Fish and seafood are a source of good quality protein which is essential for health (Kim and Lall, 2000). Fish is usually cooked in different ways such as boiling, smoking, baking, frying, and grilling. These cooking methods result in enhancing flavor, taste, and improve the digestibility and inactivate the pathogenic microorganisms and enzymes (Kocatepe et al., 2011).

During cooking of fish, some chemical and physical reactions take place such as protein denaturation that increases its digestibility and improves the nutritional value. However, the contents of fat-soluble vitamins or polyunsaturated fatty acids are often reduced (Alizade et al., 2009).

Deep Fat Frying (DFF) is a major cooking method and is considered to be one of the oldest methods of food preparation. It is a cooking method of immersing foods in hot oil at a temperature above the boiling point of water. The oil temperature usually varies from 130 to 200 °C. During frying, there are many chemical reactions such as browning, gelatinization, and denaturation due to the elevated temperature of the product (Tangduangdee, Bhumiratana, and Tia, 2003).

Pampus argenteus (White pomfret) is one of the most preferred indigenous aquaculture species in Iran due to better growth. Nutritional information is only available for fresh fish (Figure 1). The study was carried out to determine nutritional composition (protein, fat, ash, moisture, and energetic values) of brined, boiled, and fried Pampus argenteus.

These cooking methods result in enhancing organoleptic properties and improve the digestibility and inactivate the pathogenic microorganisms (Kocatepe et al., 2011). Method cooking of fish, lead to some chemical and physical reactions take place such as protein denaturation that increases its digestibility and improves the nutritional value. Meanwhile, the contents of fat-soluble vitamins or polyunsaturated fatty acids are often reduced (Alizade et al., 2009). Deep fat frying is a major cooking method and is mentioned as one of the old methods of food preparation. It is a cooking method of immersing foods in plant hot oil at a temperature above the boiling point of water. The oil temperature usually is around 180°C. During frying, there are many chemical reactions that take place such as browning, gelatinization, and denaturation due to the elevated temperature of the product (Tangduangdee, et al. 2003).

Salting is the oldest, applied, and commonly used processing technique for fish preservation all over the world because of the simplicity of the process and low production cost (Martínez-Alvarez and Gómez-Guillén, 2013). Salt is effective as a preservative compound because it reduces the water activity of fish fillets, consequently, microorganism growth and enzymatic spoilage are inhibited. Also, the aim of salted fish is to improve sensory properties more than preservation (Mujaffar and Sankat, 2005).

Results for proximate composition analysis (protein, fat, moisture, and ash) for fresh, boiled, fried, and brined fish Pampus argenteus are presented in Table 1.

Determined protein content for P. argenteus was 13.60%, 29.40%, 38.17%, and 33.56% for fresh, boiled, fried, and brined fish respectively (Table 1). Protein content was significantly increased (p <0.05) is processed (boiled, fried, and brined) fish. Lowest protein levels were nevertheless observed in boiled fish. The highest and lowest levels of fat were recorded in brined (9.94%) fish and were significantly different from unprocessed (fresh) fish. Brined and fried fish had significantly high levels of ash (17.70% and 9.20%) (p <0.05). There were significant differences (p <0.05) in moisture content for P. argenteus where the lowest levels were recorded in boiled fish (25.20%).

Chemical composition of fish varies widely depending on the fish species, age, nutrition, size and sex, growth place, catching season, and the environmental conditions (Manthey, Karnop and Rehbein, 1988). The increase in the fat content of the fried fish fillets is related to oil absorption during the cooking process. Also, reported that the increase in dry matter content was observed in the fried and grilled fish fillets. The decreased moisture content was noticed in all the cooking methods except for the boiled fillets. Increased ash content was noticed in all the cooked fillets when compared to raw fish fillets. That deep-fried fish had the highest protein value comparing other cooking methods. Water loss, occurring during cooking resulted in increasing protein content in the fried fish samples (Gökçe et al., 2004; Gokoglu, Yerlikaya, and Cengiz 2004). The higher lipid content of fried fish than other methods is mainly due to the absorption of oil by the fish and losing moisture during frying process (the apparent higher ash content of a fried sample is due to more loss of moisture took place during deep-frying cooking comparing with another method (Küçükgülmez et al., 2006)). A higher level of protein in this study agrees with earlier reports (Saguy and Dana, 2003).

Cooking methods that require heat can affect the nutritional composition of fish depending on the intrinsic composition, temperature, and time the product is exposed and the method used. The water loss may be attributed to the evaporation, dehydration of the fillet fibrils, and probably to some heat-induced protein denaturation during boiling, which causes less water to be entrapped within the protein structures (Chukwu and Shaba, 2009; Costa et al., 2013).

Quality properties of processed fish at the time of consumption and eating greatly depends on different factors such as; the freshness and fish quality, the cooking methods, and the storage or preservation conditions followed before cooking. Fish spoilage is usually more rapidly than other animal muscle foods and this spoilage is primarily bacterial. Therefore, good preservation techniques must prevent the bacterial spoilage of fish without affecting its quality and nutritional value (El-Lahamy et al., 2018).

Marimuthu et al. (2012) showed that the increase in dry matter content was observed in fried fish fillets. The highest moisture content (77.2%) was recorded in raw fillets. Increased ash content was noticed in all the cooked fillets when compared to raw fish fillets. Moisture loss was also found in baked fillets of snakehead fish. However, the dehydration rate comparatively was lower than during frying. These changes were similar to those reported by Gokoglu, Yerlikaya and Cengiz (2004) in rainbow trout and García-Arias et al. (2003) in sardines. Water losses, occurring during frying resulted in higher protein content in fried fish as compared to the raw fish fillets (García-Arias et al. 2003). Accordingly, the increase in ash, protein, and fat content found in cooked silver catfish fillets is explained by the reduction in moisture. These results were similar to our present study.

Abraha et al. (2018) reported that generally fish processing methods (high and low-temperature treatments) including, chilling, freezing, canning, smoking, drying, salting and frying, and various combinations of these, to give the fish product a form which is attractive, fresh to the consumers and prolong and suitable storage life. These processing methods have different applications, techniques, and significant influences and effects on the chemical, physical, and nutritional composition of processed fish. This is because heating and exposure to a high concentration of salt lead to chemical and physical changes. Ultimately different quality could be obtained via these methods, hence subsequent effects on processed fish’s shelf life also vary (Magnussen et al., 2008; Díaz-Tenorio, García-Carreño and Pacheco-Aguilar, 2007).

The results obtained from this study showed that the highest protein (38.17%) and moisture (40.40%) contents found for fried fish Pampus argenteus, while lowest moisture content (25.20%) found for boiled fish. The least lipid content (9.94%) found for brined fish (p <0.05). Fish provides the most dietary animal valuable protein to people in Iran. Findings also confirm earlier reports that processing alters nutrients content in fish. Although fried fish are mostly liked by many people, results in this study have demonstrated that more nutritional benefits could be obtained when fish are processed through the boiling method.

Lower amounts of lipids in boiled samples were presumably the result of the spread of fat into stock during boiling. As far as fat is concerned, the loss introduced by heat treatment is not as explicit as in the case of water. Boiling, both with and without the addition of salt and, caused a substantial (10% on average) drop in the amount of fat. The loss of fat and nutrients in boiling was less than frying in oil. It has been shown also that boiled fish exhibit better storage properties due to low moisture retention