Tuna Chocolate Blocks 101 |no fish = no future|

I have a tremendous amount of respect for sushi chefs and today by coincidence, I was at a restaurant and suddenly I hear my name being called. I turn around and the chef says, “its very tasty here”, and I replied “yes” as he stands to greet me. I discovered his shop 10 years ago, and now he’s a Michelin stared chef, so I stopped going. I still consider him a very capable chef, elegant and quiet, almost always happy and smiling, Hasaguchi san.

While I am not a chef, and certainly not a sushi chef, I am still avidly interested in sushi, as are many sushi aficionados. Both as a food form, the technical aspects and the tuna’s life. I am fascinated by tuna and especially “hon maguro”, those fish that are true bluefin and fished off the shores of Japan and used exclusively by top chefs.

I feel deeply sorry that tuna has been abused by the fishing industry but not only. Customers all over the globe eat what they think is sushi, but it’s no more than frozen junk food that is trampled on by Taiwanese and Chinese clippers. Then you have the Europeans and their humungous Spanish fleets who are destroying the tuna population by taking as much as 3000 tons in a single trip: https://www.youtube.com/watch?v=BA7enHKa5As We must do something to stop this way of fishing, or our seas are doomed.

In Europe the seas are being destroyed and in the Aegean fish is more and more scarce. Fishermen continue to use small sized trawlers, and fish suffer tangled in their nets depleting themselves of oxygen. You can imagine the quality of a sushi grade fish that spends hours suffering in fish nets. Letting off toxins into their blood stream, the fish’s flesh is less than suitable for sushi and should be thrown away. Very sadly it still satisfies sushi junkies.

I am sorry to disappoint people but most people eat over priced jetsam. We must be more responsible and selective in enjoying sushi otherwise our ecosystems will not survive and there will be no more tuna, or other species of fish: fish:https://www.youtube.com/watch?v=UlOtjxufybM

Tuna is a wonderful species, and deserves the respect. Intelligent and strong swimmers never stop swimming until caught, Tuna can’t stop or they will suffer supply of oxygen and die. Tunas have the most interesting life at sea feeding on the yummy smaller fish, they also eat squid, crab larvae, skip-jack, flying fish, puffer fish etc.

One might have wondered why they are so tasty, and why such warm water fish such as tuna appear in Northern Japan, even in winter. The answer is to be found in the ocean’s current. The Tsushima Current is a warm-surface ocean current, a northward-flowing branch of the Kuroshio (Japan) Current, which runs along the Japanese side of the Japan Sea and averages 24°C degrees.

At the northern end of Honshu, the current divides, one branch continuing northward along the west coast of Hokkaido and flowing into the Sea of Okhotsk, while the other, called the Tsugaru Current, brings its warm and relatively salty water into the Pacific Ocean through the Tsugaru Strait. The migrations of warm-water oceanic fish follow this current. On the Asian mainland side of the Japan Sea, three relatively cold and fresh-water currents, the Liman, the North Korea and the Mid-Japan Sea flow southward.

While currently suffering from incredible demand, tuna was until the 1970’s, a sport fish commonly known as “horse mackerel” and sold to companies for cat food or thrown away. To have some deeper perspective Maguro was generally treated as a low-class fish, even earning the nickname neko-matagi “cat bait” for its ability to attract fish-loving felines. The reversal of the value of the akami (lean red meat) is thought to have occurred in the mid Showa period (1926-1989). Now, its fatty belly meat, known as “toro” is one of the more expensive prized for its taste, texture, and rarity.

Funnily sushi’s popularity lies in the USA, where people obsess about their sushi. But it wasn’t until the 1980s that Americans discovered that all tuna didn’t come in a can. Since then, the popularity of fresh and frozen tuna is rampant and everywhere you look, you find someone serving some version of what they call sushi. Sushi is one of the most abused foods, most often not made by Japanese chefs, and in some cases by sushi mongrels:  https://mesubim.com/2014/12/23/worst-sushi-2014/

Tuna served in restaurants is generally one of two different species, the bluefin tuna Thunnus thynnus, traditionally known as hon maguro, and the yellowfin tuna Thunnus albacares, known as “ahi” which is a fattier species. Yellow fin tuna may also be labeled ‘maguro’ but more often than not. In fact many times sushi chefs, (not in Japan) substitute ahi for toro.

The Japanese tuna we see today in markets is often caught by tuna longliners who set their lines at night, fishing waters more than 300m deep because that’s where the larger tuna fish are found. The old-fashioned “clippers” those sea vessels that travel long distances in the pacific waters, searching for the ultimate catch.

