(part of the Secret Pro Techniques ramen technique series)

@Elvin Yung <@shikaku.ramen>

Fushi or bushi (節, not to be confused with 武士 bushi, as in "bushido") roughly means sectioning. For the purposes of this article, it's a method of preservation which involves smoking and drying a food item, and then optionally applying a specific kind of mold on it and letting it ferment.

(A note on nomenclature: although it's technically more correct to call it fushi when referred by itself without rendaku lenition, it's more commonly called bushi in English literature, so I'll be adopting this convention for better consistency, which will hopefully be useful when cross-referencing terms against other literature.)

Katsuo (鰹) is a type of fish that is sometimes translated as bonito and sometimes translated as skipjack tuna. If you don't have access to the exact kind of fish, the bushi process is generally applicable to an exceedingly large variety of seafoods, and should work with most kinds of fish or food in general — indeed, a few people have already succeeded in replicating the same process with meats and vegetables.

Most of the kinds of "katsuobushi" you have access to outside Japan are actually usually a product called hanakatsuobushi (花鰹節), which is the retail name for a dried, smoked, but *non-*fermented fish product called arabushi (荒節). This product is created during the early stages of katsuobushi production, and is therefore much less costly to produce.

Although it may be near-impossible to make true katsuobushi at home because of a lack of access to the correct mold for the fermentation process, the general process is replicable, and various hacks have been tried to replace the fermentation step.

In Japan, it's common to combine various kinds of bushi in order to achieve a certain balance and variety of qualities. Motenashi Kuroki, for example, has been known to use a mix of hongarebushi for umami, soudabushi for fuumi (風味, roughly "distinctive flavor"), and hanakatsuobushi for aroma. In this way, a dashi becomes yet another dimension for dialing the proportions and extractions of an endless variety of ingredients to create an optimal product to fit the purpose.

The production process is roughly as follows:

1. Namakiri (生切): After removing the heads and guts of the fish, the fish is sectioned into three fillets (三枚おろし, sanmai-oroshi).
2. Kagodate (籠立て): Lay the fillets in a container in preparation for boiling. Traditionally, a rectangular metal basket sometimes called a nikago (煮籠, "boiling basket") is used. The goal here is to keep the fillets as straight as possible, since any curvature here will be hard to reverse after the cooking process, and will negatively impact your ability to shave it.
3. Nijuku (煮熟): the nikago is lowered into boiling water and the fillets are cooked. The water temperature can be anywhere from 75-99°C (usually above 90°C), and the cook time can tend to be anywhere from 60 minutes to 2 hours.
4. Hone-nuki (骨抜き): remove the remaining small bones from the cooked fillets. This step also tries to remove the remaining fat and other scum. To avoid overly damaging the fillets, this step needs to be done incredibly carefully.
   1. Optionally, some katsuoubushi makers may also fix any minor mistakes by "painting" the fillet with a paste that consists of a mix of cooked and raw fish (~2:1 ratio). This process is called shūzen (修繕, roughly "touch-up").
5. Baikan (焙乾): The fillets are smoked, then rested overnight (あん蒸, anjo), which allows interior moisture to equalize and diffuse out to the surface. To dry out the fillet further, and the smoking and resting steps are further repeated, generally daily for at least a week and sometimes for upwards of a month. Generally, each smoking step involves smoking at 80-90°C in a chamber called baikanshitsu (焙乾室) for about an hour. Each smoking step is numbered with the word 火 (hi, "fire") as a counter, so the first smoking is ichiban-hi (一番火), the second smoking is called niban-hi (二番火), and so on.

At this point, this product becomes arabushi (荒節), sometimes called oni-bushi (鬼節), and has ~25-30% of the remaining moisture. This arabushi is ready to be shaved and used in a dashi, but can be further refined into karebushi (枯れ節):

1. Hyomen-kezuri (表面削り, sometimes just called kezuri 削り, not to be confused with the process of shaving the block into thin shavings), this step involves grinding/sanding off the "tar" and smokiest parts of the fillet on the surface of the block. At this point, the block is sometimes called hadaka-bushi (裸ぶし) or "naked" bushi.
2. Kabi-tsuki (カビ付き): the katsuobushi-kabi (鰹節カビ) or "katsuobushi mold" is applied to the surface of the block, and fermented at a specific temperature (sometimes cited to be ~28°C) and a specific humidity (sometimes cited to be around 95%) for between 1-2 weeks. The primary function of this mold is to dry out the katsuobushi even further, but obviously it has a fermentative effect as well. (It's not perfectly clear whether this process also breaks down protein into amino acids like Aspergillus oryzae/koji does, but it's a reasonable assumption.)
   1. The mold is mostly (but not exclusively) comprised of Aspergillus glaucus. In Japanese literature, the strain is often called by an alternative name, Eurotium herbariorum (ユーロティウム・ハーバリオラム) or Eurotium (ユーロチウム or ユーロティウム) for short. People have been known to acquire this mold by rubbing it off an existing katsuobushi block. Aspergillus oryzae (koji tane or koji-kin) also seems like it's probably an adequate substitute, but there have been reports that the mold might not survive as well in the same conditions.
3. Hiboshi (日乾): the block is sundried for a few days to further dehydrate the block.