Katsuo irori is a liquid seasoning made from bonito or skipjack tuna (Katsuwonus pelamis), a medium-sized fish in the tuna family that is known in Japanese as katsuo. The lean fillets of this fish are used to make katsuobushi, a category of dried, smoked, and sometimes fermented bonito products that impart umami flavor the typical broth known as dashi, as well as to a variety of other dishes.
The product from which katsuobushi derived 4-5 centuries ago dates back at least 1,300 years and is known as shio-katsuo, or “Bonito preserved in salt” (this product is not smoked). Katsuo irori developed around the same time as shio-katsuo (both are mentioned in ancient documents as valuable products that were used to pay taxes), and is made by boiling bonito bones, heads, and trimmings (but not the organs) until a concentrated, dark, syrupy liquid is obtained. The boiling process takes about 5 days, and an initial volume of 300 liters of water yields only 4 or 5 liters of katsuo irori, which is then used as needed, dissolved in hot water to make a flavorful broth for meat, fish, or vegetables. The leftover bones and other solids are used as fertilizer.
Katsuo Irori is said to be the oldest liquid seasoning using bonito in Japan, it was the most widespread processed product used as a source of umami flavor. However, over time, fermented products and fermentation techniques, many of which originated in China, became more popular and widespread, leading to the decline of both katsuo irori and shio-katsuo. In the early days of katsuobushi being adopted as the main processed bonito product, producers continued to use bonito heads, bones, and scraps to make katsuo irori, but this practice almost completely died out during the Meiji era (1868-1912).
Today, there is just one remaining commercial producer of katsuo irori, based in Tago on the Izu Peninsula, and they make only about 50 kilograms a year. There is a similar product from Kyushu called senji, but this often contains added starch as a thickener. Katsuo irori is one of three rare bonito-based Ark of Taste products from the Izu Peninsula (the others are shio-katsuo and Izu tagobushi), and an important reflection of this region’s gastronomic culture and history.
It is in desperate need of recognition and revitalization.