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One sign of really good fresh well-roasted coffee beans is foam. When you pour hot water into the French press, it foams, often forming a head up to 2" high. And when you use an espresso machine, you get a nice foam called "crema".

However, if you pour hot water into a teapot and see foam, that's a sign of terrible tea and you should throw it out.

Questions: what chemical reaction is taking place in each of the two cases (coffee and tea), and why does coffee get less foamy when it gets older whereas old tea gets more foamy?

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2 Answers 2

A partial answer to start...

Crema isn't just foamy coffee. When you make espresso, hot water is forced through the grounds at high pressure, which extracts normally insoluble oils from the grounds and creates an emulsion of oil droplets and strong coffee.

Offhand, I can't find anything that explains why older beans don't produce as good a crema as fresh, it seems to be one of those things that "everyone knows."

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Could it be that old bean's oils have gotten rancid (oxidiced)? Rancid lipids smell and taste different, but I don't know if/how it would affect the emulsion. –  J.A.I.L. Nov 20 '12 at 7:49

During roasting, carbon dioxide is generated (the beans are burning, after all) and trapped in coffee beans. According to the book Espresso Coffee, a kilogram of freshly roasted beans contains as much as 10 liters of carbon dioxide. This carbon dioxide is the main gas component of espresso foam. Over time, unused beans will release that gas into the air, or de-gas, leaving less to create the crema on your espresso.

They say that coffee is not best immediately after roasting, but needs a few days to reach its peak. I've heard that this is because of the excess carbon dioxide, and I've heard that the "flavors need to develop." I'm not sure what the truth is. I don't normally get my beans that soon after their roasting, so I can't speak personally to the differences.

I don't know much about tea, so I can't help you there...

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I also couldnt find any definitive answers about foaming in tea, but dbenton is spot on about carbon dioxide causing the "foam" in coffee. When pouring hot water directly onto ground coffee its called a bloom and in an espresso the crema is actually coffee oils suspended in the carbon dioxide. –  leon Nov 5 '13 at 10:16

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