I'm sure many of you have been seeing the trend swirling around the internet lately called Dalgona Coffee (or whipped coffee). It's sort of like a backwards latte: milk on the bottom, frothy coffee on the top.
Being the curious food scientist that I am, I tried digging into how this worked and I couldn't find any articles about it online, so I did my own research into espresso crema, surfactants, instant coffee composition, and food foams. Based on everything I learned, I hypothesize that this is how it works:
Instant coffee is made up of 23% protein and contains a small amount of fatty acids. Combining 2 Tbsp water with 2 Tbsp of instant coffee and 2 Tbsp sugar creates a very highly concentrated beverage with enough protein to stabilize a foam.
As the mixture is whipped, the agitation forces tightly clumped protein molecules apart and incorporates air. The proteins rearrange forming a matrix around the air bubbles. The hydrophilic (water-attracting) end of the molecule attaches to the liquid phase and the hydrophobic (water-repelling) end moves toward the air. This is how the foam is created.
The sugar is needed to help stabilize the foam. Sugar is hygroscopic (water-binding), and since it is in the mixture at a high concentration, it binds the water between the air bubbles forming a syrup that prevents the water from leaking out.
It's potentially further stabilized by the small amount of fatty acids in the mixture acting as surfactants (lowers surface tension). This is what happens in espresso crema, but the level of fatty acids in instant coffee is very low.
The small air bubbles in the foam lead to a light fluffy texture, and very pleasant mouthfeel. 2 tablespoons is a lot more than one serving of instant coffee, so the foam is a little more bitter than you might expect.
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