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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?

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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.

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Both instant coffee and regular ground beans "foam" when pouring hot water over them.

Here you can see a coffee cupping (quality assessment), where they pour fresh off the boil water onto coffee grounds:

Cupping

Most of the gas released during any kind of coffee brewing is the CO2 created during roasting. While brewing filter coffee (in a V60 for example) this initial phase of degassing is called the bloom.

Instant coffee also degases a bit when brewing, here is an extreme example:

enter image description here

When either of those coffees is left alone for a while all of the trapped gas escapes, similar to a soda which is left open. When the gas has escaped and the "foam" disappeared you cannot create a foam on top again, unless you whisk it really really fast.

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