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Species identification in canned tuna is much more challenging than for processed fish in most cooked foods.  This is because the DNA is substantively degraded during the canning process.

In this paper (purchase required) the authors present a protocol to increase concentration and purity of DNA extracted from canned samples. The experiment mainly consists of: (1) drying the canned tissue in paper filter, (2) washing it with a PBS solution, (3) store in ethanol 96 % at −20°C, and (4) perform DNA extraction.

They report that the pre-treated samples showed an increase of both DNA concentration and purity indicating that some of the inhibiting molecules were successfully removed. These differences between the two treatments were statistically significant (p < 0.01). At the amplification level, the pre-treatment allowed the recovery of complete fragments of the barcode region COX1 with approximately 650 base pairs.

The authors recommend their approach should be used in combination with other methodologie such as mini-barcoding.

Photo by Grooveland Designs on Unsplash

The canning process has a degrading effect on DNA, making the species verification of canned tuna more challenging than for raw fish.

In this paper (open access) the authors optimised and compared three PCR approaches: real-time PCR (RT-PCR), mitochondrial control region (CR) mini-barcode, and multiplex PCR.  They tested 24 samples labelled as either albacore, yellowfin, skipjack or light tuna.

They reported RT-PCR as having the highest identification rate (100%), followed by CR mini-barcoding (33%) and multiplex PCR (29%). They consider that the success of RT-PCR may have been due to the short (<100 bp) DNA fragments targeted. In comparison, multiplex PCR and CR mini-barcoding targeted slightly longer fragments of 127–270 and∼236 base pairs, respectively. Regarding species identification, CR mini-barcoding and multiplex PCR confirmed the presence of albacoreor yellowfin tuna in several samples; however, both methods struggled with the identification of skipjack tuna.

CR mini-barcoding enabled sequencing-based detection of a range of species in the products. The authors conclude that a combination of real-time PCR and CR mini-barcoding is the optimum approach for rapid screening of target species along with sequencing-based confirmation.

Photo by Grooveland Designs on Unsplash

A study conducted by Universidad Nacional Autonoma de Mexico (UNAM) researchers found traces of dolphin meat in three out of 15 samples of tuna cans on sale in Mexico. 

Lead researcher Karla Vanessa Hernendez Herbert used DNA probes with polymerase chain reaction (PCR) to identify dolphin meat adulteration.

The full report has yet to be published in a journal, but the Herbert and Professor Francisco Montiel Sosa disclosed the results in an interview with Mexican newspaper, Excelsior. The original article can be read here in Spanish., or a summary of the article from SeafoodSource can be found here.

On 11 August, the environmental protection service of the Spanish civil guard SEPRONA announced the seizure of 45 tons of illegally treated tuna fish. Four people were investigated and face possible criminal penalties of up to four years in prison for endangering public health, as well as administrative sanctions. The investigation has so far uncovered three companies and three fishing vessels involved in the fraudulent scheme.

Investigators found that frozen tuna only suitable for canning had been illegally treated with substances that enhance the colour and then been diverted to the market to be sold as fresh fish. This treatment can pose a serious public health risk associated with allergic reactions to histamine.

The investigation was coordinated by EUROPOL under the OPSON VII operation, in collaboration with the European Commission and other Member States, which was previously reported on the Food Authenticity Network in May 2018.

Criminal investigations are ongoing.

For more information on this case including the European Commission's contribution and information on other successful outcomes for EU coordinated cases.