Introduction
The presence of water-borne off-odour compounds in fish, such as geosmin and 2-methylisoborneol (Abd El-Hack et al., 2022), or less commonly reported feed-borne compounds, such as beta-ionone, butanoic acid, or indole (Podduturi et al, 2017; Mahmoud et al., 2018), can drastically reduce its palatability, leading to consumer rejection and lower market demand of recirculating aquaculture systems (RAS) products. A common off-flavour mitigation strategy is to purge the fish in flow-through systems prior to harvesting while ceasing feeding. However, this procedure results in the loss of fat content and precious market weight, as well as in a far greater consumption of water. Other technological strategies utilise intense ozone and ultraviolet (UV) light, which further increase costs in RAS farms (Abd El-Hack et al., 2022). In one recent study, feeding was observed to increase excretion rate of geosmin in Nile Tilapia (Schram et al., 2021), with important implication on reducing the negative impacts of standard off-flavour purging methods. The aim of this study was to isolate the effect of water born vs feed born off-odour odours in juvenile male Russian sturgeon (A. gueldensteadtii) reared in RAS as well as to understand the effect of feeding during standard purging practices.
Methods
A commercial scale experiment was designed to trial four groups of test fish: two groups reared in clean flow through (FT) water and two in RAS water. In each water type, one group of animals was starved for 3 weeks and one fed at standard feeding regime with a commercial feed previously in use. After 3 weeks in experimental conditions, three animals were sampled per test group, euthanised, filleted and frozen at -20°C. Subsequently, the dominant off-odour compounds in the two water sources, in the commercial feed and in the fish fillets of each test group were determined via olfactometric detection of single odour-active volatile compounds coupled with instrumental analytical methods. Aroma distillates were obtained from fish fillets by using solvent assisted flavour evaporation (SAFE), the dominant off-odorants ̶ i.e., exhibiting higher sensory detection thresholds and concentrations ̶ were determined by aroma extraction dilution analysis (AEDA) and finally identified by gas chromatography-mass spectrometry/olfactometry (GC-MS/O) as described by Mahmoud et al., (2016).
Results
Dominant off-odour compounds present in the fish fillets were sourced from both commercial feeds and water types. RAS water was found to be more diverse in off-odour compared to FT water, and ubiquitous dominant off-odours to be detectable with higher AEDA dilution factors (FD) in the RAS water. Geosmin and 2-MIB were present in both water types yet reaching higher FD in RAS water. The dominant off odours present in the commercial feed comprised feed-borne compounds previously published (see Podduturi et al, 2017; Mahmoud et al., 2018), being dominated by butanoic acid (“cheesy, sweaty)”, methyl butanoic acid (“sweaty”), thymol (“thyme-like”), heptanoic acid (“pungent, sour, wax-like”) and 4-hydroxyphenyl acetaldehyde (“horse stable-like) notes. Juvenile male Russian sturgeons reared in RAS water were more diverse in dominant off-odour compounds compared to those reared in FT water. Geosmin and 2-MIB were observed in all four test groups with different FD factors, being in general higher in RAS water. Within each water type, feeding was observed to reduce the FD levels of highly dominant odour compounds, being a strong indication of reduced concentrations, while increasing on the other hand the overall diversity of the odorants, albeit all with relatively low FD factors. Accordingly, feed-borne off-odour compounds were absorbed into fish fillets either by ingestion or by excretion-reabsorption via respiration from RAS water, yet these substances were not detectable in the group starving in FT water. Of the four test groups, the animals fed in FT water yielded the least concentrated dominant off-odours and the highest organoleptic palatability.
Points of Discussion
In line with the results published by Schram et al. (2021), this study confirms that feeding during standard RAS off-odours purging practices in FT water helps reducing the levels of highly dominant off-odour compounds, such as geosmin. Therefore, feeding during purging should be taken into consideration by aquaculture farmers to reduce the economic loss of standard purging practices. In general, we are convinced that the lack of understanding of the biochemical mechanisms improving off-flavour depuration during feeding should receive greater attention in future research. At the same time, the uptake of highly unpalatable feed-borne odorants in fish fillets is confirmed in this study, in accordance with findings of previous studies. As feed-born volatile organic compounds are obviously absorbed into fish fillets, this source of off-odours should be addressed by feed manufacturer in the future, and the potential for flavour modulation should be considered. Finally, the possibility to create “purging” fish feed formulas resulting in highly palatable fish products from RAS is a promising strategy as demonstrated by this study.
References
Abd El-Hack M. E., El-Saadony M. T, Elbestawy A. R., Ellakany H. F., Abaza S. S., Geneedy A. M, Salem H. M., Taha A.E., Swelum A. A., Omer F. A., AbuQamar S.F., El-Tarabily K. A., (2022). Undesirable odour substances (geosmin and 2-methylisoborneol) in water environment: Sources, impacts and removal strategies, Marine Pollution Bulletin, Volume 178, 2022 113579, ISSN 0025-326X, https://doi.org/10.1016/j.marpolbul.2022.113579.
Mahmoud MAA, Tybussek T, Loos HM, Wagenstaller M and Buettner A (2018). Odorants in Fish Feeds: A Potential Source of Malodors in Aquaculture. Front. Chem. 6:241. doi: 10.3389/fchem.2018.00241
Podduturi R., Petersen M. A, Mahmud S., Rahman Md. M, and Jørgensen N.O. G., (2017). Potential Contribution of Fish Feed and Phytoplankton to the Content of Volatile Terpenes in Cultured Pangasius (Pangasianodon hypophthalmus) and Tilapia (Oreochromis niloticus). Journal of Agricultural and Food Chemistry 2017 65 (18), 3730-3736. DOI: 10.1021/acs.jafc.7b00497
Schram, E., Kwadijk, C.J.A., Hofman, A., Blanco, A., Murk, A.J., Verreth, J.A.J., Schrama, J.W., 2021. Effect of feeding during off-flavour depuration on geosmin excretion by Nile tilapia (Oreochromis niloticus). Aquaculture 531 (2021) 735883 https://doi.org/10.1016/j.aquaculture.2020.735883
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.