Evaluation of qualitative indicators of oil of rainbow trout waste and compared to common oils (sunflower oil and Kilka oil) in aquatic feed and human food

The rainbow trout Oncorhynchos mykiss is one of the most desirable farmed fish. The aquaculture of this fish is 811 thousand tons (FAO 2019), and about 167 thousand tons in Iran (statistics of the Fisheries Organization of Iran,1397) which has led to the production of a large volume of waste. Fish waste includes head, skin, tail, fins, intestines, and intestines that have been shown to have oil extraction potential (Ramakrishnan et al., 2013). Fish oil is a rich source of omega-3 fatty acids, especially docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), which are recommended for the possible treatment of many diseases and disorders, especially cardiovascular disease (Yue et al., 2008; Asnaashari et al., 2019). Because polyunsaturated fatty acids are not produced in the body, they must be supplied through food (Hao et al., 2015). Therefore, the use of fish oil in food and animal feed are necessary. Kilka oil is also a by-product of fish powder companies and is used in fish and poultry feedstuff (Ghasemi et al., 2014). Kika oil is a by-product of fish powder factories. Despite its high price and nutritional value, Kilka oil is very sensitive to oxidation due to its abundance of unsaturated fatty acids (Pirestani et al. 2010; Fazel et al. 2008). Due to the above, many efforts have been made to replace fish oil with other oils. Sunflower oil is one of the most common oils for human consumption. Due to the combination of fatty acids, sunflower oil has used for fish oil substitution (Dalbier et al. 2015). Previous studies have shown that rainbow trout oil has a considerable value due to its taste and color, availability and low pollution. On the other hand, due to the expansion of aquaculture products and processing industries, the oil extracted from this source will have a reasonable price. In this study, an attempt is made to compare the quality of rainbow trout waste oil with sunflower and Kilka. If the quality of the waste oil is equal to any of the mentioned oils, its use for human consumption or animal feed will be recommended.

In this study, Kilka oil was prepared from Qarahboron sturgeon farm (Mazandaran-Sari) and sunflower oil was purchased from a food store. The residual oil of rainbow trout was extracted by solvent according to Vafaei et al. (1396). In brief, about 100 g of fish waste was homogenized in a mixture of 400 ml chloroform and %2 methanol. The mixture was filtered via a filter paper and solvent was removed using rotary evaporator (Vacuum Rotary, Fara-Azma, Iran) at 40 ºC and kept refrigerated until analyses in a dark screw topped bottle. Peroxide value (PV) was determined according to AOCS (Cd 8-53), in which PV is reported as milliequivalents of active oxygen per kg oil (AOCS, 1995). Free fatty acids (FFAs) content was assessed following AOCS (Ca 5a - 406) (AOCS, 1995) and expressed as percent of oleic acid. Anisidine value (AV) was determined using AOCS (Cd 18-90) (AOCS, 1995). Color properties of the extracted oil was evaluated in a smart colorimeter (ATEBA, MAT 2000, Shiraz, Iran) based on Lab system, in which L* denotes lightness, a* was the representative of redness or greenness and b* indicated yellowness or blueness of the oil. The color index L * ranges from zero to 100 and a * and b * are values ranging from -20 to + 120. 9 points were randomly selected and the L*, a*, and b* were recorded.

Fish are classified into three groups based on their fat: low-fat, medium-fat, and high-fat fish (Strateva and Penchev 2020). According to studies, the yield of oil extraction in this was % 16.16, which was consistent with the study of Vafa et al. (1397). Therefore, the residual waste of rainbow trout, as a high-fat fish, have a significant amount of oil that can be extracted and used for food and feed. The permissible limit of PV for refined edible oils consumed by humans is less than 5 and unrefined oils less than 10 (meqO2/kg oil) have been reported (Codex, 1993). The significant difference in PV in different treatments with the standards was analyzed by One-sample t-test. Based on the results of the analysis, the amount of PV in Kilka oil was more than the standards for human and animal consumption, while they were lower in waste oil and sunflower (Fig. 1). In the process of refining the oil, all impurities and harmful compounds will be separated during neutralization, discoloration, and deodorization in order to achieve optimal quality. In this study, no purification process was applied to the oil extracted from fish waste. It seems that if this process is industrialized, along with the application of refining and purification processes, the quality of the product will be superior to the present sample. The results of TBA was the same as the PV index. In all three oils, TBA levels were lower than standard. Unlike the previous two indices, the amount of FFAs in waste oil was significantly higher than the other two samples. The presence of lipase and phospholipase enzymes is one of the most important reasons for the increase of FFAs. The process of refining oil will reduce the FFAs index in waste oil. Figure 5 shows the results for the browning, yellowness and whiteness index of oils. The index of browning in Kilka oil was significantly higher than sunflower oil and waste oil. Oxidation reactions, Maillard, and compounds resulting from the reaction of secondary products of fat spoilage with free amines will cause discoloration and darkening of the oil color (Sabzipour et al., 2019). The results of peroxide and thiobarbituric acid tests also confirmed that in Kilka oil.

Comparison of the quality of the Kilka oil, sunflower oil and rainbow trout waste oil had interesting results. Waste oil can be considered as a new and serious option for aquaculture industries due to its ease of extracting and remarkable quality. Due to the high potential of the country in order to rainbow trout aquaculture and consequently the high volume of waste (which is considered an environmental threat), the waste oil could not only be used in animal feed but also could be used in human’s food.