Why can’t tuna reach the market alive, why did tuna turn brown and How to choose good quality Tuna
Why can’t tuna reach the market alive
Most fish can survive a pretty long time out of water. However, fish like tuna almost immediately dies when they exit the water. In South Korea, fish that can’t survive out of water are called ‘hasty’.
The reason is really simple because most fish is an ectotherm (cold blooded) and they don’t have a high metabolism and could adjust their metabolism depending on the environment.
Most fish as an ectotherm don’t require high oxygen as they don’t need to produce energy to maintain body temperature and they could even slower down their metabolism.
However tuna are an endotherm, homeothermic(warmblooded) and can’t regulate their metabolism but they require large amounts of oxygen.
As tuna can’t pass through water through their gills, when they exit the water they are technically suffocated immediately and dies right away.
Like humans suffocate in water, tuna suffocate on land. Mackerels also partially regulate their temperature so they can’t also survive long outside the water.
Mechanism of tuna’s discoloration and color change turning into brown
Bluefin tuna sashimi and sushi is the best method to enjoy the rich flavor of bluefin tuna. However, restaurants and consumers are challenged by the difficulties receiving fresh bluefin tuna by the discoloration as it often turns brown and changes color during shipment.
Bluefin tuna has red meat because of the high contents of the myoglobin in the bluefin tuna meat muscle. Even compared with hemoglobin, myoglobin has an even higher affinity with oxygen.
Tuna is already rich with myoglobin, however its high contents with unsaturated fats are even highly reactive with oxygen. Thus tuna is very susceptible to oxidation.
The Metmyoglobin, which is the oxidized form of myoglobin causes the brown color of the tuna during shipment.
In live cell, enzymes called metmyoglobin reductase prevents metmyoglobin, however tuna meat is of course distributed and shipped dead so tuna meat is susceptible naturally to oxygen and oxidation.
Thus, it is natural for the tuna color turning brown, however it decreases the flavor of the tuna and make an unpleasant sour taste.
Factors that cause the tuna color to turn brown
Tuna is still edible if the brown color is light, however tuna that turned to a dark brown color is not edible. Because tuna is high in unsaturated fat and myoglobin contents, it is naturally very susceptible and very reactive to oxygen.
Thus, tuna must be kept in at least a -60 °C freezing environment to prevent the color from changing to brown.
Tuna freezers are special and have a much lower temperature than ordinary freezers at home. Thus, storage of tuna in household freezers for a long period is also a factor that tuna could turn brown.
Also, the time taken for shipment of the tuna and the temperature that the tuna was shipped is a crucial factor on the browning effect of the tuna.
Difficulty of storing tuna
The more fatty the tuna is the more susceptible the tuna is to changing color and becoming rancid. Tuna fat or oil mostly consist of poly unsaturated fat so it is highly reactive to oxygen.
Thus the more fatty parts of the tuna such as the belly like the Otoro and the Chutoro, the more difficult it is to ship and maintain quality.
Even the Otoro is much more difficult to maintain quality than the Chutoro. Also, technically the tuna starts to melt and defrost in ordinary household freezer conditions.
Even the most powerful freezers in households could go down to -30 °C. However tuna start to change and defrost as they are stored in a -60 °C specialized freezer. Thus, once received tuna, it should be consumed in a short time at households.
How to choose high quality Tuna – Bleeding
This is the most crucial factor in choosing quality tuna and even most fish in the market. Suppliers that supply high quality fish usually bleed out or drain the blood of the fish before selling to the market. The most important function of blood is transportation of oxygen to the body thus itself is very rich in oxygen.
Fat and oil of fish are usually unsaturated fats so they are much more reactive to oxygen than terrestrial animals that consist of saturated fats.
The oxygen of the blood quickly oxidates the fish and turns it rancid. Also, long time of blood exposure to muscle tissue induces necrosis which means it triggers self-digestion and the body tissues of the fish literally breaks down.
This make the fish more susceptible to bacteria disintegration as well. Thus the draining of blood or bleeding of tuna and fish is most important for higher quality. Ask the merchant or supplier if the tuna or fish was bled out or had its blood drained.
How to choose high quality Tuna – Firmness of the meat
Bluefin tuna are never distributed to the market alive. Thus when pressed on, the tuna should be firm and plump. Experts would check the body elasticity and even check the anus part especially.
If the tuna started to deteriorate, the intestines would start to disintegrate and become watery so the tuna wouldn’t be firm but it would be squishy and soft.
As the anus is directly connected with the intestines, the elasticity near the anus is thus important to check if the tuna has a certain quality.
How to choose high quality Tuna – Fat Content
If the above criteria was to determine if the tuna could be edible as sashimi or sushi(eaten raw) checking the fat content is to choose the best tuna among the selections.
Tuna auctions would display the tail of the tuna as the tail part of most fish is the least valuable part.
Thus cutting off the tail doesn’t harm the value of the tuna and the bidder could check the approximate composition of the belly and the back by examining the tail cross section.
The tail cross section would have the two belly parts and the two loins of the tuna and bidders would check how the fat is distributed in the tail of the tuna and check the quality of the lean meat on the tail.
The tail has the least fat and the lowest quality of meat of the tuna so if the quality of the tail is higher, the higher the tuna has better quality.
How to choose high quality Tuna – Shorter but fatter is the best
Tuna with higher weight is not always the best parameter to determine the best quality tuna. Like humans with the same weight have different body forms, tuna also with the same weight have different body forms and compositions.
Supposing the tuna is the same weight, the shorter the tuna and the fatter the tuna is the higher the tuna has more flavor and has a better quality.
One secret that merchants check is the head and the neck of the tuna. Tuna, if stored enough fat in the belly, starts to store fat in the intestines and other parts of the body.
If a tuna has a fat head and neck, it means that the tuna is packed with fat so this is one of the secrets that merchants have. However everything is luck and not decisive.
