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During the lockdowns of 2020, many people spent their afternoons whipping equal parts instant coffee, sugar, and hot water into a thick, peanut-butter-coloured paste known as dalgona coffee. While it looks like culinary magic, the transformation from a dark liquid to a stiff peak is a classic demonstration of surface chemistry and fluid dynamics. If you have ever wondered why regular brewed coffee cannot hold its shape but the instant version can, the answer lies in the molecules created during the roasting process.
What makes the bubbles in instant coffee?
When you beat the coffee mixture, you are physically forcing air into the liquid. Pure water has a very high surface tension, meaning any bubbles you create will pop almost instantly as the water molecules pull together. To keep those bubbles intact, you need a surfactant—a molecule that lowers surface tension.
According to a 2021 review in *Critical Reviews in Food Science and Nutrition*, coffee foam is a complex system of gas bubbles stabilized by coffee components like proteins and polysaccharides, and influenced by the roasting process. These components act as natural surfactants that lower the surface tension.
These molecules are amphiphilic. They have a hydrophilic (water-loving) head that stays in the liquid, and a hydrophobic (water-fearing) tail that pushes into the trapped air. This arrangement creates a protective molecular skin around each microscopic bubble, stopping them from bursting or merging.
Why do you need sugar for thick foam?
While the coffee compounds create the bubbles, they cannot keep the foam stable for long on their own. If you whip instant coffee and water without sugar, you will get a thin foam that collapses within minutes, much like the crema on an espresso.
This is where sugar steps in. Sugar dissolves in the water to form a highly viscous, thick syrup. In any foam, gravity constantly pulls the liquid downward, thinning the walls between the bubbles until they break—a process known as drainage. The high viscosity of the sugar syrup dramatically slows this drainage, allowing the dalgona foam to hold its stiff peaks for hours.
Why doesn't regular brewed coffee foam up?
You might have noticed that whisking filter coffee or French press coffee yields nothing but a few temporary bubbles. This comes down to concentration and chemistry.
First, the classic dalgona recipe uses equal parts of coffee, sugar and water, creating an extraordinarily high concentration of surfactants. Standard brewed coffee is mostly water, which is far too dilute to build a structure.
Second, high-quality brewed coffee contains higher levels of natural coffee oils (lipids). Oils act as antifoaming agents—they penetrate the surfactant layer and pop the bubbles. The industrial drying processes used to make instant coffee often strip away these foam-destroying lipids. Instant coffee is essentially a highly concentrated, dehydrated extract, leaving behind a powder perfectly primed for whipping.
Understanding fluids and foams
The science of surface tension and viscosity explains everything from how soap cleans your hands to why insects can walk on water. If you want to see how altering density and forces changes how fluids behave, the Fermi Simulation Lab lets you adjust variables and watch the physics happen in real time.
For more on how temperature and pressure affect the behaviour of fluids and gases, read our guide on why rapid processes are adiabatic and slow ones isothermal. You can also test your intuition with the buoyancy and density simulation to see what sinks and what floats.
