First, let’s confirm where chutoro and otoro are located on the tuna using a cross-section diagram of the fish’s midsection.
Looking at the tuna cross-section, the center part is red meat (Akami), while the surrounding areas closer to the skin are medium fatty (chutoro) and extra fatty (otoro). However, the borders aren’t clearly defined like this, so it’s merely a guideline. If the cut is closer to the tail, it will be just red meat.
Next, when the tuna is dissected, as shown in the illustration below.
The tuna is broadly divided into three main sections from the head: “Kami (Near the head)”, “Naka (Central section)”, and “Shimo (Near the tail)”. The back side is divided from the head downwards as “Sekami (se means back),” “Senaka,” and “Seshimo,” while the belly side is divided as “Harakami (hara means belly),” “Haranaka,” and “Harashimo.” Sushi chefs purchase these sections by width, measured from the index finger to the little finger. This width corresponds exactly to the size of one piece of sushi.
Now, let’s get to the main topic.
Basically, “otoro” refers to the fatty belly section of bluefin tuna or southern bluefin tuna. While various tuna species exist, this term is limited to bluefin and southern bluefin tuna. The term “otoro” isn’t applied to yellowfin or bigeye tuna, though yellowfin tuna can still be fatty. Japanese supermarkets sometimes use the term ‘toro’ for promotional purposes. In-season Albacore (bincho tuna) has exceptionally rich fat, and that cut is sometimes called “bintoro.” Depending on the cut, it can be divided into truly otoro-like otoro and otoro closer to akami-like chutoro.
Otoro generally includes all of the harakami and part of the haranaka. Furthermore, otoro also exists in the kama section near the pectoral fins. Otoro, much like marbled beef, is marbled with white fat. This gives it a rich, melt-in-your-mouth texture. However, some sushi chefs dislike it being too fatty and prepare it as aburi (seared) or zuke (marinated in soy sauce) to balance it with the shari (vinegared rice). Since otoro yields less per fish than akami or chutoro, its price is inevitably higher.
Chutoro, on the other hand, is found on both the belly and the back. It has a base of lean red meat with patches of white fat mixed in. It balances the savory flavor of the lean meat and the sweetness of the fatty tuna, making it universally popular. It consistently ranks among the top favorite sushi toppings for Japanese people. Some parts of the back section also yield a prized cut called setoro. In sushi restaurants, it is the second most expensive tuna cut after otoro.
The question then arises: where exactly is the boundary between otoro and chutoro?
In reality, there is no clear-cut definition. The distinction is made by tuna wholesalers and sushi restaurants based on years of experience and practical knowledge.
One initiative in producing regions involves branding fish. When certifying branded fish, various criteria are used. One of these is the fish’s fat content. This is objective data and significantly impacts the fish’s deliciousness. The method involves simply touching the fish with the tip of a measuring device, taking only a few seconds. For example, the peak fat content of mackerel imported from Norway to Japan is reportedly around 25-30%.
For otoro, this value is over 20% and can reach 40%. Chutoro is often said to be 15-20%, but such standard values actually don’t exist.
Sushi chefs determine whether it’s chutoro or otoro by cutting the purchased portion into akami, chutoro, and otoro. If the fat content is 15%, it should be classified as chutoro, but if the sushi chef says it’s otoro, then it becomes Otoro.
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You know toro tuna and akami, but have you heard of the more rare parts of the tuna?