Wasabi is a condiment that many of us first encountered in the context of sushi — it was largely unknown outside of Japan until the North American sushi boom of the 1980s. That unfamiliarity may explain why most of us still haven't encountered real wasabi—but we've developed a taste for the thing we think is wasabi.
Wasabi is part of the Brassicaceae family, which also contains horseradish, radishes, and mustard. They all share the same heat-producing chemical compound, allyl isothiocyanate, which is only triggered when its cells are broken down, i.e., by crushing mustard seeds, or grating wasabi or horseradish.
The compound is a volatile one that evaporates quickly, which is why we feel the heat of wasabi, horseradish, and mustard in our sinuses. It's also more water-soluble than the oily heat-producing compounds found in chilies, which is why chili heat lingers even after a glass of water, but wasabi and horseradish heat fades quickly from the tongue.
Wasabi heat and flavor also break down quickly on the plate. Fresh-grated wasabi hits peak pungency after about five or ten minutes, and loses most of that heat after fifteen or twenty. For this reason, wasabi products are often either dehydrated (powdered), or preserved in an oily paste.
But real wasabi products are tough to come by, and rather expensive, because the wasabi plant is difficult to cultivate. Wasabi originally only grew in cool Japanese mountain streams, protected from the sun by the shade of overhanging trees. It sounds idyllic; it's also very specific.
A lot of commercial wasabi now grows in riverside mud banks, but it's still a challenging plant to grow, and it takes about a year and a half for the plant to reach the maturity at which it can be harvested. In the U.S. there are a handful of wasabi cultivators in the Pacific Northwest, and most of their crop goes to restaurants. If you are lucky enough to find fresh wasabi on sale in a Japanese market, it retails for anything between $40 and $100 for a pound—though a pound is a lot of wasabi.
Like ginger, turmeric, and galangal, the part of the wasabi plant with widespread culinary use is the rhizome, the horizontal plant stem that produces the roots of the plant. In some sushi restaurants this rhizome is dashed against a coarse paddle to produce fresh-grated wasabi to order, but the wasabi served at most sushi restaurants in a green swirly blob is more likely to be horseradish and green dye than actual wasabi.
The same is true of wasabi products on supermarket shelves. I have three wasabi products in my kitchen. One is labeled "wasabi sauce," and it's really a soybean oil mayonnaise made with a "root blend" of horseradish and wasabi. It packs about as much heat as a Dijon mustard. Next is a tube of wasabi paste, and again the ingredients state that it contains both horseradish and wasabi powder. The ratio of horseradish to wasabi is never disclosed, and as horseradish grows more readily than wasabi and is much cheaper, I suspect that the horseradish dominates the mix.
Both of these products are North American; the third is powdered wasabi in a green tin with a picture of a root on the label. This powdered wasabi was imported from Japan and bought in an Asian market. Alas, this isn't wasabi at all—it's horseradish powder mixed with mustard and corn flour, with yellow and blue food dyes to produce the distinctive pale green color. Even buying Japanese doesn't necessarily mean buying authentic wasabi. Indeed, wasabi is sometimes called "Japanese horseradish," but even if you see "Japanese horseradish" listed in the ingredients, that may just mean "horseradish from Japan."
So if you decide to buy wasabi powder—or to save yourself a little effort and buy prepared wasabi paste—be aware that you're probably not actually buying much or any wasabi, and for the sake of your pocketbook that may be just as well.
Economics aside, it might not even matter if the wasabi you buy at your supermarket isn't the real deal—most of us aren't used to the taste of true wasabi, and we may even prefer horseradish, which packs more heat. Real wasabi offers a sweeter, more aromatic flavor and a more delicate pungency. Sushi should be made with just the right amount of wasabi on the rice to provide the flavor balance the chef wants to achieve, but that doesn't stop many people from dabbing more wasabi on with their chopsticks, or stirring it into their soy sauce for extra heat.
There is something exhilarating about the heat of horseradish-wasabi, and if that's what you're looking for, the most convenient way to capture that heat is to buy "wasabi" paste in a metal or plastic tube—one that won't retain any air, which hastens the loss of flavor. The paste should be stored in the refrigerator and used within a few months. Wasabi powder will keep in its tin in a cool, dark place for several months. Powder is used to make paste, but it's also great stirred into tempura batter or mixed with panko crumbs to make a crust for chicken or pork.
Wasabi paste is obviously a great compliment to sushi, sashimi, and other Japanese dishes like soba noodles, but it's also worth trying in places where you might use spicy mustard or horseradish, such as a salad dressing or on a sandwich—it's delicious in an egg mayonnaise! You can stir it into ketchup, mayonnaise, or guacamole for a hot spread or a spicy dip, or whisk it into soy sauce— the way so many people do in sushi restaurants —to make a marinade for beef.
Whatever you use it for, the product you use probably isn't real wasabi—but it probably isthe flavor you know and love.