and other variables. It is particularly difficult to estimate the time required for removing off-flavour from fatty tissues and organs. In the case of caviar production, sturgeon sometimes have to be purged for several months to produce truly off-flavour-free roe.
All these factors make the curative effect of purging rather uncertain, partly ineffective, and costly. It requires large quantities of clean water, additional tanks and production areas, and pumps. As the fish are not fed during this time, they may lose weight, resulting in financial losses for producers. Additionally, animal welfare concerns have been raised, as long periods without feeding are seen by some as compromising fish welfare and the sustainability of the operation.
Alternative strategies are often still in early stages
For the reasons mentioned above, there is intensive research into methods to prevent GSM and MIB from forming or accumulating in fish. These efforts to optimise cleaning processes are grouped under the third strategy: “alternative methods.” This area is of particular interest to the RAS sector. As systems with high stocking densities that rely on water reuse and heavily populated biofilters, RAS are particularly susceptible to the development of GSM- and MIB-producing microorganisms. Although solid waste is continuously removed from recirculating water, many organic compounds that serve as substrates for off-flavour microbes still accumulate in the system. Studies of off-flavour have shown that actinomycetes, especially streptomycetes, are particularly common in the water treatment areas of RAS. Cleanliness and regular maintenance throughout the facility are therefore essential to prevent off-flavours.
Large purging facilities are often needed to remove off-flavours from pond fish before they are marketed.
These facilities are also used for overwintering.
In addition, researchers are seeking alternative methods to significantly reduce the need for cleaning. One such method is the chemical oxidation of GSM and MIB using ozone (O₃) or hydrogen peroxide (H₂O₂), ideally in combination. These treatments are already commonly and successfully used to remove off-flavours in drinking water production. In low doses, ozone is frequently used in RAS for disinfection, reducing turbidity, and clarifying water colour. However, effective control of GSM and MIB would require much higher doses, potentially in combination with intense UV irradiation, which could be harmful to fish if not used with activated carbon. As strong oxidising agents, ozone and hydrogen peroxide attack not only the off-flavour-producing organisms but all organic substances indiscriminately. Despite this, they are seen as promising because they break down easily and convert into harmless water and oxygen after use. To enhance their effectiveness and better control the processes, advanced oxidation processes (AOP) are used. These generate highly reactive hydroxyl radicals (OH) that destroy unwanted off-flavour substances and remove them efficiently from wastewater. In AOPs, oxidants such as ozone (O₃), hydrogen peroxide (H₂O₂), and peracetic acid (PAA) are combined to strengthen the treatment against GSM and MIB. In commercial RAS -trials, concentrations of off-flavour compounds in recirculating water were reduced by up to 77 percent for MIB and 93 percent for GSM.
A completely new idea, still in its infancy, is the selective degradation of geosmin and MIB by other microorganisms. Also, not ready for practical use are the so-called “low off-flavour RAS” systems with “off-flavour bioreactors.” These bioreactors currently operate based on an upflow sludge blanket principle. However, experts agree that much better results in off-flavour treatment would be achievable with moving bed biofilm reactors (MBBR). Work is already underway on this, but commercial readiness and practical implementation in RAS will take more time.
Another method—the “final” step, so to speak—for concealing the off-flavour of a fish from the consumer is through how the product is processed and preserved. Effective methods for masking off-flavour include intense seasoning, which makes off-flavours less noticeable, as well as heavy -smoking, which can often partially -cover up the -unpleasant taste.
Manfred Klinkhardt
