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Executive summary

The South Australian Research and Development Institute (SARDI) is the South Australian Government’s principle resource for primary industries and related research and development, and is a primary source of advice on state science and research policy. In May 2005 SARDI undertook a study into the mercury content of canned tuna products from Port Lincoln Tuna Processors (PLTP). This report summarises results from 96 commercially packed cans of tuna with mixed vegetables from PLTP.

In total eight varies of commercial canned tuna products with mixed vegetables were tested in replicates of twelve. The mean total mercury content found was 0.1032 mg/kg. All product tested met the requirements of the United States (US) Food and Drug Administration (FDA) action level for methyl mercury, which is currently set at 1 mg/kg, and the Maximum Level (ML) of 0.5 mg/kg for total mercury set by Food Standards Australia New Zealand, within the Australia New Zealand Food Standards Code.

Total Mercury test results for the PLTP products were compared with US canned tuna products. Data used for comparison were those from the US Food and Drug Administration (FDA) Monitoring program for mercury in fish. Statistical analysis showed that the Mercury concentration in PLTP product was significantly lower than White Albacore canned tuna (p<10^-6)and also for one of three Light canned tuna groups (p=1.5x10^-6) tested by the FDA. There was no statistically significant difference between the PLTP Mercury results and the remaining two Light canned tuna groups (p = 0.61 and 0.64).

Introduction

Skipjack Tuna (Katsuwonus pelamis) is caught by purse seining in Australian, New Zealand and West Pacific waters for commercial processing in Port Lincoln, South Australia by Port Lincoln Tuna Processors (PLTP).

Skipjack tuna, Katsuwonus pelamis (Linnaeus, 1758) (Source www.fao.org)

Methodology

Products tested included:

Sampling methodology

Each can of tuna was prepared for mercury testing followed the procedure from the United States (US) Code of Federal Regulations, Title 21 Food and Drugs, Section 160.190 Canned Tuna. 

Analytical methodology

Analysis was performed by cold vapour atomic absorption spectroscopy (CVAAS). 

Accreditation of laboratory

Details of accreditation held through the National Association of Testing Authorities (NATA) Australia by Advanced Analytical (www.advancedanalytical.com.au) for measurement of total mercury content of fish and biota can be found at www.nata.com.au 

QA/QC of mercury analysis

A total of ten duplicate tests were performed with an average Relative Percent Difference of 15.4%.  The average matrix Spike recovery for mercury was 94.4%.

Results -- PLTP canned tuna varieties

A summary of the mercury test results for the PLTP canned tuna are presented by variety in Table 1. The mean mercury concentration across all varieties was 0.1032 mg/kg with a range of 0.07 to 0.15 mg/kg. The highest average mercury concentration was in the Chilli variety (0.1125 mg/kg) and the lowest in the Onion and tomato savoury sauce (0.09 mg/kg) (Figure 1). The horizontal dashed line in Figure 1 is the mean mercury concentration of all varieties.

Table: Summary of Mercury concentration test results for PLTP canned tuna varieties.

* Code number refers to the variety label in Figure 1.

Figure 1: Mercury concentration results for the eight PLTP canned tuna varieties tested for Mercury

Analysis of variance of the PLTP mercury concentration results highlighted statistically significant differences in the average value in two of the varieties: Chilli (p = 0.15) and Onion and tomato savoury sauce (p = 0.0007). The magnitude of the difference in the mean Mercury concentrations was small, at +0.0093 mg/kg and –0.013 mg/kg for the Chilli and  the Onion and tomato savoury sauce, respectively.   

Comparison of PLTP and US FDA (2004) canned tuna Mercury results

A comparison of mercury results in canned tuna can be made using results from investigations of product sold into the US (FDA, 2004). Although the FDA the exact details of the sampling and experimental methods are not available, the comparison is useful in placing the current testing results into perspective.

