Why does milk foam? The science behind your latte.

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Oh it’s fall in the Northeast again- the perfect time to switch from iced coffees to silky lattes, cappuccinos, and macchiatos. When I was younger, I remember wishing there was something warm that tasted as good and had a mouth feel as good as ice cream. Luckily, I’ve pretty much lost my sweet tooth and these coffee drinks have fulfilled that wish. Though espresso alone is one of the most delicious things on Earth, add some foamed milk and those tiny velvety bubbles transform it into perfection.

I was making a latte for myself one morning and my daughter asked if I could froth her milk, too. What a great idea, I thought! So these first few chilly mornings of the fall, we’ve been cuddling up and sharing some warm, silky drinks together. I’ve included some coffee-free recipe ideas for you to try with your little ones, too. It’s a great way to bond and also explain a little science. So, what is special about milk that allows it to foam?


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The Science of Milk Foam

Even though milk looks like a solution, it is actually a colloid. Colloids are one state of matter (gas, liquid, solid) suspended in another state of matter. Emulsions are a type of colloid where tiny, microscopic droplets of one liquid are evenly and stably mixed into another liquid. Milk is an example of a complex emulsion, where fat droplets are stabilized by proteins and suspended in water (remember fat and water will usually separate into two layers). The water also has a number of other proteins, minerals, and carbohydrates dissolved in it, and some of those proteins, called casein, are formed into little groups called micelles.

When you want milk to foam for coffee, you heat it, then introduce air through whipping or through the steaming process, itself. The interplay between the components of milk (proteins and fats, mainly) and this heating process will affect what your foam looks and tastes like. As the air is introduced, the proteins in the milk, mainly casein, will form spheres around the air and stabilize the bubble. The fat in the milk actually destabilizes these bubbles. Though milks will foam without heat (i.e. whipped cream), the steaming process further stabilizes the bubbles in the foam through melting the fat droplets (which, remember, are destabilizing the bubbles). If you use fat-free milk to try to avoid this problem, you will get lots of foam, but it will be dry and not really mix in with the coffee (but if that’s what you like, do it!). Also, if you heat the milk too much, the proteins will start to unwind (the proper word is “denature”), clump together, and not be able to stabilize the air bubbles.

So in brief, casein proteins in the milk arrange around the air/water interface and stabilize the bubbles. The fat in the milk destabilizes the bubbles by disrupting that protein layer, but without it, your foam will be dry. Also, if you’re using the foam in coffee, the fat helps deliver some of the coffee flavor. It’s a lovely balance of variables that is fun to play around with to discover the tastes and mouthfeel you enjoy. Also, lots of liquids with proteins will froth with stable bubbles in similar ways. An example is egg whites, and if you’ve ever whipped those before you know to not get any yolk into the whites since it will destabilize the foam. Another example is the liquid you pour out of canned or cooked beans, like garbanzo beans! You can even make meringues and pavlovas with it. Here is a link to a recipe!

Foaming milk may seem pretty complicated, and really, to get a perfect milk/espresso drink is difficult without skill and proper equipment (that’s why we will pay for good baristas, after all), but you can approximate this process easily at home. After you do it several times, slightly adjusting things like heating or your frothing process with each iteration, it will be so fulfilling to finally figure out the combination of bubble size, fat content, froth amount, coffee amount, etc. that you like the best. There is no wrong answer.

If you want to go even MORE into the science behind milk foam, check out the video at the bottom of the post where Them Huppertz, a principal scientist at NIZO Food Research, goes into incredible detail about this fascinating system.


Froth your own milk at home

If you want to make coffee drinks, there are tons of different ways to make espresso- from fancy machines to just using strong espresso grounds in your drip coffee maker. Whichever you chose will depend on your financial situation and how much you care about coffee. Right now, we just do the latter. For frothing your milk, here are several options to choose from:

Fancy Frothers

Nespresso VertuoPlus Deluxe Coffee and Espresso Maker by De’Longhi with Aeroccino, Black

You can go all out and buy an espresso maker with milk steamer or frother. An “affordable” (still $$$) and small model is Nespresso. It makes great espresso (with creama, even!) and doesn’t take up a lot of room on the counter. This also comes with an Aeroccino, which froths your milk for you. (We had this model…had…but our apartment got broken into and it was stolen!!! *crying*)

And here is the Aerocinno on it’s own. It is great at frothing milk with just a push of a button if you already have another way to make coffee or espresso.

Electric Whisks

PowerLix Milk Frother Handheld Battery Operated Electric Foam Maker, Durable Drink Mixer With Stainless Steel Whisk, Stainless Steel Stand & eBook Include

This little thing is also a milk frother, but significantly cheaper. This is what we have now, and though it doesn’t make the foam as silky as a professional frother or barista would, it gets the job done. To use this one, you should heat the milk so your bubbles will last longer. How hot you heat it will depend on what you prefer, so try different temperatures. I put about a cup of milk in a large mug for 1.5 minutes in the microwave. Some people like it hotter, some colder. After heating, just use this whisk for 15-20 seconds, and you will have foam! Here is a quick video from America’s Test Kitchen for this technique.

Jars

Yup, you can just heat milk, then put it in a mason jar or some other sealable container and shake it. Done!

Ball Pint Jar, Regular Mouth, Set of 2

No matter what way you choose to froth your milk, how you like your foam, or how you like you coffee, let this be a stress-free process. I didn’t get into specific temperatures or milk types and stuff because honestly this is all about preference. Try several things to figure out what you like, that’s my best advice. For example, my sister loves cappuccinos with tons of dry foam, but I think it’s flavorless. Go figure.


Frothed Milk Recipes for Kids

To bring your kid into this (because mine LOVES frothed milk) here are some kid friendly drink ideas you can easily make along side your caffeinated version so you can find another excuse to cuddle up this season with your little ones.

  • Drink it plain if your kid loves milk.
  • Add a spoonful of pumpkin puree, some cinnamon, and maybe some honey
  • Add some cocoa powder and sugar for a frothy hot chocolate
  • Add turmeric for yellow milk (this stains if it spills!! But my daughter loves it)
  • Add orange zest for a hint of creamsicle

Depending on the age of your kid, you can discuss the science of the process with them, too! If they are curious about proteins and what they are, try reading our book about proteins for kids, The Baby Biochemist: Proteins (Volume 3). For younger kids, you can compare milk to water, for example asking them “Do you think water would foam up like our milk just did?” and use their answer for a quick experiment (try frothing water) and discuss what is in milk that isn’t in water.


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I hope you enjoy a nice latte and a little dive into some kitchen chemistry today!


Sources:

  1. Milk Foam: Creating Texture and Stability

2.

3. Protein distribution at air interfaces in dairy foams and ice cream as affected by casein dissociation and emulsifiers

4. Observations on the Air-Serum Interface of Milk Foams

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