The entire block for each chef has those parts you savour, and as you cut the block the tuna opens up and gives you more taste and flavors.
The tuna block’s appearance is indicative of the fat concentration and the fat type. As you’ve seen from previous postings:https://mesubim.com/2016/01/30/otoro-densities/
This block’s fat varies from side to side. One side is much deeper in color and on the other side pinker and more fatty. If you watch the video below you have a better understanding of how tuna is cut. The fish is quartered and each section is then cut into blocks. The belly has two of the four quarters so the belly cut, center cut is rare.
This block is center cut and below the chef has the block squared off at the tsukiji by the vendor who takes the topside, the akami which is the red meat, closer to the bone.
The block below has matured for 5 days and has been kept cool and you can see it is rigid in appearance otherwise the chef couldn’t cut it.
The tuna meat warms up very quickly after its cut, as the chef places the two halves into his basket to acclimatize to the ambient temperature. The end result is fatty tuna, not overlay dense, but very rich in flavour and obviously expensive.