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This study (IAFP membership or purchase required) investigated whether there are environmental (i.e. non-adulterations) explanations for foreign tuna DNA in canned tuna.  It was prompted by reports that DNA of undeclared tuna species had been found in cans labeled as skipjack. Only one species of tuna is allowed in a can sold in the European Union if the muscle structure of the tuna is visible.

Previous reports had identified environmental DNA (eDNA) from various Thunnus and other species in the ocean and on the tuna catcher vessels and further tracked to the first stage of raw tuna processing in tuna canning (thawing).

The authors built on this work to further investigate if eDNA can be found in other parts of the cannery supply chain during tuna processing in the stages after thawing.

They collected 99 samples from raw/frozen and unfrozen tuna, precooked tuna, and retorted (heat preserved) tuna.  A total of 225 DNA analyses were performed on these samples using Polymerase Chain Reaction (PCR) methods and Sanger sequencing, real-time PCR and NGS-SBS.

They report that DNA fragments from other tuna species are present during tuna harvesting and processing operations. They suggest that eDNA from various Thunnus and other species could be found at multiple levels of the cannery processing chain.

They conclude that their results highlights the drawbacks of using traditional DNA barcoding for the purpose of detecting fraud or mislabeling with retorted canned tuna. They present  mitigation measures for solving the problem of multiple species of muscle meat in tuna products during the canning process.

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In this survey a total of 107 tuna cans were collected between 2020 and 2022 from 7 different Portuguese commercial brands.  Samples were purchased from main supermarket chains, large-scale distributors and local stores.  Samples were categorised by the quarter of the year when the tuna was canned (inferred from the expiry date) with each sample was tested in triplicate.  Extracted DNA was purified and iteratively tested using molecular metabarcoding methods.

The researchers report (open access) that the occurrence of different species was observed only in products canned in brine or water (i.e. all products canned in oil were Skipjack Tuna). Skipjack tuna was predominant across all canning liquids and brands analysed. Nonetheless, other species like Thunnus obesus and T. albacares, or Auxis spp. (not considered true tuna) were also detected. The use of different species was limited to cans produced during the second quarter of the year, which could reflect differences in seasonal availability of different tuna species or in sourcing strategies/market preferences of each company. For four brands, multiple species were detected inside the same can.  This violates current European legislation.

The researchers conclude that these results provide the first broad assessment of species used in the Portuguese tuna canning industry and showed the inclusion of vulnerable species is limited.

Photo by Grooveland Designs on Unsplash