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Why Use Cold Milk to Froth a Cappuccino?

post #1 of 3
Thread Starter 
There is a direction for all cappuccinos that I froth that I need to use cold milk to make the froth. I have no problem with this, and I see the results are better when I do. I'm not disputing the need to use cold milk, but rather I was wondering if anyone knows why it works so much better?

I guess the science geek in me just wants an explanation, and I have not been able to find one on my own so far. Thanks!
Deglazed
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post #2 of 3
Milk froth isn't stable compared to whip cream for example. To help it stabilize, you need to slow down molecules motion. The easiest way to do that is to cool them down.

Frothing also adds heat through friction so it won't be as cold when you're done frothing as when you started so the colder the better.
post #3 of 3
For once, the answer is simpler than you might think. We use cold milk, as cold as possible, to keep from cooking the milk.

Making good cappuccino or latte milk is a four part process. Froth, roll, bump, and polish.

When we froth, we hold the tip of steam wand at the top of the milk, so the holes are partially submerged. This gets a lot of air into the milk, mostly at the top.

We roll the milk, by submerging the wand all the way (or nearly all the way) to the bottom of the pitcher. Sometimes we angle the picture to facilitate the "rolling action." That is, the milk swirls around the picture -- if you don't see it happening, it isn't. During this phase more air is being introduce into the milk, the air is distributed more easily, and the size of the bubbles are reduced.

To bump the milk, bang the pitcher a few times onto a hard surface. The point is to collapse any big-bubble froth floating on the surface (big bubbles are less dense, and float).

Finally, swirl the pitcher a few times to evenly distribute the bubbles through the milk.

Okay -- so you knew the what already, you wanted to know the why. Unfortunately it was necessary to break down the steps a bit -- you'll see why when we get to the "nuances." But first the biggie: After about 150F, milk doesn't heat -- it cooks. It cheeses. It curdles, i.e., starts to separate into solids and water. And it also starts to smell something awful. As we say in law, "not a good result."

On the more nuanced side, bubble size is influenced by the heat of the liquid. Hotter liquid makes for bigger bubbles -- especially at atmospheric pressure (on top of the milk) when you're blowing the steam in sideways. If you're doing latte art you want the milk to end up with the smooth texture you get form tiny bubbles, what's termed "chrome." That means very small bubbles. So, you stop frothing at around 85 - 90F. On the other hand, for cappuccino, you want definite, stick-on-your-lip froth, which takes larger bubbles; so you'll take the frothing stage up to around 100F.

Pracitcal notes about temperatures: If you're using an analog thermometer, understand the thermometer will have a little lag time -- instant read sizes tend to run about 5F slow. You can get pretty good by taking the temperature with your fingers. Touch the pitcher about half way down the level of the milk. "Warm" is around 90F; "hot" around 120F; and "can barely tap it," around 145F.

Colder milk plus chilled pitcher equals more time to control the process of gettng air in. More time means large pitchers take longer, which is not a good thing; or less skill required which is. Some machines produce a lot of steam, some wands have big holes; in combination they don't give you a lot of time. That either makes steaming large pitchers easy, or makes it so small pitchers require a lot of skill. "Skill" sounds positive at first blush, but "skill" means making mistakes to gain it, then paying attention to use it. Neither thing is particularly desirable for making coffee at home.

Hope this helped,
BDL
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