Wasabi or Japanese Horseradish (Wasabia japonica)

Wasabi has probably been grown in Japan for a thousand years. Wasabi cultivation in Japan ranges from Russia’s Sakhalin Island, north of Hokkaido (the northern most major Japanese island) to Kyushu (the southernmost major Japanese island). But the Shimane region has the largest collective area of wasabi production and breeding research in Japan. It is now being grown in many countries in the world including New Zealand, Taiwan, Korea, Israel, Brazil, Thailand, Columbia, near Vancouver in Canada and Oregon, USA. The Ministry of Agriculture and Fisheries introduced wasabi into New Zealand for experimental cultivation in 1982.
Wasabi is a member of the Cruciferae family, which also includes cabbage, cauliflower, broccoli, sprouts, watercress, radish, mustard and horseradish. The European horseradish, Armoracia rusticana is known as a distinct cousin of wasabi and is sometimes preferred as a substitute for wasabi by chefs.
Wasabi leaf

Wasabia japonicais a glabrous, perennial aromatic herb that grows about 45 cm high, producing leaves on long petioles from the crown of the plant. As the plant ages the rhizomes start to form and at maturity after 18 months the wasabi plant has a distinctive thickened stem (or rhizome) connected to the heart shaped leaves by long, thin petioles. Rhizomes are the most highly demanded plant part of wasabi ranging in length from 50 to 150 mm.
Two types of commercial cultivation methods are used, upland or soil grown wasabi or it is grown in flooded fields. In Japan wasabi plants grown using the upland soil production method are harvested primarily for leaf and petiole products while wasabi plants grown using stream cultivation are harvested mainly for their enlarged stem and used in products derived from this. It is believed in Japan that flooded systems produce superior high quality enlarged stems, which are highly sort after and therefore command higher prices.

Wasabi grown in soil
Soil grown wasabi

Upland wasabi requires an air temperature from 6 to 20°C with 8 to 18°C considered optimal. Soils of pH 6 to 7 are considered best. It is most often grown on well-drained soil under mulberry or plum treesin Japan. Whereas in New Zealand wasabi is usually grown in soil but in shade houses rather than under trees.

