# Off-flavors in Aguacultured Fish: Origins and Implications for Consumers
## 5 Depuration of Off-flavors in Fish  
Numerous processes have been studied and tested to eliminate or reduce the formation of GEO, MIB, and other TOCs in RAS, as well as ozonation [71], advanced oxidation processes (AOPs) [72], algicides [73], adsorption, i.e., with activated carbon [69], zeolites [74], and ultrasonic methods. According to [PERSON] et al. [75], ultrasound-induced cavitation significantly reduces TOCs in tap water and both freshwater RAS and saltwater RAS. However, it is still necessary to perform studies of this treatment in RAS to assess its economic and technical viability. Photocatalysis-based methods, such as modified TiO\({}_{2}\) with sunlight and palladium-modified tungsten trioxide photocatalyst, have been studied [76; 77]. This technique uses degradation by means of light (oxidation/reduction), which is caused by the activation of a catalyst resulting from ultraviolet (UV) or visible radiation to which it has been subjected. Currently, this method is considered promising, since it has been used to remove GEO and MIB in drinking water [78]. However, the implementation of this technique on a full scale in RAS is still unfeasible due to the impossibility of carrying out the process with a batch reactor and the high cost of the process [79]. Despite the diversity of methods presented, depurating using clean water is still the most effective process available to eliminate off-flavors [65; 80]. This occurs because it is a relatively simple and highly effective method, based on the idea that with the circulating aqueous environment and a concentration gradient, the bioaccumulated compounds will be diffused freely out of the fish [68; 81]. Depuration can take from days to weeks, depending on several factors, such as preliminary concentrations of GEO and MIB, the volume of water available in the process, the species, and the size of the fish [69], in addition to other factors such as water temperature [81; 82; 83; 84]. According to [PERSON] [85], the concentration of MIB and GEO in the dorsal and ventral tissue of Japanese sea bass (_Lateolabrax japonicus_) decreased by 50% after 10 days of deprivation. The lipid content and weight of fish are directly correlated with the ability to absorb off-flavor compounds such as GEO and MIB, since these compounds are lipophilic [81]. During the deprivation process, fish are not fed, ensuring a good water quality and having the most effective deprivation possible, which can lead to significant weight loss and generate financial losses [80]. Therefore, it is essential that the deprivation is performed as quickly and efficiently as possible. According to an experiment carried out by [PERSON] et al. [85], the feeding of European whitefish (_Coregonus lavaretus_) during the process of deprivation of TOCs resulted in a significant increase in the rate of elimination of GEO from the ovary--this being a more rapid elimination compared to hungry fish. In addition, the retention of food during long periods of purification causes a reduction of fish weight, lipid content, and color of the fillet, resulting in lower quality products [69]. Lipophilic compounds such as off-flavors are eliminated from fish by passive diffusion through the gills and/or skin, or by metabolizing these compounds into more polar compounds that are excreted in the urine (via the kidneys) or the feces (secreted in bile--gallbladder) [86; 87]. Pharmacokine studies of MIB in catfish have shown a rapid total clearance of the compound [82; 83], suggesting that urinary excretion is responsible for only a minor portion of total loss and elimination through the gills or rapid biotransformation with subsequent clearance of metabolites. Sensory analyzes showed that approximately 6 days of purification in clean water were required for the GEO concentrations to decrease below the concentration of the sensory threshold, going from 90 \(\upmu\)g/kg to 8 \(\upmu\)g/kg in channel catfish [83; 88].