I once asked a waiter at a restaurant how he managed not to mix up cups of regular and decaf coffee when bringing them to various tables. He replied that he could tell by looking at the bubbles – in one type, the bubbles linger, whereas in the other they disappear quickly (of course, I can't remember now which one he said was which).

However: I can't find any evidence or research that supports this, but I don't have any reason to believe that the guy wasn't being serious.

Is there any truth to this?

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    The only visual difference I personally know of is the label on the container the coffee is shipped in. However, waiters have other tricks to give the right drink to the right person. One of the most common is to stage the drinks on the tray in the same physical order as the guests at the table, from a reference position--usually where the waiter tends to stand for that table. This is what I did in my long ago youth. It also works for sodas which also tend to all look the same. – SAJ14SAJ Feb 12 '13 at 21:22
  • Even without the container, you can tell by the roast, and even when you make the coffee. Decaf tends to be darker and you need to grind finer when making espresso. But to tell after the shot is pulled? I suspect that the decaf does generally have less creme due to the extra processing. But it's been years since I had any decaf beans ... – Megasaur Feb 13 '13 at 10:05
  • @WayfaringStranger Can you post your answer using the 'answer' function rather than using a comment. Thanks. – Robert Cartaino Feb 14 '13 at 0:29
  • @RC Done, as requested. – Wayfaring Stranger Feb 14 '13 at 3:12

Coffee foam/bubbles is made up of a mixture of proteins, sugars, oil droplets, caffeine etc, and the lifetime of a bubble is highly dependent on the composition of its membrane. Given that decaf is coffee that's been solvent extracted, it wouldn't be at all surprising to see its different composition affecting bubble lifetime. However, no one seems to have conducted definitive studies of the matter. – answer upped from comments as requested.

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Background: I'm no expert, but I'm very picky about coffee. I prefer dark, smooth roasts that aren't too bold. I used to grind my beans by hand with a mortar and pestle prior to brewing in my French press, but I got lazy and addicted. However, I still pay close attention to the appearance, smell and taste of my coffee. Now I grind with the machine at the store and use a Keurig. I just had decaf coffee for the first time. I bought two bags of self-ground coffee beans: a bag of Sumatra and a bag of decaf Sumatra (Swiss water method, not solvent-extracted). I have had this brand/roast many times before as recently as a couple of days ago.

When I brewed my first cup of decaf, I freaked out because it looked weird coming out of the Keurig! There were teeny bubbles (not quite foam) swirling around as it poured, then they kind of grouped in the middle once it finished. They finally dissipated about halfway through the cup. But coffee DEFINITELY looked different.

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In some restaurants the decaf is instant. Instant and brewed coffee will look very different in terms of things like bubbles, or a little sheen on the surface. After all, the waiter wasn't telling you how to tell them apart in all circumstances, just how he tells them apart at work.

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