## 4 Discussion
### Oil Extraction  
The high-temperature extraction is the most analogous to the traditional extraction method. This was even more effective when compared to the cold extraction method. This is due to the higher temperature, allowing for the mechanical separation of the solid and liquid fractions, the rupture of adipocyte tissue, and the subsequent release of the oils in the liquid phase. The use of enzymes such as pepsin allows for the breakdown of the tissues at lower temperatures; this method seems to be more effective in the liberation of the oil globules in the adipocyte tissue [52; 53].  
Much like the collagen extraction process, limitations regarding the maceration and filtration of the material are also a very considerable bottleneck in the retrieval of this material. Time-related issues may be more problematic for lipid extraction when compared to collagen extraction as processes occurring at room temperature or higher for a prolongedperiod may influence the quality of the final product considerably. This means that for larger-scale procedures, more efficient and temperature-controlled procedures may need to be implemented in order to produce products of the highest quality. TOTOX values indicated that samples treated with high temperature exhibited total oxidation values that were considerably higher than those at lower temperatures (CT and PE). Samples extracted from assorted muscle also exhibited higher TOTOX values; this may be indicative of accelerated oxidation of the lipids due to the presence of natural components present in muscle and viscera, especially iron, myoglobin, hydrogen peroxide, and ascorbic acid, which are known to accelerate lipid oxidation. These act as catalysts that induce the formation of reactive oxygen species [54].  
The Global Organization for EPA and DHA Omega-3s (GOED) gives a global target for the TOTOX value that unflavored fish oils must have. It indicates that a TOTOX value exceeding 26 is unsuitable. From the analysis of the oils obtained, only Mix-CT and SO-PE fell within these parameters, whilst the others exceeded it. Mix HT and AM PE/WT were by far the highest in TOTOX value; this indicates that muscle and viscera content, coupled with high-temperature extraction, considerably affected the quality of the oil [54].  
Oils obtained using muscle tissue (both thermally and non-thermally extracted) have been found to have a high concentration of both primary oxidation and secondary oxidation products. This is mainly attributed to the presence of a higher concentration of iron (as hemoglobin) present within the muscle tissue. The results obtained are consistent with the literature, given that the major factors affecting the rate of oxidation are the degree of unsaturation, the amount of oxygen, temperature and light, and the presence of metals (mainly transition metals such as Fe and Cu) [55; 56; 57]. [PERSON] and [PERSON] demonstrated that trace amounts of Fe, Cu, Cr, Pb, and Cd contribute oxidative effects to lipid peroxidation [58].  
The presence of these trace metals enhanced the rate of oxidation of edible oils by increasing the generation of free radicals from fatty acids and hydroperoxides. On the other hand, it was found that thermal extracted oils derived from skin had a lower degree of oxidation; this was mainly attributed to the lack of iron and other hydrolytic enzymes that might be present in the viscera used in the general waste. This could be due to several reasons; however, a number of studies have shown that fish skin contains a significant antioxidant effect, derived from the presence of proteins shown in studies carried out on Alaska pollack skin gelatin hydrolysate [59], yellowfin (_Thunnus albacares_) fish protein [60], and snakehead fish skin [61].