Katsuobushi

Katsuobushi(Japanese:鰹節)issimmered,smokedandfermentedskipjack tuna(Katsuwonus pelamis, sometimes referred to asbonito). It is also known asbonito flakesor broadly asokaka(おかか).
Shavedkatsuobushiand driedkelp—kombu—are the main ingredients ofdashi, a broth that forms the basis of many soups (such asmiso) and sauces (such assobano tsukejiru) inJapanese cuisine.
Katsuobushi's distinctumamitaste comes from its highinosinic acidcontent. Traditionally madekatsuobushi, known askarebushi, is deliberately fermented withAspergillus glaucusfungus in order to reduce moisture.Katsuobushihas also been shown to impartkokumi(a term translated as "heartiness").
The fish is beheaded, gutted, andfilleted, with the fatty belly, which does not lend well to being preserved, trimmed off. The fillets are then arranged in a basket and simmered just below boiling for an hour to an hour and a half, depending on their size.
The rib bones are then removed and the fillets smoked for up to a month usingoak,pasania, orcastanopsiswood. They are smoked for five to six hours in one session, left to rest one day for the condensation to rise to the surface, then fired and smoked again the next day. This smoking and resting cycle is repeated 12–15 times in total. At this point, the fish is dry and hard, and the built-up tar from the smoke and excess fat is cleaned from the surface using a grinder.At this stage the fillets are calledarabushi(荒節)and most commonly found in stores shaved and packaged for sale under the namekatsuo-kezuri-bushi(鰹削り節)orhanakatsuo. They are not truekatsuobushiwithout the last fermentation stage, but still valued as a good substitute.
The last stage of creatingkatsuobushiis to allow the fish tosun-dryusing the assistance of mold. The fillets are sprayed withAspergillus glaucusculture and left for two weeks in a closed cultivation room. The mold ferments the fillets and also draws out any residual moisture.
The mold is continually scraped off, with further sun-drying increasing hardness and dryness until the fillet resembles a piece of wood, with less than 20% of its original weight. By definition, only fillets that have been treated in this manner may be referred to askatsuobushi. After repeating this process of mold growth and sun-drying at least twice, thekatsuobushican also be calledkarebushi(枯節,lit.'dried fillet'), and fillets repeating this process more than three times can be calledhonkarebushi(本枯節,lit.'true dried fillet'). When tapped together lightly, they sound almost metallic, and unlike their dull beige outer appearance, when broken open they are a translucent deep ruby color inside. Rarely, very high-endhonkarebushirepeat this drying process for over two years.
In the Edo era, it was common forkatsuobushito go through an extra step, the so-called tebiyama style(手火山式,tebiyama-shiki)process. After the fillets are boiled and their rib bones removed the fish are put in steaming baskets stacked atop one another for one to two hours a few meters above a burning wood fire.  These are rotated to assure an equal exposure to the smoke. The result is more flavorful and resistant to deterioration. Due to the extra cost and facilities required only a few factories followingtebiyama-shikiremain.
Traditionally, chunks ofkatsuobushiwere shaved as needed with an instrument similar to awood planecalled akatsuobushi kezuriki.
Todaykatsuobushiis typically sold in bags of small pink-brown shavings, which vary by thickness: smaller, thinner shavings, calledhanakatsuo(花鰹), are used as a flavoring and topping for many Japanese dishes, such asokonomiyaki, while the larger thicker, calledkezurikatsuo(削り鰹), are favored for making the widely used dashi stock.
In addition to making dashi, other popular uses ofkatsuobushiinclude:
Themycotoxinbeta-nitropropionic acidhas been found onkatsuobushias well as inmisoand insoy sauce, two other Japanese fungal fermented products. Certain strains ofA. glaucusare reported to produce mycotoxins.
Due to the smoking process which involves tar and charcoal, amounts ofbenzopyreneexceeding EU standards, as much as 37μg per kilogram, have been detected in commercially soldkatsuobushi.As a result, they have been banned for sale in the European Union.