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.