The FDA test results for mercury taken during 2003 are reported for two classes of canned tuna: White (Albacore) and Light (most likely Skipjack (ref) and Long tailed tuna (Thunnus tonggol)). Each of these two classes is further divided into four for the White meat: Albacore chunk white, Albacore solid white, Albacore and Albacore white solid and three for Light meat: Light chunk Tongol (sic), Light chunk and Light solid. A summary of PLTP and the FDA White and Light canned tuna results are presented in Table 2. The mercury results in Table 2 are for total mercury only and do not include methylmercury results (cf Burger, Stern and Gochfeld, 2005).

Table: Summary of PLTP and FDA (2004) Mercury concentration results for canned tuna

FDA canned tuna results for 2003 only

* Code number refers to the variety label in Figure 2.

The lowest average mercury result is for the Light chunk Tongol (0.08271 mg/kg) followed by the PLTP canned tuna result of 0.1032 mg/kg. The difference in mercury concentrations between White and Light tuna is clearly apparent in Figure 2. White tuna (FDA1 – FDA4) have average mercury concentrations of between 0.3 and 0.4 mg/kg with a maximum concentration of 0.85 mg/kg.  These mercury concentrations are 3 to 4 times greater than the average PLTP value. What is also clear is that some Light tuna samples have relatively high mercury contents, upto 0.533 mg/kg in the case of the Light solid (FDA7) samples. 

Figure 2: Graphical summary of Mercury concentration in PLTP and FDA canned tuna. White canned tuna are labelled FDA1-DFA4 and Light canned tuna are labelled FDA5-FDA7. 

Analysis of Variance comparing the PLTP canned tuna and the seven FDA results highlight that each of the four White canned tuna samples (FDA1-FDA4) and FDA7 had statistically significantly (p < 10^-6) higher mercury concentrations than the PLTP product. Inaddition no significant difference could be found between the PLTP results and the lowest Light groups, Light chunk Tongol (FDA5) and Light chunk (FDA6).

Discussion

Results of PLTP canned tuna are low (average 0.1 mg/kg) and are consistent with FDA survey results for Light canned tuna (FDA5 and FDA6 in Figure 2). The results also highlight the substantial difference in mercury concentrations between canned tuna made from White Albacore and Light Skipjack tuna. Statistical analysis of the PLTP and FDA results for White tuna showed that the average mercury concentration maybe as high as four times greater for the Albacore sourced product.

Effect of dilution of vegetable ingredients on Hg results

Within the PLTP product statistically significant differences were found for two of the varieties: Chilli and the Onion and tomato in savoury sauce. The differences, at least in the case of the Onion and tomato in savoury sauce product may be due to dilution the added vegetable content. The onion content of this variety is reported as 20% of the total content of the can. As the majority of the mercury found is expected to be in the tuna, the presence of the onion or other vegetables will result in a decrease in the measured mercury concentration.

In all cases the cans were drained and compressed following methods described in the FDA canned tuna documents. Draining and compressing the solid material in the can will remove most of the free springwater and canola oil.

Conclusion

Results of this study show that the mercury levels in PLTP canned tuna are consistent with the lowest measured groups of “Light canned tuna” in the FDA survey of canned tuna. As Skipjack tuna (Katsuwonus pelamis) is used in the PLTP product and is the same species as is commonly sold as “Light canned tuna” in the US (Burger and Gochfeld, 2004) the results reported in this study are consistent.

References

Burger J and Gochfeld, M (2004) Mercury in canned tuna: white versus light and temporal variation. Environmental Research, 96, 239-249

Burger J, Stern A H and Gochfeld M (2005) Mercury in commercial fish: Optimizing individual choices to reduce risk. Environmental Health Perspectives, 113(3), 1-6

FDA (2004) Mercury in Fish: FDA Monitoring Program (1990-2003). Available http://www.cfsan.fda.gov/~frf/seamehg2.html [Accessed 10 May, 2005]