The Science of Steamed Milk

Microfoam maestro or foamy newbie, while turning our milk into a glossy cloud we often find ourselves focusing on:

The visual - how good is my latte art? How velvety and smooth is the milk?

The texture - how big are my bubbles, and how long do they stick around?

The temperature - did I denature or scald the milk?

But delving a little deeper we find that there are dozens of very scientific variables that influence the way our foam behaves. Some of this science is quite easy to remember, and can make your milk steaming experiences simpler and more consistent. 

We know you know how to steam milk, so we'll get into our top two science-based facts on why milk steams so that you can make informed adaptations to your method or ingredients, to get the perfect steamed milk every time. 

1. Protein.

Milk protein is something that's discussed often in barista circles, but did you know that different types of milk contain different protein levels?

Average Protein Content of Different Types of Milk

Milk % Protein
Skim milk 3.4
1% milk 3.4
2% milk 3.3
Whole milk 3.2
Soy milk 2.7
Almond milk 0.4


Beyond this, you'll find protein content also varies based on what the cow ate, whether the milk was collected in winter or summer, the breed of the cow, how the milk was processed....

So why does this matter? Because proteins are what binds the fat and the air together in tightly emulsified bubbles - the holy grail of microfoam. As we have all learned when overheating milk, when the proteins denature, your foam is ruined. On the inverse, if there's too little protein, your milk won't foam at all.

Morten Münchow, a Danish coffee scientist, has studied extensively the results of varying levels of protein and fat on milk for steaming. Münchow is adamant that the most important factor for stable foam is a high protein content - so much so that a British dairy company set out testing the milk of cows all around the UK to find a herd with the highest protein milk so they could release the perfect milk for coffee. 

How can we use this to help us create more consistently steamed milk? Luckily, protein is one of the macronutrients that must be listed on nutritional labels. You'll want to find milk with as close to 4% protein as possible - which will work out to roughly 10g of protein per 250ml of milk.

2. Fat.

We all know a barista who swears by skim milk for steaming, or the others who will look down their nose at someone who asks for anything but regular.

In the skim milk camp: as anyone who's left a little yolk in their white before whipping a meringue will know, fat is the enemy to volume. This is because the hydrophobic part of protein is as likely to attach to fat as it is to air, thus the more fat = the less air the protein will 'hold'. As such, it is true that lower fat content milks can get 'foamier', however you may find this foam to be almost 'too stable' - less smooth to pour, with a propensity to just plop into the cup: the latte artists greatest nemesis. 

In the full fat camp: you'll find those willing to sacrifice a little volume for a smoother, glossier and tastier microfoam. Full fat milk will create a velvety textured milk, while skim milk may come across as 'dry'. Not to mention the flavour carried by fat - and as full fat milk is only around 3.25% compared to skim milks 1-2%, we think it's worth the splurge!

Next up, discover how alternative milks stack up and read which ones we recommend for brewing Prana Chai!