A common complaint among consumers is the earthy taste of farmed fish. The intensification of aquaculture comes with its own set of challenges, and one of the most prominent is the production of unpleasant flavors in fish.
The release of an excess of nutrients into the aquatic environment by farmed fish can lead to a deterioration of water quality, creating a favorable environment for the growth of microorganisms, including species that produce undesirable flavors.
Although fish farmers have been grappling with this issue for 20 years, a definitive solution to the problem of bad taste has not yet been found.
Now, researchers from the University of Copenhagen have been able to pinpoint exactly when these unpleasant flavors arise. This discovery could make it easier to deal with compounds that deter people from consuming cage-raised fish.
The published study aimed to determine the levels of taste and odor compounds in Nile tilapia and their impact on the flavor quality of fish produced in net cages in reservoirs in São Paulo, Brazil.
The Origin of Bad Fish Taste
Fish with a musty, earthy, or muddy taste is never a crowd-pleaser. You may have encountered it in trout caught in a fishing pond. The bad taste is typically found in freshwater fish such as trout and carp but also in pangasius and tilapia, two of the world’s most lucrative aquaculture species.
The primary culprit behind the muddy taste is geosmin, a compound produced by aquatic microorganisms found in the water where fish are raised. While harmless to humans, the presence of geosmin poses a challenge to the production of farmed fish, which are a significant global source of protein.
“Geosmin is the compound that gives beetroot its characteristic earthy taste. Except that’s what you expect a beetroot to taste like. In fish, it’s an undesirable unpleasant flavor that discourages people from consuming an otherwise healthy and often sustainable food. Fish farmers have known about this bad taste for many years, but it’s still challenging for them,” says Associate Professor Mikael Agerlin Petersen of the Department of Food Science.
Now, Petersen and his colleagues from Denmark and Brazil have determined exactly how much geosmin is needed to affect the taste of tilapia using refined measurement techniques. This is the first time this has been possible. The studies were conducted with tilapia from various aquaculture facilities in the state of Sao Paulo, Brazil.
Identifying Unpleasant Flavors in Fish
The study employed Gas Chromatography-Mass Spectrometry (GC-MS) analysis to investigate taste and odor compounds, including geosmin and 2-MIB (2-methylisoborneol), in both the fish and water from six different farms. Geosmin concentrations in the water ranged from 1 to 8 nanograms per liter (ng/L), while geosmin levels in fish flesh ranged from 40 to 750 nanograms per kilogram (ng/kg). Similarly, the level of 2-MIB in water varied from 2 to 25 ng/L and from 0 to 800 ng/kg in fish.
The GC-MS analysis revealed the presence of more than 100 volatile organic compounds in the fish flesh, including aldehydes, alcohols, benzene derivatives, hydrocarbons, ketones, and other compounds. Notably, the study found that a sensory panel detected flavor notes related to geosmin and 2-MIB in all fish samples. Furthermore, a high correlation was observed between the chemical analysis and sensory evaluations.
Small Amounts Lead to Earthy Flavors in Fish
A common complaint among consumers is the earthy taste of farmed fish. The intensification of aquaculture comes with its own set of challenges, and one of the most prominent is the production of unpleasant flavors in fish.
Geosmin is a compound formed by various bacteria and algae in the water where fish are raised. As farmed fish absorb geosmin through their gills, the compound begins to accumulate. Once there is enough of it, it imparts an earthy taste to the fish’s flesh.
“Our study shows that fish acquire an unpleasant taste from geosmin even when there are extremely low concentrations in the water. We found concentrations in the fish that are 100 times higher than in the water. Due to the lack of methods to measure such low concentrations, little has been done about it for a long time, but now we have the measurement techniques to do it,” says Niels O. G. Jørgensen, Associate Professor Emeritus at the Department of Plant and Environmental Sciences and co-author of the study.
