1. Why Grating Wasabi Creates a Sharp Sting in Your Nose
When you grate wasabi, you feel a sharp stimulation in the back of your nose. This sensation comes from volatile mustard oils, a type of substance called isothiocyanates. "Volatile" means the substance easily moves into the air. According to documents from Shizuoka Prefecture, wasabi cells contain a substance called glucosinolate, which is the precursor to mustard oil. If physical force, like grating, breaks the cells, enzymes within the wasabi break down this precursor to create mustard oil [1]. This means that while the plant is growing, the spicy precursor and the enzyme are separated. They only meet and create the sting when you grate the root and break the cells.
Because of this, the way you grate changes the spiciness. Materials from JETRO (Japan External Trade Organization) explain that the finer you grate, the more cells are destroyed, leading to more spicy compounds and a stronger kick [2]. There is also a measure of quantity. The most common mustard oil, making up about 90% of the total, is allyl mustard oil. The materials state that in 100g of raw wasabi, there is about 0.3g (0.3%) of this substance [1]. Since a 1-yen coin weighs 1g, this is about one-third of a coin. This figure indicates the amount contained in 100g of raw wasabi. It does not mean this amount of finished mustard oil sits in the root; rather, it is what is produced from the precursor when grated. The plant holds only the materials for the spice, waiting to be grated. What kind of place is needed to grow this plant?
2. Why Wasabi Grows Best in Spring Water With a Constant Temperature
Among wasabi relatives, the water wasabi (or sawa wasabi) grown in water is said to originally grow wild in cold water flows. JETRO materials state that a water temperature of 10–15°C is suitable throughout the year [2]. A publication by the Ministry of Agriculture, Forestry and Fisheries explains that wasabi is weak against both low and high temperatures and requires a large amount of spring water at around 15°C all year. Spring water with a constant temperature protects wasabi from winter cold and summer heat [3]. In the Amagi foothills of Izu, cultivation uses spring water at 13–15°C [6]. Spring water is chosen for wasabi fields because its temperature changes little with the seasons. However, Shimane Prefecture’s cultivation guide points out that just pouring cold water is not enough. What is missing?
3. How Stone Beds Bring Water and Oxygen to Wasabi Roots
In summer, when water temperature rises and dissolved oxygen (oxygen dissolved in water) becomes insufficient, soft rot disease increases. Shimane Prefecture’s materials explain that while water coming from upstream contains plenty of oxygen, this oxygen is consumed and drops suddenly if the water stagnates [5]. A method called the "tatami-ishi method" (layered stone-bed method) answers this problem. According to Ministry of Agriculture, Forestry and Fisheries materials, the ground is dug out, and large stones, small stones, and sand are laid in that order. Wasabi is planted on top, and spring water flows from the upper level [3]. The application for a Globally Important Agricultural Heritage System (GIAHS) organizes the effects of this method. Larger stones are used in lower layers, and spring water is flowed through, allowing water to pass not just over the surface but inside the bed. This filters out impurities, stabilizes water temperature, and delivers nutrients and oxygen. Because the water is cold and rich in oxygen, soft rot is less likely, and root growth is better [4]. The tatami-ishi method was developed in Izu around 1892. It then spread throughout the Izu region and to the Shizuoka region [4]. Wasabi cultivation itself is said to have started about 400 years ago in the Yutoku area of Shizuoka during the Keicho era (1596–1615) and later spread to Izu [4]. In about 400 years of history, the stone-bed method was added in the recent 130 years. Still, it was a big step for stable annual cultivation. So, when are these roots harvested?
4. Why Wasabi Takes Over a Year to Harvest and Is Stingier in Winter
It takes time to grow wasabi from seed to harvest. Izu City materials state that the cultivation period is 18 months for the main Izu variety "Mazuma" and 12 months for other varieties [6]. This means the plant grows in the same field, bathed in water, for 1 to 1.5 years. Roots can be harvested all year. However, wasabi that grows slowly from late autumn to winter is said to have a better spicy flavor [3]. This environment is becoming harder to maintain. Shizuoka Prefecture’s preservation plan states that in recent years, more wasabi fields have become difficult to cultivate, likely due to climate change impacts like global warming. Wasabi is easily affected by changes in water temperature and volume. It is difficult to raise seedlings during the hot summer, and a shortage of seedlings is also a challenge [7]. Note that this is described as a "possible" cause. For that one moment of grating, spring water and stone beds spend over a year growing the materials for the spice. When you eat a bite of wasabi, you can remember that long time.
5. An Activity: Comparing Coarsely and Finely Grated Radish
What you can do today: Take a radish from home and grate it two ways: "coarsely grated" and "finely grated." Taste them little by little to compare. This is an experiment to check with your own tongue if the "finer grate, more spicy" rule from wasabi materials applies to radishes too. Materials do not say the result will be the same. That is why it is worth checking. To avoid cutting your fingers on the grater, do this with a family member. If you want to read materials: Shizuoka Prefecture’s website has an agricultural information page explaining wasabi components [1]. Izu cultivation and the tatami-ishi method can be read in the Ministry of Agriculture, Forestry and Fisheries publication [3] and the GIAHS application [4].