The recent return of a food fraud technique involving tuna triggered alarm bells in European control organizations. Due to suspicions raised in the industry, hundreds of tuna pieces had to be examined.

Some of the tuna samples had been adulterated using unauthorized additives in order to alter the final color of the piece. 

At this point, readers might wonder: why would the dealer be interested in adulterating the color of the tuna? How do they do that? In case of ingesting a fraudulent piece of tuna, do we run any risk as consumers?

In this article, we will try to answer these questions from the point of view of food chemistry.

Why is tuna the color of tuna?

The characteristic color of tuna is due to its hemoglobin and myoglobin content, complex organic molecules responsible for oxygen transport. In the case of the genus Thunnus, the myoglobin content has been shown to be higher than that of hemoglobin, which is why we will focus on its study.

In the structure of myoglobin, there is an iron atom in the oxidation state, responsible for binding to the O₂ molecule. The resulting structure is called oxymyoglobin and has a very bright red color that is attractive to the consumer.

Once the animal is captured and slaughtered, if it is deep-frozen at -40 ⁰C, its color is maintained with hardly any apparent variation. However, this treatment is not common and generally, the post-capture freezing temperature is -18 ⁰C.

When thawing and filleting the tuna, the myoglobin is exposed to the air. This causes a color transformation from the initial deep red (due to oxyhemoglobin) to a much less attractive reddish-brown for the customer. This change in color is linked to the oxidation of the iron atom, which goes from being in an oxidation state of +2 to +3. This new molecule is known by the name of metmyoglobin.

It is evident that the longer the time of exposure to the environment, the greater the degree of appearance of metmyoglobin. Therefore, the worse aspect the piece will present to buyers.

This is a good strategy to identify the tuna catch date. A darker piece would be, a priori, less advisable due to the appearance of certain potentially toxic compounds, among which are biogenic amines. Of this group, perhaps the most significant would be histamine, a nitrogenous compound originating from the decarboxylation of histidine, a very abundant amino acid in tuna.

Why adulterate the color of a tuna?

There are two reasons behind the adulteration of the color of tuna: on the one hand, the prolongation of its useful life. On the other, make cheaper species pass for more expensive ones.

The first would seek to extend the time period during which the tuna could be for sale, because its appearance would remain palatable for a longer time.

As for the second, it consists of passing a piece of yellowfin tuna (Thunnus albacares) or bigeye tuna (Thunnus obesus), species of lower price and quality, for a bluefin tuna ( Thunnus thynnus ) or bluefin tuna. The latter has its own coloration consisting of a characteristic very bright red that differentiates it from the rest of the cheaper species.

How is tuna color adulteration carried out?

The main factor that causes a variation in the color of tuna is the oxidation of myoglobin. Avoiding that step can prevent such browning. But how?

There are different strategies, among which are:

1. Addition of nitrate and nitrite salts (NO₃/NO₂).

Its incorporation into the product provides a high concentration of these species in the tuna meat. In this environment live a series of microorganisms capable of reducing nitrate to nitrite. This nitrite, in turn, is capable of generating highly reactive NO radicals, which can mimic the function that O₂ performed on myoglobin.

Thus, the color of oxymyoglobin would be stabilizing artificially.

2. Treatment of tuna with carbon monoxide.

Carbon monoxide, CO, fulfils the same role as the nitro compounds mentioned above. In the presence of CO, myoglobin would capture it, forming highly stable Mb-CO complexes with a very bright and striking red appearance.

3. Addition of plant extracts.

Since the nitrate ion is very common in different plant species, its addition to fish such as tuna would generate an effect similar to that of directly adding nitrate salts.

In addition, thanks to the presence of pigments of a color very similar to that desired in beet, its extracts are particularly useful as far as the adulteration of tuna is concerned.

Are you at risk if you eat adulterated tuna?

The aforementioned procedures have been banned within the European Union due to their ability to mislead consumers and their potential health effects.

In the cases of tuna samples treated with carbon monoxide, this risk is lower since the amount of this substance that is necessary to have a visible effect is very small (<1% in CO already produces a significant effect).

On the contrary, if the adulteration has been through the addition of nitrate or nitrite salts, the risk may be greater. Nitrite has been shown to be capable of generating reactions for the formation of nitrosamines, compounds related to the appearance of different types of cancers.

In addition, the consumption of tuna outside the recommended period may involve certain dangers due to the appearance of the aforementioned biogenic amines. We previously highlighted histamine, since it is related to the appearance of the syndrome called scombroidosis. This is characterized by suffering from tingling, vomiting, dizziness and a burning sensation in the mouth after having consumed a contaminated piece.

conclusion

The emergence of a growing world market for tuna has generated excess demand that is difficult to cover. As a result, different strategies have been detected to be able to make available to the consumer foods that do not comply with European legislation.

Its accidental and sporadic consumption does not pose a great danger to the population. In addition, it is important to convey a message of reassurance to the consumer: that articles like this can describe the fraud detected demonstrates the efficiency of the official controls that offer us safe food.

This article was originally published on The Conversation.

Image Credit: Getty

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