The researchers used a combination of advanced chemical analysis and keen human senses of smell. First, they used gas chromatography and mass spectrometry to identify and measure the presence of geosmin and other compounds in both the water and the fish. Next, a panel of people trained to recognize geosmin tested what concentration of the compound was necessary before it could be smelled and tasted in tilapia fillets.
The Role of Water Quality
The results demonstrated that, as expected, geosmin is the primary culprit, but 2-methylisoborneol also plays a significant role in the bad taste. Other volatile organic compounds also contribute, although to a lesser extent, and the researchers emphasize that all these compounds are harmless to humans.
“These methods have allowed us to determine the exact concentrations of different bad-tasting compounds and how much of them is needed before people can taste them. So, in principle, producers can use this as a practical tool to determine whether their fish are safe for the market, either by conducting the analyses themselves or by paying laboratories to do so,” states Mikael Agerlin Petersen.
The study indicates that water quality plays a key role in the development of these compounds. In the Brazilian net cage farms used for the study, fish live in large net cages located in hydroelectric reservoirs.
“The problem in Brazilian net cages is that, even though there are thousands of fish, there is very little water circulation. When so many fish gather in a small area, uneaten food and feces enter the water, leading to a local deterioration of water quality. We can see that where the water hasn’t been cleaned well or lacks a flow of fresh water, geosmin quickly becomes a problem,” says Niels O. G. Jørgensen.
Potentially Sustainable Fish
The bad taste of farmed fish not only affects the income of the aquaculture industry but also impacts the promotion of a more sustainable protein source.
Nearly 90% of the world’s wild fish populations are fully exploited, overexploited, or depleted. As such, aquaculture plays an increasingly important role in global food supply. Currently, aquaculture accounts for half of all fish consumed worldwide.
“The focus on sustainability in aquaculture is growing, but there are still numerous challenges, including wastewater management and sourcing feed from wild fish populations. However, it is possible to make the sector sustainable and climate-friendly compared to both wild-caught fish and other animal products. But no place is safe from the potential issue of fish with earthy flavors,” says Niels O. G. Jørgensen.
Mikael Agerlin Petersen adds, “And if you want to produce more fish for the global population, it needs to be something with an acceptable taste if people are going to eat it. That’s why knowing how to manage it is essential.”
Conclusion
Intensive fish production in Brazil, particularly Nile tilapia, has significantly contributed to the country’s fish consumption. However, the industry faces a significant challenge in the form of unpleasant flavors, primarily attributed to geosmin and 2-MIB. The comprehensive analysis in this study sheds light on the presence of these compounds and their impact on the flavor quality of farmed fish.
Addressing these unpleasant flavors is vital to maintaining the quality and consumer acceptance of aquaculture products. It is clear that a holistic approach, encompassing both chemical and sensory assessments, is essential for understanding and managing undesirable flavors in farmed fish. Additionally, the findings underscore the need for nutrient management in aquaculture systems to mitigate the production of unpleasant flavors.
The study was funded by the São Paulo Research Foundation (FAPESP) in Brazil and the Ministry of Higher Education and Science, the Strategic Research Council of Denmark.
Contact
Mikael Agerlin Petersen
Associate Professor
Department of Food Science
University of Copenhagen
dn@food.ku.dk
+45 23 72 30 67
Niels O. G. Jørgensen
Associate Professor Emeritus
Department of Plant and Environmental Sciences
University of Copenhagen
nbj@plen.ku.dk
+45 40 35 88 25
Reference (open access)
Raju Podduturi, Gianmarco da Silva David, Reinaldo. J. da Silva, Grethe Hyldig, Niels O.G. Jørgensen, Mikael Agerlin Petersen. 2023. Characterization and finding the origin of off-flavor compounds in Nile tilapia cultured in net cages in hydroelectric reservoirs, São Paulo State, Brazil, Food Research International, Volume 173, Part 2, 2023, 113375, ISSN 0963-9969, https://doi.org/10.1016/j.foodres.2023.113375.
Crafted with information from the University of Copenhagen and the scientific article.
