A surfactant is a compound that lowers the surface tension between two substances. Surface tension is the force that makes molecules of liquid at the surface stick together. It’s the same force that makes water hold together to form droplets. Bubbles in pure water burst immediately, because surface tension pulls the liquid back together.
The best way to stop a bubble from bursting is to wrap it up in a surfactant. In foam chemistry, the name for this wrapping process is adsorption, where a substance adheres to another one and makes a film across it. Don’t confuse it with absorption, which involves a substance moving into the spaces inside another one — adsorption happens only on the surface. In coffee beverages, surfactants make bubbles less likely to burst because they reduce the surface tension between the milk and the air. Low surface tension means more elasticity in a foam. For example, if you try to stretch a bubble surrounded by liquid, you reduce the concentration of surfactant around the bubble. This raises the surface tension which requires a lot of energy. When you’ve got the right amount of surfactant, the system is more likely to spring back to its original size rather than bursting. Elastic foams with low surface tension gives the latte artist more time to pour while making the pattern last longer.
The principal surfactant in milk foam is the protein β-lactoglobulin. This very elastic whey protein — on its own, without other protein present to mess things up — contributes to the production of extremely long-lasting foams. The key is ‘elasticity’. Imagine each foam cell being a fresh rubber balloon – when you push it or squeeze it, it deforms and recovers its shape. If the rubber is a bit old and brittle, it can’t so easily respond to stresses and will pop. As discussed in this Foam Elasticity app, the factors controlling the elasticity are complex,