Why does instant coffee foam when stirring it? I have seen that when preparing coffee without milk (just water and coffee), instant coffee (at least Nespresso) foams when stirring it just a bit. However, regular coffee grounds do not.
Does someone know the reason for this?
I have noticed the same thing. I believe it occurs because minute gas particles in the instant coffee, trapped during the production process, are suddenly released. This gas (air or something inert) rises to the top and bubbles or foams. It dissipates immediately, unlike crema.

I don't think it's caused by any physical chemistry stuff, like heat from dissolution or any actual reaction, though I do notice it more with very hot water.