Clippers are fleets of long-line tuna boats that freeze their fish at -50°C for the Japanese sashimi market and after they reach their quota, they are back to shore. Many clippers were originally owned and operated by Japanese companies, but today most are now operated by Korean, Taiwanese and Chinese companies.

The tuna shown below is a good illustration of the tuna loin, once it has been cleaned. The topside is the red colored tuna that is very tasty, however much less popular than the fatty types. A block taken from the centre of the fish’s belly, “hon maguro” a domestically caught blue fin tuna fish (113kg) caught in kochiken in the north pacific ocean.

This cut is a block of 10kg, and after being cleaned divides up as follows: 30% akami,50% chutoro, and otoro 15-20%. The akami is attached to the chutoro (topside not shown) and akami the meat is perfectly red and delicious./my favourite/


Life as a tuna is about swimming fast and maintaining the speed to feed the amount of myoglobin in a tuna’s muscle, as it determines the color. The more myoglobin, the redder the flesh. The amount of myoglobin is a function of a tuna’s age, physical activity and species. After the flesh of a tuna is exposed to air, an iron ion in the myoglobin molecule will start to oxidize, which turns the meat brown.

I circled the otoro in three areas and if you look carefully it has changed color and is slightly brown. This doesn’t effect the taste negatively, however too much concentrated browning means the fish should be consumed. In this case it has little to no effect as the tuna matures it gains more flavour.

This tuna was not mishandled, and the small spots of browning are due to the fact that the chef holds the block for a week or more. This browning is more surface slight browning and once the chef cuts the tuna to prepare nigiri, the browning is insignificant having no impact on the flavour and just the opposite.


You can see below the nigiri is swiped with nikiri (below) and the oxidation is not relevant, and while there is dark area in the photo (below) it is the swipe of nikiri from the chef. A sushi chef adds all the necessary flavors before the nigiri is handed over to the customer. Each chef has his own nikiri and uses it as the final and finishing stroke. The nikiri is a thin glaze that is brushed on fish just before serving it. It negates the need for a small dish of soy sauce to be provided, as the chef has already seasoned the fish for you. Each chef has his own ratio of ingredients and some its 5:1:1, and some its 10:1:1, shoyu being the largest ingredient and sake, or mirin, or dashi is used. In this case its sake, mirin and shoyu mixed together and slightly boiled.


So when it comes to tuna, colour is what it’s all about. Until recently, frozen tuna had to be held at ultra cold temperatures such as -50°C to retard oxidation, preventing the tuna meat from turning brown. In fact the browning is more common in fresh caught tuna when it is handled and then flash frozen. The brown colour in frozen yellowfin is called “chocolate” tuna, and affects the price negatively.

However exposing red tuna meat to tasteless smoke for up to 12 hours can fix the red color, so that tuna held at conventional cold storage temperatures of -20°C will not turn brown. Tuna that are not properly iced after catching and left in the sun can produce histamines poisoning which is rarely fatal, but can be extremely uncomfortable.

So color in tuna is key and in some cases tuna suppliers use carbon monoxide to fix any discolouration in frozen tuna’s red color. However by using carbon monoxide the color can also be enhanced, allowing processors to make a lower grade of tuna appear higher than it is. The FDA requires that seafood exposed to either tasteless smoke, or carbon monoxide be labeled accordingly, but in Japan it is just forbidden and is not used.

Carbon monoxide in the USA is also used in packaging fresh meat products such as beef, pork, and fish to keep them looking fresh. The carbon monoxide combines with the myoglobin forming a carboxymyoglobin, a bright cherry-red pigment. Carboxymyoglobin is more stable than the oxygenated form of myoglobin, oxymyoglobin, which can become oxidized to the brown pigment metmyglobin, characteristic brown colouration that occurs as the meat ages.

But akami is naturally red and right now (spring season) it is most flavourful while the pink flesh of the otoro is less compared with winter’s end. The meat of akami is what many sushi afficandos consider the true taste of hon manguro.


From left to right; Hara means belly, Shimo mean under, Kami means upper, Kama means head (used in only fish), Saki means top.

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Harashimo: under edge of belly
Haraheso: Navel
Haranaka Sanban: #3 under middle of belly
Haranaka Niban: #2 under middle of belly
Harakami: Upper belly edge
Kamasaki: Top of head


The last image shows the fat striation and this is different from fish to fish, and depending on the migration and behaviour. The fat density is what adds most of the flavour, and for some the otoro’s suppleness o is the ultimate expression, while for others, its the chu toro, or the akami.

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