Fresh tuna has is a nice red colour, however it quickly turns brown when exposed to the air.
Tuna is easier to sell red than brown so those involved in the trade are faced with a temptation to reverse the discolouration. In 2017, an estimated CHF 200 million was made tricking retailers and consumers in this way, according to 20 Minutes.
One way to turn brown tuna red is to treat it with carbon dioxide. Tuna treated in this way was discovered in Europe in 2016. Another way is to treat it with nitrites. Both deceptive practices are illegal.
Illegally treated tuna may contain high amounts of histamine which can cause serious allergic reactions – histamines result from spoiling, something masked by faked reddening. Nitrites may also result in the formation of carcinogenic substances known as nitrosamines.
Detecting these additives is not easy, however, Switzerland’s federal government asked a laboratory in Basel to run tests on samples taken from tuna sold in Switzerland. Close to half of the 13 samples tested contained nitrite.
Testing is challenging because the use of nitrites is often masked by adding vitamin C which makes nitrites difficult to detect. To get around this trickery, the scientists in Basel searched for nitrous oxide, a substance found in the presence of nitrite.
Food laws in Switzerland require importers and retailers to run their own check, however, their checks have been found wanting.
In addition, some species of tuna can have high levels of persistent organic pollutants (POPs). These substances take a very long time to break down and accumulate in the environment. Our main exposure to them is via food. Avoiding them completely is impossible, but wise food selection can reduce exposure. Concentrations increase up the food chain. Plants have the least. Concentrations in meat, the fat in particular, rise when moving up the food chain. Unfortunately, predatory fish like tuna are high in the food chain.
The only sure-fire way to avoid potential “fresh” tuna nasties seems to be to avoid all of it.