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August 29, 2020

Steaming Milk - Theory Behind the Microfoam

Using  the steam wand of an espresso machine is the most common way of steaming milk. But what really happens when introducing heat, air and tiny water droplets into the milk?

FAQ about steaming milk 

Why hot steam creates bubbles in milk? How are the bubbles created? 

The air is captured by the milk proteins which act as a surface-active component in the liquid.
The fat content and the generated foam structure define how shiny and smooth your foam is.
Also, more cold the milk, more viscosity there is.

Can milk be steamed for too short or too long time? 

Yes. You need enough time to get the milk to +55-65 Celsius. This is this optimum for most of the milks. Below that, the sweetness is not yet blossoming and the drinking experience is not nice due to luke-warm coffee. 
Also the fat content of the milk plays a role. The more fat, the longer it takes to create foam as fat globules compete for space on the surface of air droplets. 
When getting over 65 Celsius (or not even there when steaming non-dairy milks), the proteins start to unravel more and more and you will end up with clumpy milk that smells weird and burns your tongue. This is also the point where the foam structure loses its shiny appearance – even with fat in the milk – if you overheat the milk you loose the shiny, smooth microfoam structure as the protein starts to denaturate. 
And you might have noticed, that hot milk tastes sweeter than cold? This is because the human tongue is naturally more sensitive to sweetness when things are hotter. This also explains why a cold soda tastes refreshing and balanced, but a warm one is cloyingly sweet.

How coffee and milk interact in a cup? Any differences between coffees and milks? 

The main reaction from the barista point of view is between the acids of the coffee and the proteins of the milk. 
Also, fat can block some flavors and also enhance some. Milk and coffee generate also new flavors when combined. Especially pleasant flavor characteristics such as creamy and caramel notes can be detected when you use milk with more than 1,5 % fat. Fat in milk also helps to reduce astringency. 
Note that when choosing plant based drinks you should evaluate which of them are best for your coffee of choice. One coffee can be great combined with dairy but turn into misery when combined into some vegan milk.

Which kind of milks work best for steaming? 

The more fat, the easier it is to create microfoam that is shiny and silky. On the other hand too much fat is not appreciated with all the customers and the fat content also relates to the flavor and mouthfeel. Prefer milk with at least 1,5% of fat to get the positive influences of milk fat for both texture and flavor. With reasonable fat content you will gain nice and shiny microfoam and it is easier for you to pour latte art.   
Fat free milk gives you big and rigid foam, much like shaving foam actually! Not usable for latte art and definitely not nice mouthfeel. 
Once again: proteins. It is not about having them a lot but having correct proteins and having them in balance. With the correct balance the foam stays stable longer meaning not separating into liquid and airy bubbles. 
Proteins react differently to heating. Casein is pretty heat tolerant. Whey on the other hand is not but it foams well and is more water soluble. Whey also coagulates a bit already during steaming but when keeping the temperature below 65 c, coagulated whey maintains the foam structure. If going higher than 65 c, whey structure changes and all the foam disappears. Note that proteins are very different in cow milk and non-dairy milk products!
For example in Finland there is 32-40g proteins per a milk litre. Globally the content can be over 50 g! 
The more carbohydrate, the more sweetness. Note that enzymized lactose free milk tastes very sweet because the carbohydrates contain glucose and galactose that are very sweet in nature. You might want to consider if sweetness is “the thing” in all your espresso based drinks or can less be more?
Milk might make your coffee taste a whole different!

Microfoam without a steam wand? 

Based on this, it is easy to understand why we cannot create microfoam when heating the milk on a direct heat source such as a hotplate. We can for sure add air with an eggwhisk or one of those small automatic frothers. And there are many milk frothing devices in the market, too. However the cheapest and easiest way to do that is to use a French press. After heating up the milk, pour it to a French press decanter. Then pump the piston up and down pretty fast until you get a nice foam. 
Now that you know what happens in the pitcher when steaming, you can take the next step and start preparing a delicious cappuccino.


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