Bubble Analysis (LOL)

(I can't believe I just spent an hour writing about bubbles in coffee while doing my laundry. Regarding a forum post.)

Well, first you need to nail down what it is we're looking at. Big bubbles? Ok - at least we can agree on that. So what is a bubble, and what are bubbles doing in your milk? (And is that a 4.5oz Acme porcelain cup?)

A bubble (for the purpose of this discussion) is just a bag of gas. The gas is inside the bag, the bag itself is made of liquid. If the liquid is all of the same type - i.e. it's evenly distributed - then the particles (molecules) of liquid are all pulling on each other with equal force and the membrane of liquid is stable. But if the liquid becomes uneven because of temperature, mechanical, or chemical changes, then some parts of the bag are pulling at other parts of the bag in an uneven fashion, and the bag will break - i.e. the bubble will burst. Sometimes when you have lots of little bubbles, some of them will break just enough so that their contents will combine - two bubbles become one.

Your latte (they're all lattes - I don't care if you prefer to call them capps or flat whites or magics)... has at least two types of bubbles in it. There are bubbles from the espresso (a), and there are bubbles from the steamed milk (b).


The bubbles from the espresso are there because roasted coffee has gasses trapped in it, mostly carbon dioxide from bits of the bean being burnt during roasting. These gasses are extracted from the bean by the espresso machine, and they end up in espresso. Between roasting and brewing, gases in the bean will slowly escape... so if the bean has been freshly roasted, it still has lots of gas in it, compared to the same bean a few weeks later.

The bubbles from the milk are there because some people like the texture of bubbly milk. Many people prefer the texture of milk that is steamed in such a way that the bubbles are so small that you cannot see individual bubbles - "micro-foam" is the slang for it. A lot of lattes are served like this. (Skipping the bit about how to get the bubbles into the milk. :))

Back to your picture - it looks like there are regions of micro-foam, and then there are larger bubbles here and there. The larger bubbles seem to be evenly distributed in size... so it looks like they were all created in the same way at the same time. It also looks like the larger bubbles are mostly in the brown parts of your drink, not in the white parts. Well now we're getting somewhere... because the milk foam by itself seems to be holding up just fine. It's the espresso that's giving you big bubbles.

What makes the bubbles in some espresso more unstable, than the bubbles in espresso? It goes back to how fast the espresso is changing. Espresso "crema", or the bubbly bit, is largely composed of oils that float on top of water - order an espresso, and play with it for a few seconds, and you will be able to see this upfront. But what's in the water under the crema? All sorts of sugars, acids, salts, and more. Maybe you'll remember this from school... or maybe just from personal experience... but acids and oils just can't leave each other alone... they tend to react with each other, and turn into all sorts of other things. That sounds like a fight waiting to happen - in fact, highly acidic coffees will have less stable crema for this very reason. The degree of acidity in a coffee depends both on the green bean, and how the green bean was roasted.

So in conclusion...

(a) the freshness of roasted beans tends to give you more or less crema, but you can have a lot of very stable crema;

(b) the milk micro-foam is stable in your picture; if there were random sized bubbles in the milk, we might suspect an uneven frothing to be the cause; if there were evenly distributed large-bubbles of even-size in your milk, then we might suspect the chemical composition of your milk (try steaming a processed soy-milk - it often behaves like detergent); but these don't seem to be the cases today;

(3) it's probably the case that your coffee had a high "ratio of acids to oils" present it in after the roast stage; but that doesn't have to be a bad thing, unless you're addicted to stable crema.

At the end of the day, you should tell us - how did the coffee taste? :)

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