Proximate chemical composition and lipid profile of Indian mackerel fish

Indian mackerel (Rastrelliger kanagurta) belongs to the family Scombridae and is commonly found in the coastal areas of the Indian Ocean. Mackerel fish are also found in the waters of the Red Sea and the Arabian Gulf and are abundant in summer. Mackerel arrives in India and forms the basis of pelagic fisheries after sardines, representing 7.2% of total marine fish (Sofi et al., 2016). Indian mackerel is the most available and economic fish sold in markets in different countries and is included in many local dishes. Indian mackerel is characterized by a blue-green color on the back, silver flanks and a golden tint. The size of Indian mackerel varies in length, reaching 21.7 cm at 8.6 months old. Additionally, Indian mackerel are characterized by dark muscles because of the high contents of myoglobin and lipids. Mackerel is a fatty fish and a good source of marine lipids; thus, increasing the consumption of this fish is good for human health (Abdussamad et al., 2010; Chaijan et al., 2004). The high levels of amino acids, fatty acids, minerals and vitamins in Indian mackerel play a positive role in human nutrition (Viji et al., 2015; Corapci & Guneri, 2020).


Introduction
Some of the most available protein sources for human beings of fish and fish products, which represent approximately 80% of animal protein. There is strong evidence that fish are rich sources of vitamins such as B12 and B6 and minerals, especially iodine and fluorine, which are required for the prevention of goiter and promote strong teeth and bones (Håstein et al., 2006).
Indian mackerel (Rastrelliger kanagurta) belongs to the family Scombridae and is commonly found in the coastal areas of the Indian Ocean. Mackerel fish are also found in the waters of the Red Sea and the Arabian Gulf and are abundant in summer. Mackerel arrives in India and forms the basis of pelagic fisheries after sardines, representing 7.2% of total marine fish (Sofi et al., 2016). Indian mackerel is the most available and economic fish sold in markets in different countries and is included in many local dishes. Indian mackerel is characterized by a blue-green color on the back, silver flanks and a golden tint. The size of Indian mackerel varies in length, reaching 21.7 cm at 8.6 months old. Additionally, Indian mackerel are characterized by dark muscles because of the high contents of myoglobin and lipids. Mackerel is a fatty fish and a good source of marine lipids; thus, increasing the consumption of this fish is good for human health (Abdussamad et al., 2010;Chaijan et al., 2004). The high levels of amino acids, fatty acids, minerals and vitamins in Indian mackerel play a positive role in human nutrition (Viji et al., 2015;Corapci & Guneri, 2020).
The objective of this study was to investigate the chemical composition, saturated and unsaturated fatty acid contents, and fatty acid fractions in Indian mackerel muscle and to compare the obtained results with previously available data.

Collection and preparation of Indian mackerel samples
Indian mackerel (R. kanagurta), 17 ± 3 cm in length, were purchased from the central fish market in Jizan, which is caught from the port of Jizan, Saudi Arabia. Fish samples were transported to the laboratory in an ice box as soon as possible and were analyzed within 24 hours of catching them.
Fish were prepared by cutting the samples using sterile cutters and homogenizing obtain three samples of Indian mackerel muscle for chemical analysis, Lipid Fractions and Fatty Acids analyses.

Chemical analysis
Moisture, ash, protein, and fat were estimated in Indian mackerel by the methods described by the Association of Official Analytical Chemists (2012). All analyzes were performed in three iterations. The carbohydrate content was calculated as follows (Mathew et al., 2014

Determination of lipid fractions and fatty acids analysis
Fatty acids were obtained from total lipids by acid-catalyzed transesterification according to (Christie & Han, 2012). amount of 100 mg of the extracted lipid was dissolved in 1.5 mL dichloromethane and 3 mL methanol-sulfuric solution (200:3 at a volume ratio) and shaken vigorously. The reaction mixture was heated at 100 °C for approximately 1 h and then cooled to room temperature. Subsequently, 3 mL hexyl hydride and 1 mL distilled water were added to the mixture. After vortexing and layering, the entire upper phase was collected. One gram of anhydrous sodium sulfate was added to the collected sample and then stewed for overnight stratification. After filtration, the sample was evaporated until its weight turned to be constant under a stream of nitrogen. Heptane was added to the sample to make up a final concentration of 50 mg mL -1 . The qualitative analysis of Fatty acids was by column chromatography (CC MSQP5050A System: model 2001, Shimadzu, Japan; autosampler, Shimadzu, Japan; serial number, C111437075). The quantity of each fatty acid was determined by calculating the peak area relative to the total area of the peak. These values are indicated by fatty acid content (%).

Statistical analysis
Data are presented as the mean ± standard deviation (n=3) and were analyzed by one-way analysis of variance (ANOVA) using Minitab 17 software. The significance level was p < 0.05.

Proximate chemical composition
The average chemical composition of the tested Indian mackerel samples purchased from different markets in March was investigated and is shown in Table 1. The average moisture, protein, fat, ash and carbohydrate contents were 74.33 ± 0.39, 20.89 ± 0.07, 3.23 ± 0.26, 1.26 ± 0.14, and 0.29 ± 0.14 g/100 g, respectively.

