A while back, Copenhagen's Coffee Collective wrote about skimming the crema off the top of an espresso to improve its flavour. Crema is bitter, they argued, and coffee tastes sweeter without it.
But the Istituto Nazionale Espresso Italiano says that a ''certified'' espresso has a ''hazel-brown to dark-brown foam - characterised by tawny reflexes - with a very fine texture''. So no crema, no espresso.
(I tried it. For a few days I ordered two espressos at a time and skimmed one. Sometimes it tasted better, sometimes not. Depends on the beans. But espresso is definitely creamier with the crema. Duh.)
So what can crema tell you about your coffee? Veneziano Coffee's Craig Dickson is president of the Australasian Specialty Coffee Association. He's been an Australian barista championship judge for 10 years and a world championship judge for four, and judged the last two semi-finals.
''We used to look for dark brown or hazelnut crema with reddish highlights, but what judges expect has changed over the last couple of years,'' Dickson says. ''With lighter roasts you can get a lighter-coloured crema, so now we want baristas to describe how the crema will look.''
Crema consists of carbon dioxide bubbles in a film of fats, oils and sugars. Good crema is an even layer of fine bubbles that is ''elastic'', says Dickson.
To judge elasticity, you tilt the espresso cup to 45 degrees; the crema should stretch to cover the surface of the coffee, and re-form as an even layer when the cup is set right.
Good crema is an important part of espresso flavour and texture, even of milk coffee. ''A lot of the flavour in milk coffee comes from the crema,'' Dickson says.
And while some say that natural-process coffees produce more crema and washed coffees less, he doesn't reckon you can tell much about processing from crema.
What you can tell, he says, is freshness. If the coffee is too fresh - only a couple of days post-roast - there is too much carbon dioxide in the beans, and the crema dissipates quickly. If the coffee is stale - as early as 14 days post-roast, but certainly by 21 days - the crema will be thin.
How do you get good crema at home? Use freshly roasted beans from your local coffee roaster (not ground supermarket coffee), and grind fresh for each cup - don't leave the beans sitting in the hopper.
What about skimming? ''If you're removing the crema, you're getting into a filter-style thing,'' says Dickson. ''I'll leave that one for the real geeks.''
How does a strong cup of coffee make you feel?