# Production and Refinement of Omega-3 Rich Oils from Processing By-Products of Farmed Fish Species
## 3 Results and Discussion  
### Chemical Characterization and Oxidative Stability of Fish Oils  
The results of the chemical and compositional quality characterization of oils from tuna by-products, tuna liver, seabass and gilhead seabream by-products during four refining stages and their comparison to cod liver oil (control) are presented in Table 1. Despite the high temperature used during the oil production (cooking at 95 \({}^{\circ}\)C for 12 min) the temperature was found to have a poor influence on oil quality parameters. During preliminary studies we inspected quality parameters in oils extracted at different temperatures (from 65-95 \({}^{\circ}\)C) and compared it to Bligh and Dyer extracts [20] and found no significant difference in oxidative parameters [21]. It has been previously found that oil extraction temperature is weakly linked to the oxidative quality of produced oil, but strongly affected by omega-3 content of the raw material, providing confirmation of our finding [22]. To control the quality of fish oil properties, which is very labile to hydrolytic spoilage and oxidative deterioration, numerous standards with variable acceptable levels have been established [23]. The express lipolysis and oxidation of fish oils are the results of high autolytic activity and high content of PUFAs in fish tissues. It is expected that this process is even more susceptible for fish by-products. For this reason, fish oils usually have high free fatty acid (FFA) content. In this study, the results of FFA values of crude oils were low confirming that short cooking periods during the oil extraction, even at higher temperatures, did not cause significant hydrolysis. Only in tuna liver oil the FFA values were over 3. The refinement process ensured an additional decrease of FFA by 3.7, 32 and 47% in tuna by-product, tuna liver and seabass/seabream oil, respectively. Among the three studied oils, seabass and gilhead seabream by-product oil had the lowest FFA values, even lower than the control. The allowable limit of FFAs value for crude fish oil is in the range of 1-7% of oleic acid, usually 2-5% [24], but the general recommendation is that FFA values of edible oils should be \(\leq\)3.0%. This is important since FFAs have an impact on the oil organoleptic properties as well as oil compositional quality [25], can act as pro-oxidants which initiate the oxidation mechanism [26] and high FFA values are problematic during omega-3 extraction and biodiesel production [27]. Primary oxidation of oil used to monitor hydroperoxides formation is determined by PV and should be \(\leq\)5 meg O2/kg for fish oils intended for human consumption [28; 29]. Despite the increase of the PVs for studied oils after degumming step, final values obtained for the refined oils were below the limit of 5 meq O2/kg (Table 1) which opens the possibility of using these oils for human consumption. The influences of the refining steps on oil oxidation status and products expressed as \(p\)-AV, TOTOX and TBARS are presented in Table 1. Among crude oils, \(p\)-AV and TOTOX values for tuna by-product oil were the highest, followed by those for tuna liver oil and seagrass and gilthead seabream by-product oil. The processing steps of degumming, neutralizing and bleaching caused reduction of \(p\)-AV and TOTOX while these parameters increased after the deodorization step. The highest \(p\)-AV value was detected for refined tuna by-product oil (19.5), while TOTOX values of refined tuna liver and seagrass and gilthead seabream by-product oils were higher than those obtained for crude oils, 27.7 and 24.7, respectively. This suggests that used adsorbents (activated carbon and Fuller's earth) have the capacity to adsorb primary and secondary oxidation compounds [14]. The allowable limit of \(p\)-AV for acceptability of fish oil for human consumption is \(\leq\)20 [30]. Although the \(p\)-AV obtained for oils in this study were under this limit, they were significantly higher than the control, thus additionally influencing the higher TOTOX value.