Using GC–MS to Analyze Geosmin in Tilapia

Nile tilapia (Oreochromis niloticus) is a species of fish that are commonly found in the Levant region, particularly Israel and Lebanon. These fish can reach up to two feet in length and weigh up to 11 lbs. (1). Tilapia are a common type of fish found in many supermarkets. Thanks to an increased understanding of the health benefits of consuming fish over alternative meat products, the demand for tilapia and other types of seafood has increased. As a result, it is important that the fish caught in the wild and prepared for consumption does not contain volatile or off-flavor compounds that are unsafe or undesirable for humans.

The importance of using analytical instrumentation to monitor and quantify levels of volatile compounds in fish products is paramount. A recent research collaboration between researchers from Denmark and Brazil tackled this issue by exploring how gas chromatography–mass spectrometry (GC–MS) (2). The findings of this study were published in Food Research International (2).

Using a fishing net, catch fish. Baby tilapia, tilapia fishbreeding and culture. | Image Credit: © TL23 photo - stock.adobe.com

The research team (comprised of scientists Raju Podduturi, Gianmarco da Silva David, Reinaldo J. da Silva, Grethe Hyldig, Niels O.G. Jorgensen, and Mikael Agerlin Petersen) conducted their study in São Paulo State, Brazil, where there is a thriving aquaculture industry. The cultivation of fish constituted, according to the researchers, 860 × 10³ tons in 2022, contributing to approximately 87% of total fish consumption (2). Of that 87% Nile tilapia is approximately half of that, and most tilapia farms are known for using floating net cages (2). Their main research objective was to use GC–MS analysis to determine the concentration of geosmin, 2-MIB, and other volatile compounds in the tilapia that were in these aquaculture environments (2).

According to Mikael Agerlin Petersen, one of the researchers of the study, geosmin “is the compound that gives a beet root its characteristically earthy flavor. Except, that’s what one expects a beet to taste like. In fish, it is an undesirable off-flavor that discourages people from eating an otherwise healthy and often sustainable food” (3).

Geosmin, therefore, is an off-flavor compound that negatively impacts the sale of fish on the market, because odor is an important variable to consumers when selecting fish and other food products. Because geosmin is a compound that gives healthy fish an earthly odor, the result is that the presence of it could lead to food waste (4).

Meanwhile, 2-Methylisoborneol is the companion to geosmin, as it also is an off-flavor compound that accounts for a significant portion of biologically caused taste and odor outbreaks in food products such as fish and drinking water (5).

The study conducted by the research team examined the water and Nile tilapia from six aquacultural farms. Using GC–MS, the researchers detected geosmin concentrations in the water to the tune of 1–8 ng/L, and in fish flesh at levels between 40 and 750 ng/L (2). For the 2-MIB concentrations, it was 2 to 25 ng/L in the water, and 0 to 800 ng/kg in the fish (2).

The GC–MS analysis also revealed much more than the presence of the abovementioned off-flavor compounds. Within the fish flesh, the research team noted the presence of more than 100 volatile organic compounds (VOCs) (2). Some of the notable VOCs were alcohols, benzene derivatives, ketones, aldehydes, and hydrocarbons, to name a few (2).

Overall, the study revealed that geosmin had a greater presence in the tilapia compared to 2-MIB (2). Water quality was found to significantly impact the presence of off-flavors, likely influenced by fish production's release of nutrients promoting the growth of off-flavor producing microbes (2). The authors suggest that better monitoring and water quality management could minimize off-tastes and increase the sale of tilapia from net cages (2).

References

1. Fair Fish Database. Oreochromis niloticus (Linnaeus, 1758) Nile tilapia. Available at: https://fishbase.mnhn.fr/summary/Oreochromis-niloticus.html (accessed February 21st, 2024).
2. Podduturi, R.; David da Silva, G.; da Silva, R. J.; Hyldig, G.; Jorgensen, N. O. G.; Petersen, M. A. Characterization and finding the origin of off-flavor compounds in Nile tilapia cultured in net cages in hydroelectric reservoirs, São Paulo State, Brazil. Food Res. Int. 2023, 173, Part 2, 113375. DOI: 10.1016/j.foodres.2023.113375
3. EurekAlert, Pinpointing the Emergence of Muddy Flavors in Your Fish. Available at: https://www.eurekalert.org/news-releases/1005238 (accessed February 21, 2024)
4. American Chemical Society, Molecule of the Week Archive: Geosmin. Available at: https://www.acs.org/molecule-of-the-week/archive/g/geosmin.html#:~:text=Geosmin%20is%20a%20natural%20bicyclic,as%20low%20as%205%20ppt (accessed February 21st, 2024).
5. Jüttner, F.; Watson, S.B. Biochemical and Ecological Control of Geosmin and 2-Methylisoborneol in Source Waters. Appl. Environ. Microbiol. 2007, 73. https://doi.org/10.1128/AEM.02250-06