The effect of cooking on the heavy metal residues in some fish species of Caspian sea

Environmental pollution represents a major problem in both developed and underdeveloped countries. These metals are toxic to aquatic life at low concentrations, particularly in soft-water Such metals may be accumulated from water to higher levels in fish (USEPA, 1977). Fish species can accumulate heavy metals in their muscles. The content of metals in fish tissues and organs indicates the concentration of metals in the water and their food. Therefore, the problem of heavy metal contamination in fish is increasing global attention. Meanwhile, different fish tissues have different abilities to accumulate these metal elements due to the different chemical composition. The heavy metals concentrations of fish fillets can be affected by processing or cooking methods and therefore, it is important to determine the concentrations of heavy metals in raw and cooked fish fillets and it is possible to reduce the heavy metal concentration in fish fillets by choosing a suitable method of cooking (Atta et al., 1997; Ersoy et al., 2006; Diaconescu et al., 2013).

Specimens of white fish (Rutilus kutum), carp (Common carp), mullet (Liza aurata) each in three numbers in equal sizes were purchased freshly from Sari fish market in December 2017 and after unloading The viscera and scales were immediately transferred to the laboratory of the Department of Food Science and Engineering of SANRU and stored in the refrigerator. Fish samples were prepared by four different cooking methods. For this purpose, a sample weighing 100 g was removed from the inner tissue of each fillet and in the next step, each sample was cooked for the required time for optimal consumption. These conditions are for microwave (Solardem model, made by LG company) for 10 minutes at 900 watts, grilling (in a one-floor electric oven made by Baking Industries Company, Mashhad) for 20 minutes at 180 °C, roasting (in an electric oven one Construction floor of Mashhad Baking Industries Company) was 180 degrees for 20 minutes and frying (made by Pars Khazar Company) was 180 degrees for 4 minutes. Measurement of heavy metals: For this purpose, the acid digestion step was performed with the help of nitric acid. To digest the samples, the first one gram of each sample was dissolved in 10 ml of 1 M nitric acid, then boiled until complete digestion. In the next step, the filtration operation was performed and the resulting precipitate was washed with 1 M nitric acid and transferred to a 25 ml test tube and the rest was made up to volume with distilled water. The desired elements (lead, cadmium, zinc, copper, nickel, chromium, and manganese) were read by the plasma induction diffuser (ICP_AES model 4100).

Regarding the effect of the cooking process on the cadmium element, cadmium was decreased in all treatments and the least amount of cadmium was observed in roasted white fish. In the white fish, the lowest chromium content was observed in grill and microwave treatments. Grill and microwave processes were reduced in chrome in the white fish. The highest amount of nickel element in the white fish was observed in the roasted type and the lowest amount of grilled sample. In the white fish, only the roasting method caused a significant decrease in copper. In mullet fish, all treatments reduced this element significantly, but the lowest amount was found in frying. Zinc content in mullet and carp fish was significantly higher than other elements. The roasting process resulted in a decrease in this element. The roasting and grilling process significantly reduced Mn. In the white fish, the least amount was related to the frying method. In mullet and carp fish, the highest decrease was in fried samples.

Overall, whitefish and carp fish were the unhealthiest fish among the three species. In comparison between different cooking processes, roasting and after that grilling resulted in the highest reduction in the sum of three heavy metals and seven elements, in contrast, in the most cases microwaves preserved these elements in cooked fish.