The insight that 'knowledge is knowing that a tomato is a fruit; wisdom is knowing not to put it in a fruit salad' is widely attributed to Miles Kington, the late British newspaper columnist who wrote with wit and originality on a variety of subjects, but not extensively on rhubarb.
Rhubarb is the tomato's polar opposite, defined as a vegetable but eaten as a fruit - except, according to The Oxford Companion to Food, in Iran, Afghanistan and Poland, where it is used in savoury dishes.
In the US, it officially ceased to be a vegetable in 1947 when the customs court in Buffalo, New York, decided that, because Americans used it mostly for puddings and jams, it must be a fruit. The court did not adjudicate on the wisdom of putting it in drinks, but judging by the popularity of the Rubabu in the seven international outlets of Rainer Becker's Zuma chain, the group's chief mixologist, James Shearer, must be quite a wise man. It is the top-selling cocktail in all of them.
Shearer created the Rubabu for the opening of the first Zuma in London in 2002, and has since taught the mixologists in all the other outlets how to make it.
'We are based on fruit flavours and freshness,' says Zuma's Hong Kong Bar and Lounge manager Ian Wong. 'We're not out to make very alcoholic-tasting cocktails. Balance is very important, and we try to incorporate some Japanese and Asian influences.'
The key ingredient of a Rubabu is rhubarb-infused daiginjo sake. The bar handles the infusion process itself, and it involves chopping up the stalks, and boiling them in water for two hours, adding sugar at the halfway point. Sake is added to the resultant pulp, and the liquid is then strained back into the bottle.
Fresh passion fruit pulp provides aroma and texture, while 42 Below vodka gives the drink some structure and the drinker a buzz.