If it weren't for foam, what fun would espresso with milk be? (Not very.) Foam is not only what gives us the warm and comforting feeling that a beautiful cappuccino or latte has going down, but it also is what lets us have beautiful designs on top of those drinks.
Today, we honor the magic of foam by sharing a few facts about its creation, its deliciousness, and why not all of it is created equal.
Why does milk foam when you steam it?
Steam is created when water vapor and air are forced into milk via the steam wand of an espresso machine. But it's not enough that air is introduced; the milk also needs to be heated for that foam to stick around.
You know how an egg starts to get solid and opaque in a hot skillet? The same things that cause those changes in an egg are what happen in milk when you heat and steam it: milk proteins, which start out bundled up like little balls of yarn, will unfold (denature) when exposed to heat, allowing them to interact with each other more strongly.
The proteins chains in milk are polar: one end of the chain is hydrophilic (attracted to water), and the other is hydrophobic (repelled by water). Because milk is mostly made up of water, as soon as those proteins unfold, exposing their ends, the hydrophobic ends immediately try to get as far away from that water as possible. If you were to look at a single tiny bubble in a cup of foamed milk, you'd see that the hydrophobic ends of the milk proteins are all pointed inwards, towards the water-free interior of the bubble, while the hydrophilic ends stay put in the aqueous environment the bubbles are suspended in.
This structure helps keep the air bubbles intact for a long time after the steaming process, all the way into your cup (and into your happy, soon-to-be caffeinated mouth).
Just like with those eggs in the pan, there's a right and a wrong time to add the air and create the foam. You wouldn't want eggs to start to set in a skillet before scrambling them, and you don't want the milk proteins to become so denatured that they will prevent large air bubbles from being broken down into the smaller ones that make microfoam. (More on microfoam in a minute.) Eggs that are stirred and scrambled right from the moment they hit the pan will create smaller curds than eggs that are allowed to set a bit before scrambling. Same with milk: milk that has air introduced when cold will form smaller bubbles than milk foamed after heating.
Does skim milk foam better than whole milk?
Well, remember that bit about the hydrophobic and hydrophilic ends of the proteins I mentioned before? Hydrophilic ends are also attracted to fat particles. Just as they allow air and water to form a stable mixture in a foam, they allow milk fat and water to form a stable fat-in-water emulsion in a carton of milk. The more fat you have in the milk, the more viscous it feels.
So what does this mean for foaming? The absence of milk fat in skim milk means that those proteins wind up orienting themselves with their hydrophobic ends pointed inwards towards the air bubbles being injected into the liquid by the barista, in order to get as far away from the water as possible. This regular structure helps strengthen the bubbles' walls.
While this means drier foam, it doesn't necessarily mean better foam. For my money, the best type of foam is that which will easily and thoroughly mix with the espresso for the optimum coffee-milk balance in the cup. Dry foam tends to plop (or be plopped via a barista with a spoon) out on top of the coffee like a raft in a pool, which means that the drinker experiences his or her coffee in layers rather than as one cohesive beverage.
What is 'microfoam' and what's so great about it?
Now, see, microfoam is the kind of quality foam we want in a cappuccino or a latte, and while it might be harder to create with skim milk, it's most definitely possible—and muy appreciated by skim drinkers most of the time.
The term "microfoam" describes a quality of frothed milk in which the bubbles are so small and so numerous that they can't be seen, but they can be felt on the palate—namely as a texture that is something like liquid velvet. Or, you know, heaven in a mug.
This type of tight, luscious steamed milk will marry completely with the espresso when poured correctly, rather than separate into distinct layers of thin liquid milk and dry, stiff froth that lays on top of the coffee in question.
Why do people like "bone-dry" cappuccinos?
I have no idea.
No, really, I have no idea: I'm actually not trying to be snarky. If really dry foam tasted like whipped cream, I would get it, because everyone loves scooping whipped cream off of hot drinks and eating it. But really dry foam, as far as I can tell, just tastes a bit like hot stale milk. The dryness of it means that the foam structure is mostly made of air, which doesn't taste like anything, and the addition of so much excess air overwhelms the natural sweetness that milk normally contributes.
If it's a texture thing, though, I guess I can see that. Mostly I think people order bone-dry cappuccinos for three reasons: Either they believe that is the way that the drink is 'supposed to be', or they are trying to avoid the problem of not having enough foam on their cappuccinos and don't know a better way to specify that to the barista, or they are trying to cut calories by making the beverage lighter in milk liquid overall.
If you're a bone-dry cappuccino fan, please do speak up: I want to know what you like about 'em.
Is foam really the only difference between a latte and a cappuccino?
It depends who's asking, sort of.
Many folks claim that a "traditional" Italian cappuccino needs to be equal parts espresso, steamed milk, and foam, but many Italians actually prefer theirs senza schiuma, or without foam at all. So it might be the foam, but not necessarily. (Italians might also insist that the beverage is traditionally not ordered after noon, since it's taken in place of breakfast for many busy city dwellers.)
In Australian and New Zealand espresso culture, a cappuccino is served in 6- or 8-ounce handled ceramic cup with healthy microfoam and a dash of chocolate powder, while lattes (which have more foam than their U.S. and European counterparts) come in glass vessels. So it might be the cup, but not necessarily.
Most North American cappuccinos throw the idea of diminutive drinks out the window, and so "cappuccino" has come to mean "more foam than a latte," or, even, "espresso with a little steamed milk and a ton of foam." In specialty coffee cafés, however, and increasingly in more mainstream shops, cappuccinos tend to be more densely textured with foam but with the idea of balance in mind: A marriage of coffee, milk, and froth where no one element overwhelms the others. So it might be the relationship between the components, but not necessarily.
(There are, of course, also cappuccino "mixes" one can buy, wherein powdered milk and soluble coffee-like flavor substances dissolved into a kind of weird frothy concoction that will keep you warm if you're cold and keep you awake if you're tired, but they're about as much a cappuccino as I'm the Queen of England.)