On Milk and Foam

Drat! I accidentally deleted an email from a Greek pastry student that contained an interesting question. That is: why will cold milk foam but warm milk won’t? The answer is butterfat again, even though milk doesn’t have near enough of it to form a foam that has any long-term stability. It’s still fat crystals (along with some miscellaneous proteins) that are helping to reinforce the bubble walls…temporarily. But butterfat crystals can only form in cool temperatures, so warm milk won’t foam nearly as well.

The worst thing about losing this email is that this particular Greek student offered to share some good recipes with me. So please do write back…because I’m greedy!

12 thoughts on “On Milk and Foam”

    1. Good point. That’s done with steam, right? I’m not a coffee drinker so I don’t really know. However as I mentioned, milk protein molecules will gather around bubbles, giving them some measure of reinforcement. But the effect is temporary. Those foams melt into cups of coffee within a few minutes.

      Thanks Eva!

      – Joe

  1. Speaking of milk and foams, I once read some tips from coffee baristas, who said they used fat-free milk for the froth. Works like a charm. I even had an argument with my Italian friend, who said that’s not possible. At the time I looked for an explanation (Corriher, McGee), but couldn’t find anything. Any thoughts on the science behind that?

    1. Hi Anna!

      Check the answer below. Milk proteins (especially whey proteins like lactoglobulin) will hold up a low fat foam…but only for a very short time. My guess is that the thinness of the milk is more conducive to the frothing process (thick liquids like, oh say, honey, don’t whip up very well) and the proteins in the milk reinforce the bubbles…but only for a very short time.

      – Joe

      1. Really interesting, as I’m an addictive latte drinker and often froth milk – I even tried to experiment with several milks and write down their fat and protein content to find out any correlations. There was none! One milk didn’t make foam at all while another with similar fat and protein content behaved much better (at home I use this tool http://www.amazon.com/HIC-Brands-That-Cook-005/dp/B0002KZUNK, in office there is steam wand). But the general impression was that higher fat milks froth better, and I see that baristas here are usually using about 3,5% fat milk.

        1. Hello again! I think in general that’s true, though there are several variables at work here: temperature, fat content, degree of frothing, method of frothing…you could really go nuts with this — and it’s clear that some people have!


          – Joe

    2. Baristas here certainly don’t use fat free milk for froth unless requested by the purchaser. Fat free milk will give you a short-lasting, big bubbled froth quite easily, but what a good barista wants is a long-lasting silky microfoam, almost indistinguishable from whipped cream to look at. Normal 4%-ish milk, started off cold, is the trick. I suspect that starting off cold is actually to give it a decent amount of time in which to foam before it gets too hot to use, rather than any big difference in foaming ability between warm and cold milk.

  2. Over here you can find a good explanation about foaming milk and the differences in fat content: http://www.coffeegeek.com/guides/frothingguide/milk

    Milk for latte/cappuccino is indeed steamed to foam it, but it takes a while before the milk is indeed hot (especially since you want your milk as cold as possible when starting). So in the beginning, while it is still (quite) cool, you get the biggest increase in volume, and after that you’re just heating the milk so that you don’t get cold coffee. And when you overheat it, the milk will collapse/get a weird taste, because the proteins collapse.

    The weird thing is that my manual milk frother advices to heat the milk first and then foam it. And indeed, if I foam it first and then heat it, I get less foam…

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