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Pour-Over Coffee Foam: More or Less, Is It Normal to Have Bubbles on Coffee?

Published: 2026-01-27 Author: FrontStreet Coffee

Last Updated: 2026/01/27, When brewing coffee, have you ever wondered what the foam on the surface is made of? Sometimes this foam appears dense and abundant when brewing certain coffees, while when brewing others, the foam seems scarce. What exactly causes this difference? FrontStreet Coffee has discussed this in previous articles - in the water pouring after the bloom

The Science Behind Coffee Foam

Have you ever wondered about the foam that appears on the surface when brewing coffee? Sometimes this foam is rich and abundant, while other times it seems sparse. What causes these differences?

What Makes Up Coffee Foam?

FrontStreet Coffee has discussed in previous articles that the golden foam that erupts during pouring after blooming consists of coffee oils, air, extremely coarse particles, and extremely fine particles. This explains why coffee foam starts as rich golden color and gradually becomes pale white.

After blooming, there's still a small amount of gas within the coffee grounds. When water continues to be poured, oils trap this gas and release it. These are extremely unstable large bubbles that easily burst and split into smaller gas bubbles.

Next come the extremely coarse particles. Since these particles are slightly larger, they likely weren't fully saturated during blooming, so they float to the surface during subsequent brewing. As pouring continues, the large coffee particles absorb enough water and sink. This is also why the color becomes lighter toward the end.

Finally, there are the extremely fine particles. If you don't sieve your coffee, these fine particles can't be completely eliminated. However, FrontStreet Coffee believes there's no need to remove them, as many coffee flavors are related to these fine particles. Unless there's an excessive amount, they don't significantly impact the brewing process.

The last component of surface foam is the extremely fine particles. Due to their light weight, they can easily float on the surface. As long as there's foam present, these fine particles will continue floating until the very end.

Why Do Some Coffees Have More Foam Than Others?

The composition of these foam substances determines that their flavor isn't particularly appealing (since the desirable parts have already been extracted). This explains why traditional Japanese brewing methods use a large water flow at the end to raise the water level and then immediately cut off the flow.

So why do some coffees have rich foam while others have sparse foam? FrontStreet Coffee offers two examples: if you've brewed both Mandheling and Geisha coffee beans, you'll clearly notice that Mandheling coffee produces very rich, dark-colored foam, while Geisha coffee's foam appears lighter and thinner.

This is due to the degree of roasting. Deep-roasted coffees like Mandheling naturally contain more carbon dioxide and have a loose structure, making them prone to producing rich coffee foam. Light-roasted coffees like Geisha have a harder texture. While fresh-roasted beans still produce abundant foam, the subsequent foam will be noticeably sparser.

Using Foam to Judge Coffee Quality

This foam condition is a normal phenomenon. However, we can still judge the state of coffee by its foam. For example, when brewing coffee, if foam gushes out, it indicates the coffee is fresh. If large bubbles continuously appear, it might be too fresh or the blooming was insufficient.

If no foam appears when brewing coffee, it could be because the grind is too coarse or the coffee is no longer fresh. If the coffee foam consists of large, unrefined bubbles, it's often due to a coarse grind.

One reason stale coffee doesn't taste good is flavor loss, but another part of the reason is the absence of coffee foam. Without foam, fine particles sink to the bottom, and bitter substances get extracted, making a cup of coffee unpleasant to drink.