The percentage of ash during the 6th and 9th months after canning process was lower than the control sample. The highest percentage of ash was observed in the 11th month of the experiment. Statistical analysis of the data showed that there was a significant difference between all experimental treatments (p<0.05).
The results of measuring the concentration of heavy metals in fresh and canned tuna fillets after 6, 9 and 11 months after production are shown in the following Figures 3-6:
The Figure 3 showed that the iron concentration in canned fish in the 6th month of the experiment was higher than other experimental samples. The lowest content of iron was observed in the control sample. The results showed a significant difference between all experimental treatments (p<0.05).
The results of present study in Figure 4 showed there was a significant difference between the concentration of zinc in all experimental treatments (p<0.05). The highest was observed in the 9th month with a concentration of 10.83 (mg / kg) and the lowest was observed in the 11th month of the experiment with a concentration of 5.73 (mg / kg).
The results of the experiment in Figure 5 showed that the concentration of copper in canned tuna fillets over time significantly increased (p<0.05). The concentration of this element in the control treatment was higher than the 6th month of the experiment, but it found significantly lower than other experimental treatments. The highest copper content was observed in canned fish in the 11th month of the experiment (p<0.05).
According to the following Figure 6, it can be stated that the concentration of mercury in the 9th and 11th months after canning comparing to the 6th month storage increased, but this increase in concentration was significantly different comparing to the other samples in control and 6 th month storage (p<0.05).
The results showed that decrease the moisture content (%52.5) in the control treatment to %50.33 in the 9-month storage treatment led to an increase in fat content from %14.91 in the control to %18.1 and a decrease ash content from %1.11 in the control to %0. 63 in treatment 9 months storage.
The concentrations of iron, copper and mercury in the control treatment from 11.03, 7.11, 1.71, 0.03 mg / kg, respectively, increased to 22.06, 10.83, 2.76 and 0.04 mg / kg for 9 months storage treatment. This indicated that the remaining elements in the ash of the samples should be reduced after 9 months of storage.
The comparison the moisture contents in treatments of control and 11 and 6 months of storage, shown that because the amount of moisture has increased, the amount of ash has also increased, while the comparison of moisture in treatments of control and 9 months of storage showed that because moisture content was low, the ash content had also been decreased. The among the tested elements, the amounts of iron and zinc in the 6-month treatment had increased, but the amount of copper decreased and the amount of mercury did not change.
In this study, the average concentration of mercury obtained in all samples was lower than the global standard, which was 0.5 mg /kg. Gochfeld and Burger (2004) evaluated 168 canned tuna in terms of mercury concentration. Their results showed that the average total mercury content in tuna fish was 0.456 mg/kg and they stated that the total mercury content was 25% higher than the world standard. The maximum concentration of mercury obtained in their report was 0.956 mg/kg. Emami Khansari et al. (2002) stated that the amount of mercury in a sample of canned food investigated in Iran was less than the standard. Although in the present study, the concentration of mercury in canned tuna increased over time, but its amount was lower than the values of international standards, and obtained results of present study were in line with the results obtained from the study of other researchers.
In another study, the concentration of mercury in the analyzed canned fish was 146.65 ppb, which was lower than the global standards of EPA and FDA, which stated that the permissible amount was 1 μg per g (Salar Amoli and Isfahani 2010) which was consistent with the results of this study. Research on the concentration of heavy metals in canned tuna has results with different yield. Accumulation of heavy metals in fish muscle varies according to ecological and biological conditions as well as metabolic activities of fish (Canli and Atli 2002). Study of Ikem and Egiebor (2005) showed, the concentration of copper metal in the analyzed canned fish was less than the MAFF standard, which was consistent with the results of present study.
Velayatzadeh et al. (2010) investigated the heavy metals in some canned tuna in Iran. Their results showed that the highest amount of iron was 7.63 ± 0.04 mg and 5.36 ± 0.82 mg /kg and the lowest amount of iron was 4.29 ± 0.45 and 2.84 ± 0.42 mg /kg. In the present study, the concentrations of zinc and iron in canned fish showed a significant increase comparing to the fresh samples. Zinc accumulates mainly in the bones and skin, but is also found in significant amounts in the liver, gills and kidneys (Ismaili Sari 2002). The place of absorption and the mechanism of its transfer to the fish body depends on factors such as the chemical form of the metal (ionic or its salts).
The canning process can change the concentration of heavy metals in the product. Atta, El-Sebaie et al. (1993) reported that the concentration of some heavy metals decreased during the cooking and frying process. A study by Ezzatpanah et al. (2010) found showed that canning steps, including defrosting, baking, and sterilization, significantly reduced the concentration of heavy metals. In a recent study, the copper content in canned tuna in the 6th month of the experiment and the amount of zinc in the 11th month of the experiment was significantly reduced comparing to the fresh sample (p<0.05).
Iron content had the highest among other elements measured in fresh and canned tuna fillets, while mercury was the lowest concentration in the samples. These findings were consistent with other researchers' findings which the element iron had the highest concentration in different organs of the fish (Mahboob et al. 2016), which was in agreed with present study results.
According to study results of Celik and Oehlenschlager (2007), the lowest and highest levels of zinc in canned fish tested in Turkey were in the range of 33.8 - 556 μg/g. The level of this element in Black Sea fish samples, Turkey, was in the range of 52.9-40.32 μg/g (Tuzen 2003). One of the reasons for the different amounts of heavy metals in canned fish in different countries was the difference in the environmental conditions of ecosystems and the type of fish species.
The high slightly content of metals in Iranian the canned fish in this study may be due to environmental pollution and raw materials or due to secondary pollution due to improper handling of raw materials, containers and processing steps on land and / or at sea.