Chemical composition of lipids from native and exotic fish in reservoirs of the state of Ceará, Brazil

. Current study analyzes the chemical composition of lipids in fish commonly found in the dams of the state of Ceará, Brazil, namely Pterygoplichthys pardalis (bodó), Hoplias malabaricus (traira), Cichla ocellaris (tucunaré), Prochilodus brevis (curimatã) and Oreochomis niloticus (tilapia). The animals were collected during the summer and Folch extraction procedure was used for the extraction of lipids, whilst Iupac methodology (International Union of Pure and Applied Chemistry [Iupac], 1987) was used to methylate the fatty acids. Methyl esters were analyzed by GC/MS and the different components in fish oil were identified. Palmitic acid, C16:0 (35.71-45.02%), was the saturated fatty acid with the highest percentage, while oleic acid, C18:1 Δ 9 (10.62-25.29%) had the highest percentage among the unsaturated fatty acids. The chemical composition of analyzed freshwater fish lipids revealed low levels of polyunsaturated fatty acids.


Introduction
Aquaculture is the farming of aquatic organisms.It is not merely an efficient way to produce food but it is also an economical asset.Moreover, food production must also be characterized as environmentally and socially sustainable (Dias, Simões, & Bonecker, 2012).In fact, several health benefits are attributed to the consumption of fish, recommended as a feature in a balanced diet (Domingo, Bocio, Falco & Llobet, 2007).Research from various countries suggests an inverse relationship between fish consumption and an incidence of cardiovascular diseases, increasing attention on the significance of polyunsaturated fatty acids in human nutrition (Prato & Biandolino, 2012).
Several dams have been built in the state of Ceará, Brazil, to tackle the problem of drought in the state.Since another factor linked to drought is food scarcity, pisciculture in the dams is a strategy for the utilization of this relevant government investment.Thus the introduction of native and exotic species in the reservoirs meets an urgent and important need of the local population.However, the lipid composition of native fish has not yet been evaluated.
The fishes mentioned above should be included in the diet since they have low fat and high protein rates, or rather, important sources of unsaturated fats (UFA) and polyunsaturated fats (PUFA) (Marichamy, Badhul Haq, Vignesh, Shalini & Nazar, 2012).Furthermore, it is known that the oils produced by fresh or sea water fish are an important source of long-chain PUFAs, such as C18:2 (ω-6) and C18:3 (ω-3), with reduce several risk factors associated with atherosclerosis, and improvement of retina and brain development, also decreased incidence of breast cancer and rheumatoid arthritis (Ozogul & Ozogul, 2007).Therefore, oils of various fish species have been analyzed for their protective effects.
The fatty acid profile of freshwater fish is still limited to a few species and few studies have been published highlighting storage conditions.PUFA chemical composition may vary between species and little attention has been given for this aspect when selecting fish for diets (Weaver et al., 2008).Thus, when fish are suggested for a healthier diet, their PUFA profile should be taken into account.
Current study analyzes the chemical composition of lipids of the fish species bodó (Amazon river), traíra (native), tucunaré (Amazon river), curimatã (native) and Nile tilapia (Gurgel, 1990), commonly found in the dams in the state of Ceará, Brazil, and usually included in human diet.

Material and methods
Fish were collected during the dry season (summer) in Crateús, Morada Nova and Fortaleza, Ceará, Brazil.The fisherman use small boats and leave a fishing net for a few hours until capture.After removal from the fishing net, the fish are gutted and put in ice at a ratio of 1: 2 (kg ice kg fish -1 ).When the boats come ashore, the fish are placed in plastic baskets, weighed on a scale and classified to separate those with gross changes.They are then stocked until processing.
Lipids were extracted following method by Folch, Lees and Stanley (1957), using 8 g meat samples and 10 g fish skin samples, in triplicate.Fish samples were mixed with chloroform and methanol solution (2:1 v v -1 ) in an Erlenmeyer flask and homogenized for 30 min with a magnetic stirrer.A saline solution (NaCl 1.5%) was used to better separate the phases.The organic layer with the extracted material was transferred to a 100 mL volumetric flask and the volume completed with chloroform to determine total lipids and cholesterol.
Fatty acids were methylated according to Iupac method (1987), with adaptations: 500 mg of lipid, hexane (5 mL) and 0.1 M KOH in methanol (5 mL) were mixed then added to 30 cm test tubes.The test tubes were placed in a water bath at 50°C for 1 hour.Hexane (5 mL) and 5% HCl solution (15 mL) were added and the mixture transferred to a separating funnel, where the hexane phase, containing the methyl esters, was separated, dried with Na 2 SO 4 and stored in a refrigerator until analysis by Gas Chromatography / Mass Spectrometry (GC/MS).
Chemical analysis by GC/MS of methyl esters was developed on a Shimadzu Q P-5050 instrument with a fused silica capillary column DB-1ms with dimethylpolysiloxane (30 m x 0.25 mm id x 0.25 mm); carrier gas: He (1 mL min -1 ) at constant linear velocity mode (47.4 cm s -1 ).

Results and discussion
Table 1 shows lipid percentage contents (g 100 g -1 ) of meat and skin from different fish species ranging between 1.2 and 2.6%.Total fatty acid methyl esters percentages ranged between 90.43 and 94.83%.In their studies of fish in the Amazon region, Inhamuns et al. (2009) quantified the meat lipids of the peacock bass species, between 0.8 and 2.1 g 100 g -1 during the rainy and dry season, respectively.The average variation found in the lipid fraction in typical freshwater fish, such as the tilapia, was 1.33-3.19%(Oliveira et al., 2008).Low fat was evident in freshwater fish species.
Table 2 shows fatty acid percentages identified in different fish species.The experimental retention indices (KI) of compounds were obtained by a linear regression equation, using gas chromatogram retention times (in min) and the literature KI (Kovat´s index) of main fatty acids.
Intake of ω-3 rich oils decreases triglycerides and LDL levels, whereas the incorporation of ω-3 EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid) in the muscle tissue of tilapia fed with diets supplemented with different levels of head silage shrimp Litopenaeus vannamei significantly increased EPA and DHA absorption levels in the fillet (Costa et al., 2011).High levels of palmitoleic acid C16:1Δ 9 (ω-6) are characteristic of freshwater fish.Research indicates that ω-6 increases HDL and reduces risks in Type II diabetes.Ramos Filho, Ramos, Hiane and Souza (2010) demonstrated that high levels of palmitic, palmitoleic and oleic acids have been found in fish from natural rivers of Brazil.As mentioned above, curimatã provides linoleic acid content -C18:2 Δ 9,12 (ω-6) -1.57%.
Analyses of fish samples in current study demonstrated higher contents of C-16 and C-18 fatty acids similar to other fishes, reported by Jabeen and Chaudhry (2011).SFA, UFA and C-17 fatty acids were also present, the latter being a ω-9 type FA.

Conclusion
The freshwater fishes from Ceará State reservoirs are basically formed by native species, species from Amazon River and Nile tilapia, which is very well adapted to Brazilian weather conditions.SFA with the highest percentage were palmitic acid, stearic acid and, to a lesser extent, margaric acid.Among UFA oleic acid presented highest percentage.Other fatty acids followed with lower but significant percentage such palmitoleic and octadecenoic acids (W-7).In general, the chemical composition of lipids from freshwater fish in tropical region, including the state of Ceará, provided low PUFA levels.

Table 1 .
Lipid content of different fish species in State Ceará dams.

Table 2 .
Percentage composition of fatty acids identified in freshwater fish species in dams of the state of Ceará, Brazil.