There is evidence of wasabi being used as far back as 14,000 BC in ancient Japan, and it is still most associated with the country’s cuisine today, particularly sushi, as the antibacterial properties of the plant can kill bacteria in raw fish. It is part of the brassica family, similar to horseradish, and while generally thought of as a root it is in fact a stem. The whole plant can be eaten, including the leaves and flowers, and it takes two years to grow big enough to be harvested, hence its high price.
Wasabi is very hard to grow commercially, particularly outside Japan, but some companies in the UK are using watercress beds to mimic the moist environment needed to cultivate it. Fresh wasabi is very different to the pastes available in the supermarkets; the stem loses both flavour and pungency after just ten minutes of grating and pastes generally contain less than three percent actual wasabi – the rest is a combination of food colouring and horseradish.