If, by now, you have not heard of Dalgona coffee, or at least seen a thumbnail for a youtube video wherein an aesthetically pleasing cup of coffee is displayed against a (typically) white background, then you have either lost your phone/laptop or are lying and I don’t believe you. Searching for Dalgona Coffee will get you nearly 8 million results in 0.46 seconds. It is featured on youtube trending pages and littering Instagram feeds. I myself have joined the ranks and made my housemates a cold cup; I could not sing it’s praise enough… so much so I decided to write a whole article about the science and the chemistry behind this famous new coffee trend.
Foams, i.e. a dispersion of air bubbles in a continuous liquid/solid medium, can be things like polystyrene (solid foam), rising bread dough (semisolid foam) or whipped cream (liquid foam) and their stability is usually determined by the conditions of and the molecules in the solution.
When we look at the process of making Dalgona coffee, the most popular recipes tend to be volumetric, calling for equal units of instant coffee, to sugar, to hot water; once mixed in solution, the coffee-syrup gets whisked until stiff peaks form, with the finishing product resembling the stiff peaks of a meringue. The end result is a stable coffee-foam!
Foams, i.e. a dispersion of air bubbles in a continuous liquid/solid medium, can be things like polystyrene (solid foam), rising bread dough (semisolid foam) or whipped cream (liquid foam). Their stability is usually determined by the conditions and molecules in the solution.
If we look at espresso for example, a really important feature is that 10% of its total volume needs to be the coffee foam, or crema. The difference here is that the crema forms as a result of the brewing process of the coffee rather than through whipping, but the principle of the coffee foam stability can be applied to both. During the brewing process, hot pressurised water is forced through ground coffee beans, causing the cell walls of the solid coffee to break down and release carbon dioxide. The carbon dioxide is trapped as bubbles in the coffee liquid and, because the bubbles are less dense than the liquid, the foam rises to the top of the cup forming that distinctive, cream-like surface.
Now, you’re supposed to drink the espresso within minutes of it being made, however if you left it alone for longer you would see the crema slowly breaking down until you’re left with only the dark brown solution. The foam is able to last as long as it does because the gas bubbles are stabilised by a variety of surface active agents, called surfactants. These surround the surface of the bubble, electrostatically/sterically blocking the bubbles from coalescing. In coffee these could be acids, lipids, protein-like material and even solid coffee cell wall fragments. The crema however, does not last long. Over time, water drains from the foam and, in the case of the espresso crema, the fast drainage of the coffee liquid means that small amounts of oil, present at the interfaces between bubbles, will quickly interact with the surfactant barrier, leading to bubble destabilisation, coalescence and foam break down. This is why, if you try whipping the instant coffee with just hot water, the foam dissipates after only 20 minutes or so. This is why the addition of sugar is so important. It not only adds to the viscosity, i.e. thickness, of the solution so that the drainage happens much more slowly but, more importantly, the sugar crystals form a protective layer around the bubble, preventing the oil from interacting with the bubble surface. A really good demonstration for this was done by the youtube channel Flavor Lab!
So that’s essentially what’s happening! We have a sugary-coffee mixture and we’ve continuously whipped air into it forming really small, stable bubbles that we then pour over our milk, or milk substitute. My top tips, if you attempt to make it yourself: 1. use a hand mixer as opposed to a normal whisk because you will find yourself whisking for a long time in order to form the small bubbles, and 2. make sure to use instant coffee because you’ll need a highly concentrated solution in order to get enough surfactants to stabilise the foam. Otherwise, mix, whisk, sit back and enjoy!