How to optimise colour in tuna?

What makes tuna look red?

The high value of fresh tuna is largely attributed to the red colour and firm but tender texture of the thin pieces of raw flesh served as sushi or sashimi. A range of factors has been reported to affect the colour and visual appearance of the tuna flesh, along with the rate of colour change, or stability of tuna flesh colour.

The red colour of tuna flesh is primarily due to the presence of relatively large amounts of myoglobin, an oxygen-binding protein similar to haemoglobin. In the presence of oxygen, the attractive red oxy-myoglobin is dominant, but will degrade during storage to ultimately form brown metmyoglobin. Key management practices in tuna diets, fishing, farm husbandry and processing have the potential to optimise flesh colour and consistency.

Carbon monoxide treatment of tuna

This practise is banned in Canada, Japan, Singapore, and the European Union. It involves exposing tuna meat to carbon monoxide (CO) gas, which binds irreversibly to the haem group producing carboxymyoglobin giving the meat a bright cherry red colour which looks quite different to untreated tuna (this is why CO victims are found bright cherry red too). The key issue here is that consumers are eating CO, not inhaling CO, and this is quite harmless. CO makes old tuna look visually fresh and brightly coloured. This is the main reason why the countries listed above ban the CO injection (although it is used in Japan, but the product is exported to other countries that allow CO treatment!), not because of the chemical, but because of ‘fraud’. The ‘fraud’ is not about making spoiled food taste edible, you can't trick your nose or mouth, but it does trick your eyes into thinking you have something freshly caught, therefore there is concerns that consumers may incorrectly consume tuna that has high numbers of pathogenic microbes or histamine (in scombrids) that could cause food poisoning.

Of course we are always trying to extend organoleptic shelf-life through management of temperature, vacuum packaging, modified atmosphere packaging, chlorinated water/ice, etc. Another way of ‘brightening’ up meat is sodium nitrite, which also improves taste and kills microbes, i.e. bacon, and interestingly some consumers are attracted to ‘organic’ bacon which involves getting meat cured with celery juice, which actually has higher levels of nitrite (or nitrate, which turns into nitrite) than inorganic forms. Therefore, this source of organic cured meat actually exposes the consumer to higher levels of nitrite than sodium nitrite cured meats.

CO is also used to brighten the red muscle (often called ‘brown meat’) line in Seriola spp. (yellowtail kingfish), mahi mahi (Coryphaena hippurus), and other species with significant red muscle. In some cases the CO is injected into the veins post-mortem to effect the procedure. Lets not forget that CO is a natural product in wood smoke, which is also utilised to enhance colour and shelf-life in fish and terrestrial meat.

A critical issue here is that many consumers and suppliers will often discard tuna, yellowtail, etc and red meat when it has discoloured to a undesirable brown when there is no other significant organoleptic issue, i.e. smell, flavour, texture. The product may have lost its visual appeal but could still be safely consumed and marinades, cooking, etc could be used to mask the unsightly appearance. Therefore, is it important that we educate suppliers and consumers about basic organoleptic assessment, which involves not only visual appearance, but also the importance of odour?

If more countries ban the use of CO then we will have to rely on other management techniques to optimise colour retention as mentioned above. Ultra low temperature (ULT) freezing is already commonly used in the tuna industry, which involves freezing and holding tuna below -60C. This preserves colour and texture while frozen but is obviously an expensive process. Fresh tuna handled well pre-harvest and post-harvest will retain their colour for 7-10 days but if poorly handled will go brown within 24 hours.

Alastair Smart

SmartAqua is a team of aquaculture and seafood business experts with extensive domestic and international experience in a wide number of species. See www.smartaqua.com.au for more information.