People sneered back in 2006 when Coke introduced Coca-Cola Blak: the fate of the coffee-flavoured soda was sealed after Anderson Cooper taste-tested it on TV and spat it out on the floor.

But this summer, cold, fizzy coffee is the latest must-order for the caffeinated geek set, though not all bubbles are created equal.

The level of fizz depends on the fizzifier, which could be the same blast of CO2 given to pop, a subtler suggestion of mild effervescence from an infusion of nitrous oxide, or a simple, bubbling top-up of tonic water.

“We call our nitro-infused coffee ‘morning beer,’ ” laughs Rob Kettner, the owner of Victoria’s Hey Happy Coffee. “CO2 gives a sour acidic taste, but nitrogen lends itself to sweetening things up. You can compare carbonated coffee to fizzy lager and nitro-infused coffee to Guinness; it’s not fizzy, but it cascades and has a head with a softer, creamier flavour profile.”