# Off-flavors in Aguacultured Fish: Origins and Implications for Consumers
## 4 Transfer of Off-Flavor Compounds, Bioaccumulation in Fish, and Toxicity  
Off-flavors in foods may derive from environmental pollutants, the growth of microorganisms or algae, oxidation of lipids, or endogenous enzymatic decomposition in foods. Additionally, these compounds can be absorbed and accumulated in fish tissues [61]. According to [PERSON] et al. [62], earthy flavors in fish grown in tanks with phytoplankton are common, suggesting that fish can absorb off-flavors, such as GEO and MIB not only by osmosis but also by feeding on the biomass of phytoplankton (cyanobacteria). Recent studies have also shown that off-flavors were present in the stomach and intestinal mucous layer of fish, suggesting that fish can actively feed on potential off-flavor-producing organisms [23; 31; 63]. The accumulation of off-flavors may vary due to different feeding habitats. According to [PERSON] et al. [64], insignificant levels of MIB were found \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Target Gene** & **Primer** & **Sequence 5’-5’** & **Product Length** & **Reference** \\ \hline \multirow{4}{*}{_geo_A_} & geo78F & GCATTCCAAAAGCCCTGGCTTA & \multirow{2}{*}{912 pb} & [20] \\ \cline{2-4}  & geo971R & CCCCTGTTCATGATRCGGC & & \\ \cline{2-4}  & geo982R & ATGCCATGTGCCACTCGTGC & & \\ \hline \multirow{4}{*}{MIB synthase} & MIB3313F & CTCTACTGCCCCATTACCGGAGGGA & \multirow{4}{*}{913 pb} & [60] \\ \cline{2-4}  & MIB4226R & GCCATTCAAACCCCGCCGCCATCCA & & \\ \cline{2-4}  & MIB3324F & CATATCCGACGATTCAACAGAC & & \\ \cline{2-4}  & MIB4050R & CCCCAAATTGTGTACACACCATGTTGA & & \\ \hline \multirow{4}{*}{16S rRNA} & 27F & AGAGTTGATCMTGGCTCAG & \multirow{4}{*}{850 pb} & [61] \\ \cline{2-4}  & 1492R & TACGYTACCTGTTTACGATT & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Primer pairs were used for PCR and sequencing of _geo_A, MIB synthase, and 16S rRNA genes to assess concentrations of off-flavor compounds. in five important species cultivated in Hungarian aquaculture: the planktivorous silver carp (_Hypophthalmichthys molitrix_), the herbivorous grass carp (_Ctenopharyngodon idella_), the omnivorous common carp (_Cyprinus carpio_), the omnivorous Nile tilapia (_Oreochromis niloticus_) and the carnivorous African catfish (_Clarias gariepinus_), however the highest concentrations of GSM were found in the bottom-feeding common carp fillet, being higher than in the silver carp or African catfish. Furthermore, TOCs can be absorbed and accumulated in lipid-rich fish tissues, since GEO and MIB have lipophilic properties, and due to this characteristic, tend to accumulate in fishes with a higher fat content [65]. In fishes, it is assumed that the gills and skin are the main sites of GEO and MIB on uptake due to the contact with water with off-flavors [65]. The absorption of GEO through the skin, intestine, and stomach has also been proposed as an alternative source. Studies through sensory analysis have shown that the absorption and digestion of cyanobacteria containing GEO could result in the accumulation of GEO in fish flesh [63; 66; 67]. [PERSON] et al. [23] described the high abundance of GEO-producing microorganisms in the intestinal mucous and also digesta, suggesting that the digestive system in fish is a relevant source of GEO and probably other off-flavours in fish that until now, has been neglected. While GEO and MIB are absorbed and accumulate in fish quickly, compound clearance is a slow process that takes several days to reach a level below human perception; for example, sensory thresholds of 0.9 \(\upmu\)g/kg for GEO and 0.7 \(\upmu\)g/kg for MIB in rainbow trout have been suggested [67; 68; 69]. The toxicity of TOCs is frequently found in experiments with human cells and most results show low effects. According to [PERSON] et al. [70], only 2-MIB and GEO concentrations above 100 and 75 \(\upmu\)g/mL, respectively, were cytotoxic to HepG2 cells (human liver cells). Additionally, the concentrations studied were not able to induce DNA damage.