Water grown wasabi

Stream grown wasabi requires air temperatures ranging from 8 to18°C. However, a narrower range of temperatures (12 to 15°C) is considered ideal. An air temperature of less than 8°C inhibits plant growth and at less then 5°C plant growth ceases. Other factors have an effect on the growth of wasabi and need to be considered carefully e.g stable water temperature, good nutrient status in water and well aerated, neutral or slightly acidic pH of water, high dissolved oxygen level in water and a large quantity of water flow.
The unique environmental requirements and shortage of cultivable lands limit wasabi production areas to 880 hectares in Japan and 400 hectares in Taiwan, but demand for wasabi condiments is spreading from Japanese cuisine to modern western food.
The increasing interest in wasabi and the inability to expand production in Japan has seen prices rise steadily since 1970. High prices have stimulated research on soil production methods and the investigation of production areas outside Japan.
Since 1982, the cultivation of wasabi has been established in New Zealand because of New Zealand’s climate (appropriate air temperature range, high quality water, high sunlight hours), which is perfect to meet requirements for growing quality wasabi. Soil grown wasabi production has increased in Taiwan, Colombia, Canada, Korea, Thailand and the USA.
Wasabi cultivars are usually named in Japan after its region of cultivation and these cultivars are considered regionally specific in Japan. According to some Japanese farmers wasabi has eight well-known cultivars, which are, Mazuma, Daruma, Takai, Shimane, Midori, Sanpoo, Izawa Daruma and Medeka. These cultivars were developed in the Shizuoka and Shimane prefectures. Another important cultivar Hangen comes from the Kanagawa prefecture. Daruma is the most popular variety as it is known to grow well under marginal environmental conditions.
In New Zealand Daruma is used as the commercial cultivar of wasabi but preliminary studies on another cultivar, Shingen have just commenced.
Wasabi adds a unique flavour, heat and greenish colour to foods and thus it is a highly valued plant in Japanese cuisine. Wasabi’s flavour is described as ‘a sharp hot taste with pungent smell’ but the heat component in wasabi is different from chilies and the hotness quickly dissipates in the mouth leaving an extremely pleasant mild vegetable taste, with no burning sensation at all. Wasabi adds aesthetic and culinary appeal to many foods and is considered a staple condiment in the Japanese diet. Recently, it has found widespread appeal in western cuisine due to its magical expertise to change an ordinary dish to an extra special one by the addition of a spicy flavour.
All the plant parts of wasabi posses flavour but vary in sharpness they deliver and are therefore used for different purposes. Basically wasabi can be served in three ways. These are: as a condiment on the side of a dish, as a spice or herb in a dish and wasabi flavour in processed foods. Rhizomes are the most demanded tissues used to prepare fresh paste to placed in a mound on a dish next to sliced raw fish (sashimi), spread on the raw fish in sushi preparations, or served on a small dish to accompany a bowl of cooked noodles. Sometimes grated wasabi is mixed with other ingredients like soya sauce and vinegar to prepare a dip for use with raw fish other dishes according to individuals’ choice. Tofu (soybean curd) is often decorated with grated wasabi. Lower quality wasabi stems are sometimes mixed with European horseradish powder, mustard and food colour to produce wasabi paste in tubes or to sell as wasabi powder. Wasabi paste preparation in traditional Japanese cuisine, wasabi is prepared by grating the fresh stem against a rough surface, such as ginger grater, in much the same way that horseradish is prepared. Sharkskin or “oroshi” has been used as a tool for grating wasabi in Japan since the earliest times and is still regarded as the preferred method of obtaining the best flavour, texture and consistency in freshly ground wasabi. There are several types and sizes of sharkskin graters available in specialty food shops in Japan.
Constituents of wasabi Isothiocyanates (ITCs) are a group of naturally occurring sulphur compounds responsible for the chracteristic flavour of wasabi. The compounds are volatile and are evolved from plant tissues when they are disrupted e.g. in the preparation of food, grating, cutting, chewing etc. However, plant tissues do not contain ITCs but contain glucosinolates (GSLs), which are the precursors of ITCs. GSLs are a group of glucosides, stored within the cell vacuoles of all Crucifereaeplants. GSLs coexist in plants, but are not in contact with the hydrolytic enzyme myrosinase in the intact cell. When plant tissues are mechanically disrupted or injured (e.g. by chewing, crushing or grating in the preparation of food) the enzyme is released from the cell wall, and in the presence of adequate moisture myrosinase rapidly hydrolyses the GSLs to yield glucose and an aglucone. The organic aglucone is unstable and undergoes Lossen Rearrangement to produce sulphate and a variety of products. The nature of products is dependent on a number of factors, including the structure of GSL side chain, the reaction conditions (e.g. pH), the presence of cofactors (e.g., metal ions, specific protein), temperature and durationas well as the age and condition of plant tissues. Isothiocyanates (ITCs) are formed from GSLs under neutral and alkaline conditions. However, GSLs, which contains a β-hydroxyl group (I in Figure 2) in their side chain, give rise to ITCs that spontaneously cyclise to form oxazolidinethiones. Some aromatic and heterocyclic GSLs (II in Figure 2) produce ITCs which are unstable at pH 7 or higher and break down to release the corresponding alcohol and inorganic thiocyanate ions. However, once formed ITCs are more stable under acidic conditions. In weakly acidic pH’s or in the presence of Fe+2and/ or in presence of endogenous nitrile factor, nitriles are produced from aglucone by autolysis instead of ITC, with the liberation of elemental sulphur. The relative proportion of ITC to nitriles can vary widely depending upon the conditions of autolysis. Different ITCs have been reported from previous investigations in wasabi and each ITC has a specific flavour profile and so the complete taste of wasabi derived from the combined tastes and odours of all the ITCs present .
Figure 2 to go here:, Biochemical Pathways