A New View of Milk Foaming

April 07, 2023

A New View of Milk Foaming

by Prof Steven Abbott

We all agree that it’s the proteins in milk (dairy or non-dairy) that allow the milk to form a desirable foam, with “desirable” here defined as a proper silky microfoam. The proteins act as “surface active agents”, surfactants, by going to the air-water interface to stabilise it. But after that we enter a world of factoids and opinions. I confidently mouthed one of those factoids during a recent podcast, saying that we needed to denature the proteins in order to get the milk to foam. How wrong I was! Disentangling all those factoids has been a fascinating challenge. Most of them are “facts” in a specific context that then get quoted out of context and then become “well-known facts”, like the one about denaturation that I had parroted. It’s time to clean up the story. The double good news is that the new story is simple, and that it allows the coffee community new opportunities to foam creatively. Let’s start with foaming itself.

Creating lots of stable foam

A ground-breaking paper (Ref) by a top team at U Sofia in Bulgaria showed that if you create a foam properly, the amount of foam and its stability depends only weakly on the surfactant system being used. This was a great surprise (even to them) as it was well known that some surfactants are better at foaming than others. The trick is to foam properly. Many foam tests, which show big differences in foamability, create foam in a sub-optimal manner. So what’s the proper manner? Get the required volume of air as bubbles in your liquid, then quickly break the bubbles into smaller and smaller sizes. Small bubbles are automatically much more stable than larger ones (I have some apps to prove it – Refs), so you get a good foam. What this means for latte foaming is that we should find an optimal foaming technique before worrying about the milk. In my own work on fire fighting foams and on foams for cosmetics, I’ve often used a latte whisk to show that a relatively poor foam produced by “standard” techniques can become a great foam if you use a proper technique. The latte whisk has many faults, but it was a useful demo device. In my own mind I could imagine a smarter technique, but I couldn’t create it.

Separating the variables

Then I stumbled across the NanoFoamer. It seemed to me to be exactly the scientific foaming tool I needed, so I bought one on the spot. I stress that this was with my own money and was based on my scientific judgement. This isn’t some sponsored promo piece. Whether it is suitable for latte art (at which I’m a klutz) is not for me to say. Here we’re sticking with the science...