1. How 6 kg of raw bonito shrinks to about 900 g of dried bonito
Before considering the hardness, let us look at the numbers. According to a sample from the Kikkoman International Food Culture Research Center, a raw bonito weighs about 6.0 kg and is about 70% water. This fish becomes "arabushi" (rough dried bonito) weighing about 1,300 g for four pieces, with about 24% water. When it becomes "katsuobushi" (fully dried bonito), four pieces weigh about 900 g with about 15% water [1].
The source describes this as a yield of about 1/5 for arabushi and about 1/6 for katsuobushi relative to the raw fish [1]. The diagram in the source explains that 75% of the raw 6 kg fish is used, resulting in about 900 g of katsuobushi. Since 900 divided by 6,000 is 0.15, the figure of about 1/6 is a rough number that fits this calculation.
The Ministry of Agriculture, Forestry and Fisheries explains that katsuobushi is defined as having 26% or less water [2]. This 26% is an upper limit for what can be called katsuobushi, which is different from the about 15% seen in fully dried katsuobushi above. To remove this much water from a fish that is mostly water, the same steps are repeated many times. The first stage involves boiling and smoking.
2. How boiling and about 10 days of smoking make arabushi
The process starts by boiling the bonito. In an example of arabushi from a paper by Shunji Takenaka of Kobe University, the fish is boiled at 85°C for 70–90 minutes, then smoked at 80–90°C for 5–6 hours. This is repeated 8–10 times over 10 days [4]. Drying with smoke is called "baikan".
The number of times varies by manufacturer. The Ministry of Agriculture, Forestry and Fisheries states that baikan is done about 10–20 times. A product smoked only once is called "namaribushi" [3]. The number of times and days differs from the previous example, which suggests that methods vary depending on the maker.
Smoking does more than just dry the fish. According to an explanation by Makoto Koizumi of Miyagi University, smoking dries the fish at a low temperature, allowing it to dry evenly while keeping internal moisture. It is explained that the fish becomes a glass-like state [6]. However, this is from an explanatory article, not a study with numerical data, so it should be understood as one perspective.
At this stage, the water content is about 24% for arabushi [1]. However, the katsuobushi sold in shops has even less water and is harder than arabushi. The difference is made by mold.
3. How mold helps remove even more water from arabushi
Applying mold dries arabushi further. Mold is applied to arabushi, dried in the sun, and then scraped off. The Ministry of Agriculture, Forestry and Fisheries explains that katsuobushi is produced by repeating this process at least twice [3]. However, names and numbers vary by source and maker; the Kikkoman source defines katsuobushi as starting from the fourth mold application [1]. In traditional methods, this is repeated about 4 times over about 6 months, causing fat breakdown, flavor development, and color changes. The mold used is from the Aspergillus genus [4].
Note that the Ministry of Agriculture, Forestry and Fisheries states that katsuobushi takes 60–80 days, which differs from the about 6 months mentioned. The duration varies by source and method [3].
An experiment shows that mold reduces water content. In a study by the Chiba Prefectural Fisheries Experimental Station, mackerel katsuobushi had 28% water before mold application, which dropped to 16% after the fourth mold application. Lipids also decreased slightly [5]. These are figures for mackerel katsuobushi, not bonito.
How does mold reduce water? Explanations state that the mycelium growing on the surface draws out internal moisture, advancing drying [6]. However, academic backing for this mechanism in the sources reviewed here is limited to mackerel data and explanatory articles. Therefore, we accept this as an explanation that is "said to be" the case.
4. How the hardness of arabushi and katsuobushi differ
There are examples comparing hardness with numbers. In a measurement listed on the Kobayashi website, the Mohs hardness of katsuobushi is 6–7, comparable to quartz. Arabushi is 5–6, comparable to glass or a knife blade [7]. The result shows that katsuobushi with about 15% water is harder than arabushi with about 24% water.
However, this number is from one measurement example on a company website. Other bonito may not have the same numbers, and no sources were found to support claims like "the hardest food in the world." It is best to understand this as "in one measurement, it was comparable to quartz."
There is a Japanese Agricultural Standard (JAS) for the water content of shaved bonito flakes sold in shops. For bonito flakes, the water content must be between 9% and 17%. However, packs filled with inert gas in airtight containers are allowed up to 21% [8]. This standard is for flakes, not for the whole block.
Looking at this, the about 15% water content of katsuobushi falls right in the middle of the flake standard. This means a hard block is shaved into thin flakes with water content in the 10% range. When and how was this method established?
5. How the dried bonito method developed in Tosa and spread to West Izu in the Edo period
The foundation of the current manufacturing method was created in the Edo period. According to Ministry of Agriculture, Forestry and Fisheries materials, the method of drying with smoke and then applying mold was devised in Tosa (Kochi Prefecture) in the mid-1600s [3].
Materials from Shizuoka Prefecture state that the method was transmitted from Tosa to West Izu [9]. According to Kikkoman materials, "Izu-bushi" was made in Izu by applying mold many times without boiling. Then, a person named Yoichi from Tosa taught the boiling method and the method of shaving the surface. This led to the creation of "Improved Izu-bushi," which has the same steps as today: boiling, smoking, shaving the surface, and applying mold many times [1].
The same source introduces the view that the smoking method itself was generally believed to have been transmitted from the Maldives [1]. However, this is a "general view" and not a definitive fact.
Let us look at current production. In 2021, the national production of katsuobushi was 25,772 tons. Kagoshima Prefecture produced 18,921 tons, and Shizuoka Prefecture produced 6,598 tons [9]. Adding these two prefectures gives 25,519 tons, which is about 99% of the national total (this percentage is calculated from the source figures). The process born in Tosa continues today, centered in Kagoshima and Shizuoka, after passing through Izu. Thus, the accumulation of boiling, smoking, and applying mold has created the hard block.
6. How to examine a single bonito flake against the light
If you have a pack of bonito flakes at home, take out one flake and hold it against the light from a window. How much does it let light through? You can also observe how much it bends before it breaks by gently bending it with your finger.
If your home has a whole block of katsuobushi and a shaver, it is good to touch the block with an adult. Because shaving uses a blade, always leave the shaving task to an adult.
The label on the pack lists ingredients and the manufacturer. If there is an origin label for the katsuobushi, look for Kagoshima Prefecture or Shizuoka Prefecture. Do these match the two prefectures seen in the production data?