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Popping Foam Bubbles

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Carlyn asks via Twitter,

 

Carlyn Oswald

 how come when I sprinkle cinnamon sugar on my espresso, the foamed milk on top starts to “de-foam”?

 

Hi, Carlyn,

When I first read your question, I started thinking about the cinnamon. There are a lot of substances in spices that allow them to feel and taste so interesting, and cinnamon gives off a warm sensation, so with all that, I started to wonder if maybe cinnamon might have some sort of anti-foaming agent in it. As near as I can tell, it does not. Take a look at this experiment that Schlumberger Excellence in Educational Development, or SEED, ran trying to show a few different spices, including cinnamon, and their effect on milk foam. Nutmeg seems to have a strong effect, while cinnamon has very little effect.

That being said, the spice that we think of as cinnamon is usually a spice called “cassia,” so there may be an effect with regular cinnamon. It would certainly be a simple experiment to run, especially if you have a method of creating milk foam in your house, or ready access to a coffee shop. I suspect that both you and the experimenter were using cassia. How can you tell? The quickest way is if you’ve tasted red-hots, or a similar style cinnamon candy. That is what real cinnamon tastes like; it’s hot and it’s sharp. Cassia is more mellow, and is what I think of as being the flavoring of cinnamon toast. That’s kind of tautological, though, because I always used cassia on my cinnamon toast. Not much use to those who may have used real cinnamon on their toast. So focus on the cinnamon candy: if it tastes like the candy, it’s probably real cinnamon. If it doesn’t, it’s probably cassia.

There is value in re-running the experiment linked above with real cinnamon, because you never know. Though when I think about what milk foam is, and when I read your question more carefully, there is a simpler answer waiting. You sprinkle cinnamon sugar on your foam, rather than just cinnamon. The sugar is important to this, because of what foam really is.

There are two parts to the surface of any bubble, and three parts to its volume. Inside a bubble is air, which is important when you’re considering how its volume might change versus pressure and how it can float and so on, but air isn’t so important for the destruction by cinnamon sugar. Another component to milk foam is the chain of proteins. When you heat milk with a steam wand, you denature its whey proteins, making them spread out on the surface of the water of the milk. Force some air into the mix with your steam, and the air will try to escape from the liquid. When the air rises out of the surface of the milk, if the whey has been denatured enough, then it will trap the air in a net of whey and water. The whey are like the rope in a regular net, and the water stretches in-between the whey “ropes”, which keeps the air from floating out into the atmosphere. Thus, the water and the whey make up the surface of the bubble, and air makes up the inside.

The thing about sugar is that sugar really likes water. Sugar will bond with water fast, and keep lesser molecules from having anything to do with its precious water. This is especially true the more highly processed the sugar is; sugar granules are pretty happy on their own, and it takes the temptation of some hot water to get the sugar to mix with it. I mean that literally: table sugar won’t dissolve well into cold water, but heat it up and it dissolves readily.

Superfine sugar will mix much more readily with even cold water. It’s called superfine not because it’s particularly attractive to sugar, but because the crystals have been broken up into much smaller, or finer, pieces. It may as well have been so named because of how attractive it is to water, however, because it mixes very quickly. More so for powdered sugar, which for reasons we won’t get into now, as there may be youngsters reading this, is often denoted by a number of ‘X’s .

In any case, if your cinnamon sugar is made with superfine or powdered sugar, which seems likely if it’s made to be sprinkled onto a foam, then the most likely thing that is happening is that the sugar is mixing with all the water in the foam, thus collapsing the foam. 

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    TheFoodGeek | 07/08/2011

    Lower-fat milk tends to make for better foam primarily because the milk producers put extra proteins into the milk to try to make up for the loss of flavor with the loss of fat.

    I did test out regular cinnamon this morning and table sugar, but I couldn't test cassia nor powdered sugar because I was apparently out of both. The cinnamon did pop some bubbles when it hit, but I think that was primarily a mechanical thing (like poking a bubble with your finger) rather than any sort of chemical interaction. When the cinnamon sat there, it didn't cause any extra loss of bubbles, whereas the sugar-side definitely did.

    The primary difference between the different types of sugar mentioned is the size of the crystals. So if your crystals are very fine, then it'll be more likely to soak up water, though even with the table sugar, as I mentioned, it was a pretty thorough defoaming.

    I'm glad to have helped, and thanks for asking!

  • CarlynO | 07/08/2011

    Very, very interesting! We make our own cinnamon sugar with evaporated cane juice, so I don't know if that is more or less fine than normal sugar. I just tasted my cinnamon, which happens to be "saigon" cinnamon from Vietnam. Apparently, saigon cinnamon has a higher oil content from other varieties so perhaps that changes something? Anyway, it definitely had a distinct "red-hot" flavor. I'm also wondering if the fat content of the milk used (we use 1%) would effect the rate at which the milk defoams. And based on your explanation, I'm also thinking that I should try to use Turbinado sugar/raw sugar in place of the evaporated cane juice to see if the defoaming occurs slower since the crystals are so huge...
    Thanks for shedding some light on this for me!

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