Coffee foam vs. Tea foam

Question

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?

Answer 1

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."