Drawing out mercury: 'active' packaging for canned tuna

Fish is a good source of protein, which contains omega-3 fatty acids and many other beneficial nutrients. However, if mercury is present in the water it can accumulate in fish (especially those species higher up the food chain, such as tuna) as it binds to proteins in the tissues. That’s why some health organisations recommend caution with tuna consumption, particularly for young children and pregnant women.

Researchers from Chalmers University of Technology in Sweden have now developed a novel approach to packaging canned tuna infused in the water-based solution of amino acid cysteine. In studies, it has been shown to remove up to 35% of the accumulated mercury in canned tuna.

“Our study shows that there are alternative approaches to addressing mercury contamination in tuna, rather than just limiting consumption. Our goal is to improve food safety and contribute to enhanced human health, as well as to better utilise food that is currently under certain restrictions,” said Mehdi Abdollahi, Associate Professor at the Department of Life Sciences at Chalmers and coordinator of a project called Detoxpak.

The concept of so-called active packaging is to develop materials, for example a liquid inside a can, that interact with food during storage — for instance, to increase the shelf life. However, this concept has never previously been used to improve food safety.

Researchers pouring the liquid amino acid cysteine into a jar of canned tuna. Image credit: Chalmers University of Technology/Hanna Magnusson.

In a previous study, the researchers investigated the possibility of coating packages with thiolated silica to capture mercury from canned fish. What they observed was that the forces binding the mercury within the tuna tissue prevented it from being released. Proteins in tuna tissues, particularly sulfur-containing amino acids, strongly bind and accumulate mercury due to the strong interactions involving thiol groups from these amino acids. 

“By knowing that, we decided to add one of them, cysteine, to a water solution in which fish meat can be immersed. We believed this would allow some of the mercury to be drawn out and instead bind to the solution and be discarded. Further research is needed to take care of the removed mercury,” said Przemysław Strachowski, first author and, at the time of study, a postdoc at the Department of Life Sciences at Chalmers.

In the current study, the researchers discovered that the greater the surface area of fish flesh in contact with the cysteine solution, the higher the mercury uptake. The highest value of mercury reduction, 35%, was reached when testing canned minced tuna, from regular grocery stores. They also discovered a maximum threshold of two weeks, after which no further changes occurred.

The researchers did not observe any noticeable changes in appearance or smell of the tested fish samples in the current study. The researchers said the cell-based assays have also proven the safety of the developed technology.

“The beauty of this type of packaging is that it is active while the product is on the shelf. No additional production steps would be needed if a method like this were used industrially. The application of our results could increase the safety margin for fish consumption,” Strachowski said.

The study ‘New Insight into Mercury Removal from Fish Meat Using a Single-Component Solution Containing Cysteine’ has been published in Global Challenges.

Top image credit: iStock.com/RightOne