Approximately 82.1 million tons of aquatic animals are produced per year (FAO, 2020). Sustainable aquaculture, such as Recirculated aquaculture system (RAS), is envisaged as one of the solutions to achieve a further increase in global seafood production. However, even though RAS farms can produce high-quality fish, "musty" and "earthy" off-flavours often taint fish, which is a well-known issue for fish farmers, inflicting significant financial losses as well as a negative perception of the RAS-farmed fish (Moretto et al., 2022). The main volatile compounds that contribute to those flavours in fish are geosmin and 2-methylisoborneol, but other compounds may additionally play a role (Jones et al., 2022). The traditional procedure to deal with off-flavour is the depuration of the fish in cleaner water (Podduturi et al., 2021). In this project sensory analytic instrumental procedures were applied for the characterization of aroma-active compounds in Nile Tilapia (Oreochromis niloticus) fillets to determine the aroma profile and composition of RAS fish and comprehend the impact of the depuration process.
Samples were collected in March 2022 from a recirculated aquaculture farm (Gårdsfisk, Sweden). Three fish replicates were caught randomly from the production and depuration tank. All samples were directly stunned, hand filleted, skinned and frozen at -20 °C until use for analysis. For efficient extraction of the volatile compounds, the procedure was adapted from Mahmoud 2017 (Mahmoud et al., 2017). Briefly, 50 g portions from the fish fillets were taken and minced into approximately 1 cm3 cubes and extracted once with an aliquot of 100 mL of dichloromethane by stirring at room temperature for 30 min. Subsequently, the extract was collected separately and distilled using solvent-assisted flavour evaporation (SAFE). After SAFE, the sample was dried over anhydrous Na2SO4. Prior to further chromatographic analysis, the total volume of each sample was reduced to approx. 100 µL via Vigreux distillation and micro distillation at 50 °C. The aroma analysis was performed using gas chromatography-olfactometry/flame ionization detector (GC-O/FID) equipped with DB-FFAP and DB-5 columns. The comparative aroma extract dilution analysis (cAEDA) was performed using the DB-FFAP column. Moreover, gas chromatography-mass spectrometry (GC-MS) and two-dimensional gas chromatography with mass spectrometry and olfactometry (2D-GC-MS/O) analysis were carried out. The identification of the odour-active compounds was based on the comparison of odour quality, retention indices on two capillaries with different polarities (DB-FFAP and DB-5) and mass spectra with those of authentic reference compounds. Retention indices were calculated for each odour-active compound by means of a reference series of homologous alkanes (C6-C30). Furthermore, the aroma profile analysis was performed according to ISO 13299:2016–09 and complemented by the determination of the overall intensity of the sample’s odours.
The odour attributes fatty, green, mouldy, earthy, fishy and sea water-like/algae-like were selected by the panellists for the description of the fish aromas. Using combined sensory and instrumental techniques, 108 and 79 aroma-active compounds were olfactorily detected in Nile Tilapia from tank and depuration, respectively. Of these compounds, 77 were identified by comparison of the respective retention indices, mass spectra and odour qualities with their corresponding reference compounds, while 31 compounds could not be resolved due to their low concentration.
In this study, SAFE technique followed by sensory-instrumental analysis was successfully applied to identify odorant compounds in fillets from Nile Tilapia before and after depuration. A higher number of odour-active substances was detected in the fish sampled before depuration. For a detailed comparison of quantitative aspects, further studies involving quantification of the aroma substances would be required. This was, however, not the aim of the present study which focuses on a non-targeted approach for the characterization of odour-active substances that might be related to aquacultural off-odours. Recirculating water in RAS may favour bacterial growth and the accumulation of the substances they produce, often including off-flavours with earthy and musty notes. Little is known about the odorant characterization of fish and the potential impact of the individual odour-active compounds on the overall aroma in the fish matrix (Jones et al., 2022). Therefore, the identification of potent aroma compounds in Nile Tilapia is described to help mitigate the prejudice of the tainting of RAS fish.
FAO, 2020. The State of World Fisheries and Aquaculture 2020.
Jones, B. C., Rocker, M. M., Keast, R. S. J., Callahan, D. L., Redmond, H. J., Smullen, R. P., & Francis, D. S. (2022). Systematic review of the odorous volatile compounds that contribute to flavour profiles of aquatic animals. Reviews in Aquaculture. https://doi.org/10.1111/raq.12657
Moretto, J. A., Freitas, P. N. N., Souza, J. P., Oliveira, T. M., Brites, I., & Pinto, E. (2022). Off-Flavors in Aquacultured Fish: Origins and Implications for Consumers. Fishes, 7(1). https://doi.org/10.3390/fishes7010034
Podduturi, R., Petersen, M. A., Vestergaard, M., Hyldig, G., & Jorgensen, N. O. G. (2021). Case study on depuration of RAS-produced pikeperch (Sander lucioperca) for removal of geosmin and other volatile organic compounds (VOCs) and its impact on sensory quality. Aquaculture, 530. https://doi.org/10.1016/j.aquaculture.2020.735754
Mahmoud, M. A. A. & Buettner A. (2017). Characterisation of aroma-active and off-odour compounds in German rainbow trout (Oncorhynchus mykiss). Part II: Case of fish meat and skin from earthen-ponds farming. http://dx.doi.org/10.1016/j.foodchem.2016.09.172
This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 956481.