When and why are there bubbles in the cappuccino foam?

You prepare a cappuccino with a perfect and compact foam. Then you serve it and suddenly a lot of bubbles start to appear on the surface. Has this ever happened to you?

To answer this question, we’ve carried out a few tests with the help of one of our Barista trainers and Latte Art experts, Simone Celli.

When and why are there bubbles in the cappuccino foam?

For this test he has compared:

  • coffee with different roasting degrees
  • coffee with different degassing times
  • milk with different fat contents
  • milk with different agricultural production practices
  • vegetable alternatives such as soy or almond milk

Once the cappuccinos were prepared, we took a series of photos of each cup a few minutes after preparation. The images that were taken were then analyzed with a software which is able to count and measure the bubbles in the foam excluding the “micro-bubbles” not visible to the naked eye. The reference measure taken is therefore the percentage of large bubbles on the cappuccinos surface.


The tests showed that a darker roasted coffee slightly increases the creation of bubbles that tend to destroy the cappuccino foam. This is because a coffee with a darker roast produces a higher percentage of CO2 than a lighter roasted one. The same is true for fresher roasted coffees compared to those roasted a longer time ago.


No differences were found between the organic and traditional milk, while the differences between skimmed milk and whole milk are clearly evident.

In fact, the skimmed milk develops three times more bubbles than the whole one. What has surprised us is that the non-homogenized milk has a greater stability than the homogenized version of the same milk. This happens because when the air is injected into the milk with the steam wand, the milk proteins (mainly casein) surround the air bubbles, protecting them, without causing them to burst.When and why are there bubbles in the cappuccino foam?

This process leads to the creation of a microfoam, the cream of our cappuccino, which degrades over time because gravity draws the liquid coating from the surface of the bubbles making the walls thinner. These weakened micro-bubbles form larger bubbles which then collapse.

For this reason homogenized milk produces a less stable foam than a non homogenized one.

When and why are there bubbles in the cappuccino foam?


What emerges is that much less bubbles develop in the soy cappuccino than the one with cow’s milk and the cream is much more stable. In the almond cappuccino, on the other hand, the bubbles were greater, a difference given by the nature of the proteins that help stabilize the bubbles.