Composition of lipids
The obtained results in Table 2 and Figure 1 show that the tested Indian mackerel contained high levels of saturated fatty acids, reaching 27.01 ± 0.07 mg/100 g, followed by a similar content of unsaturated fatty acids, which were present at a level of 26.02 ± 0.07 mg/100 g. Unsaturated fatty acids are composed of polyunsaturated fatty acids and monounsaturated fatty acids; the level of polyunsaturated fatty acids was higher (22.01 ± 0.06 mg/100 g) than that of monounsaturated fatty acids (4.01 ± 0.01 mg/100 g). The content of trans fatty acids was 1.001 ± 0.01 mg/100 g in the tested Indian mackerel muscle.
These outcomes agree with the conclusions of Tzikas et al. (2007) and Sone et al. (2019), who recorded that the moisture, protein, fat and ash contents of mackerel were 76.8 ± 1.39%, 20.3 ± 0.68%, 1.3 ± 1.08% and 1.5 ± 0.08%, respectively, throughout the year. The fat content of the muscle increased by 2.5%, 2.8% and 2.1% in March, April, and May, respectively, compared with that in February, with a decrease in moisture content to below 75.9%. In contrast, in September and October, a low-fat content (0.4% and 0.6%, respectively) and high moisture content, reaching 78.2% and 77.6%, respectively, were observed.
It has been reported that fish lipids are mainly composed of polyunsaturated fatty acids, especially docosahexaenoic and eicosapentaenoic acids, which can prevent and treat rheumatoid arthritis, coronary heart disease, diabetes, and cancers. Fatty acids play a positive role in osmoregulation, nutrient assimilation, and transport via the cell membrane (Ackman, 1982;Osman et al., 2001). The lipid content results from this study agreed with the results of Johnston et al. (1994), as the lipid content was low in lean, starved fatty fish (0.5%) and increased to above 20% in well-fed, fatty fish. Additionally, Wickramanayake et al. (1989), Nisa & Asadullah (2012), Kumar et al. (2014) and Bahurmiz et al. (2017) recorded that lipids accounted for 0.7% to 12.0% of the muscle content of mackerel caught in the Arabian Sea.
The obtained composition results of Indian mackerel showed predominantly saturated fatty acids followed by unsaturated fatty acids, both of which were present in a similar range. Unsaturated fatty acids are composed mainly of polyunsaturated fatty acids, which agrees with many studies, including those of Marichamy et al. (2009), Ganga et al. (2010 and Azim et al. (2018). Additionally, in a study on Atlantic mackerel, the fat content increased with an expected increase in food consumption and varied among years, seasons and geographic areas (catching grounds) in addition to individual variations in weight gain, as a higher total fat content is related to the feeding period and the size of the mackerel (Romotowska et al., 2016(Romotowska et al., , 2017Calixto et al., 2020).
The obtained fatty acid fractionation results of Indian mackerel contradict those of previous studies of different species, seasons, diets, and environmental conditions (Bahurmiz & Ng, 2007;Hong et al., 2015;O'Neill et al., 2015;Wijekoon et al., 2014;Costa et al., 2020). It is well documented that docosahexaenoic acid and palmitic acid are the main fatty acids in aquatic fish (Sahena et al., 2009). Palmitic acid is needed for biosynthesis processes in fish and may play a role as a structural component of phospholipids in cell membranes (Pérez et al., 1999;Nascimento et al., 2020;Marques et al., 2020).
It has also been suggested that fish are rich in polyunsaturated fatty acids, such as eicosapentaenoic acid and docosahexaenoic acid, and have desirable components of a nutritious diet. Polyunsaturated fatty acids affect prostaglandin synthesis and hence encourage wound healing. ω-3 and ω-6 polyunsaturated fatty acids protect

Discussion
The obtained proximate analysis results of Indian mackerel are excellent indicators of fish quality. Ahigh moisture content increases fish susceptibility to microbial spoilage and lowers its shelf life (Clucas, 1982;Oliveira et al., 2020). The moisture contents obtained in this study agreed with the results of Nurnadia et al. (2011) and Gokoglu & Yerlikaya (2015), who observed 70-80% moisture in fish muscle. Ali et al. (2013) also reported 72.8-76.6% moisture in Indian mackerel from Oman.
It is known that fish protein is of good quality, containing a good amount of essential amino acids, and is highly digestible and a major source of energy (Pigott & Tucker, 1990). The results of the protein analysis of Indian mackerel were like the results of Bandarra et al. (2001), who reported 18.3 to 19.9% protein in horse mackerel. In agreement with our findings, Nisa & Asadullah (2012), Ali et al. (2013), Kumar et al. (2014) and Bahurmiz et al. (2017)    against cardiovascular diseases and cancer. Additionally, these fatty acids play a vital role in nervous, vision and reproductive system development and functions (Agusa et al., 2007;Luzia et al., 2003). Therefore, Indian mackerel is a rich source of protein and lipids. The Indian mackerel contained high and comparable levels of saturated fatty acids and unsaturated fatty acids, and the different samples exhibited somewhat similar fatty acid contents.

Conclusion
The present study revealed that Indian mackerel is a rich source of protein and lipids. Polyunsaturated fatty acids are the main constituents of lipids. The lipid profile of Indian mackerel showed high levels of palmitoleic acid, palmitic acid, myristic acid, oleic acid, 15-tetracosenoic acid, linolenic acid, and stearic acid, which exert positive effects on the body by providing the main nutrients and preventing disease in the consumer.

Recommendations
The obtained results showed favorable dietary components, mainly protein and lipids, in Indian mackerel and highlighted the importance of the lipid profile on the health of consumers and the application of different technological processes to avoid rancidity in fish. Further studies should be conducted on the amino acid, vitamin, and mineral contents of Indian mackerel in different seasons and catching areas.

Significance statement
This study estimates the proximate chemical composition, mainly those of protein and lipids of Indian mackerel and highlights the importance of the lipid profile to the health of consumers. This study will help researchers determine other nutrients, such as amino acids, vitamins, and minerals, in Indian mackerel and their differences between different seasons and catching areas, which many researchers have not been able to explore thus far. Thus, new theories on micronutrient combinations in Indian mackerel may be developed.