What happens when you throw any solid into boiling water is that the surfaces, which are very jagged on a microscopic level, provide "nucleating sites" where water molecules can latch on and form bubbles of vapor -- to see this in action, throw a bit of salt or sugar into some water just at the boiling point. Or, an even better example of this are the Mentos fountains people make by dropping Mentos candies into bottles of Diet Coke.

An interesting and somewhat related phenomenon (well, actually the opposite, I guess) happens when you super-cool a bottle of water under pressure (sealed cap), and then either shake it or open the cap and release the pressure -- it will instantly freeze up. Fun to do, just throw a bottle of water in the freezer, and pull it out when extremely cold but not yet frozen, and it will often do this when you open the cap.

The gas coming up from your beans is simply water vapor.