Fresh wasabi was once served solely in high-end Japanese restaurants - in Hong Kong, anyway - and often, only if you asked for it; the chefs weren't going to waste the expensive ingredient on people who didn't know any better. Wasabi paste - the kind you squeeze out of a tube - and wasabi powder, which is mixed with water, couldn't be more different to fresh wasabi stem grated on a sharkskin grater. The colours are similar but, if you look closely, you'll notice fine shreds in the fresh wasabi.
Many people mistakenly believe that the sinus-clearing pungency of wasabi paste (served in mid-range restaurants) and wasabi powder (used at the cheapest places) is the flavour of Wasabia japonica; a friend who has a Japanese restaurant says some of his customers complain that the freshly grated stuff he serves has no flavour. Wasabia japonica, in fact, is much more subtle; it has a mild pungency and gentle heat. The powder and paste are actually made from a plant from the same Brassicaceae family as wasabi: horseradish, which is much larger, with a stronger, more fiery taste.
One of the reasons Wasabia japonica is expensive is that it is hard to grow: it needs clean, running water. The plant is believed to have myriad medical uses - it has anti-inflammatory properties and is said to strengthen the immune system, alleviate arthritis and even cure certain types of cancer.
Fresh wasabi stems can be found in shops such as Apita and City'super.
To maximise flavour, grate it at the last minute.