carrying this warmed blood mix with the blood vessels
carrying the new oxygenated water-temperature
blood coming into the muscle from the gills. The heat
in the warmed blood is transferred to the incoming
blood, thereby preserving the heat within the muscle.
This internal heat exchange unit is very effective and
can help ensure that the internal muscles of tuna are
10 to 15°C above the water temperature in which they
are swimming. All this makes the tuna muscle work
far more efficiently.
Remember too that the tuna’s internal red muscle
is surrounded by (and insulated by) the large white
muscle – a bit like having their own internal hot water
bottle! Some of the heat in the red muscle radiates
out through the white muscle, helping warm it and
therefore increasing its efficiency as well.
INTERNAL HEAT AFFECTS TASTE
So why all this information about warm internal
muscles? This is important as the longer the muscle
stays warm after death, the more impact it will have
on its taste. To get great tasting tuna you need
to cool down the muscle in the tuna as quickly as
possible after catching it. That means you must open
up the gut cavity after you’ve killed and bled your
tuna, enabling you to chill your tuna with ice from
the inside as well as the outside. If you don’t open
up the gut cavity, the white muscle will insulate the
deeper red muscle from the chilling potential of your
ice, which means your tuna will stay warm a lot longer,
substantially deteriorating its eating quality.
The larger white muscle, which is what we eat,
consists mainly of fast twitch muscle. A lot of the
time a tuna’s white muscle is not doing much work,
however, when needed it can go very hard and very
fast, but only for a limited time.
White muscle is known as anaerobic (meaning
‘without oxygen’) as it can work really hard for short
periods using an energy pathway that doesn’t use
oxygen. But this has a cost. An end product produced
by working white muscle is lactic acid, which builds up
in the working white muscle. While tuna are very good
at clearing the lactic acid (much better in fact than
most fish), there will come a point where they have
to slow down to allow it to clear. The longer you take
to land your fish, the more the lactic acid will build up
in its white muscle – depending on how hard the tuna
is working and how quickly it can clear the build-up.
The tuna also uses valuable energy stores during
the fight, which becomes a problem because the more
acid in the muscles at death, and the lower the energy
stores, the quicker the muscle will go into rigor mortis
- which is when all the muscles go stiff. After a while
the muscles relax again, but for really good quality
meat you want the muscles to go into rigor slowly.
When rigor happens too quickly, it leads to internal
damage of the muscle structure, which reduces its
quality for eating. Meat that has been damaged in this
way will be soft and watery, and the mushy fillets can
have a metallic aftertaste. And they won’t freeze well.
When the tuna dies with a lot of lactic acid in its
system a process called lipid oxidation occurs, a
term used to describe when fats go rancid, or ‘off’.
I’ll discuss this is more detail when I explain how to
properly bleed the tuna.
The first thing you
should do after
boating a tuna is
kill it humanely and
instantly with a spike
thrust down into its
brain, which is found
just behind the top
of its eyes. Starting
in the soft depression
in the centre of the
tuna’s head, aim
your spike down and
slightly backwards.bluewatermag.com.au 75From boatside to plate