What is the foam in pour-over coffee? Are white bubbles a sign of over-extraction? What's the relationship between espresso's golden crema and foam?

When we brew pour-over coffee, we always see many bubbles appearing on the surface due to the action of carbon dioxide, right? But have you ever observed that these bubbles sometimes look different?

During the brewing process, the color of the foam sometimes appears brownish-yellow, sometimes whitish; or when brewing at different times, its quantity is sometimes abundant, sometimes sparse. In short, it's constantly changing. Do you know that this ever-changing phenomenon actually conveys a lot of information to us? Come on, grab your notebooks, FrontStreet Coffee's mini-class is starting now!

What is the foam that appears during brewing?

When the pre-infused coffee bed comes into contact with hot water, a large amount of foam instantly bursts forth. Generally speaking, the foam that appears at this moment is brownish-yellow. I wonder if you've made any connections?

That's right, the crema from espresso! It's the unpressed oils themselves. The structure of this foam consists of carbon dioxide, oils, some fine coffee grounds, and larger coffee particles. When an espresso machine extracts coffee with added pressure, this foam emerges from the bottom along with the coffee, compressed into rich golden crema!

What information does the color of the foam convey?

As brewing progresses, we'll notice that the foam color starts to turn whitish, gradually changing from the original brownish-yellow to white. Some say that white foam is a warning sign before over-extraction, but is this really the case?

There are two reasons for the appearance of white foam. First, the oils from the coffee grounds have been depleted; second, the coffee particles floating on the surface begin to sink. Both of these are the "important pigments" that color the foam. Therefore, when brewing can no longer release oils and the coffee particles begin to sink after becoming saturated with water, the foam color will change from brownish-yellow to white.

To answer the previous question: is it true that white foam is a warning sign before over-extraction? Based on these two points, we can determine that this is indeed the case. The reason coffee particles float on the surface is that they weren't fully extracted initially, which left large amounts of carbon dioxide hidden within the beans, giving them the ability to float on the liquid's surface. However, as extraction progresses, these particles are gradually penetrated by water. When hot water fills their interiors, it indicates that extraction is approaching its end. If we continue to increase extraction efficiency, over-extraction will very easily occur. Therefore, basically in the later stages of brewing, we reduce the amplitude and speed of circular pouring, increase water flow, and allow hot water to pass through the coffee grounds more quickly, avoiding excessive extraction. We can observe the espresso extraction process—the coffee liquid changes from deep yellow to yellow, then to light yellow for this very reason!

The richness of foam

The richness of foam is mainly affected by two aspects: one is the degree of roast, and the other is freshness!

The influence of roast degree

Different roast degrees create differences in the quantity of foam. Dark-roasted beans, having undergone longer roasting times, have a more porous texture and can more easily trap large amounts of carbon dioxide. At the same time, they are easier to extract. Therefore, at the moment of brewing, the amount of foam that bursts forth is very abundant.

Light-roasted beans, on the other hand, have a harder texture and difficulty storing excessive carbon dioxide. Their hard shells also make them less easily extracted. Therefore, both the degree of blooming and the quantity of foam will be far lower than those of dark-roasted beans.

The influence of freshness

FrontStreet Coffee often says that from the moment coffee beans complete roasting, they enter a continuous degassing stage. Besides aromatic substances, the main gas they release is the carbon dioxide that constitutes foam. Therefore, when coffee beans are still in their fresh period containing large amounts of carbon dioxide, we can brew very abundant foam. But when they pass their fresh period and carbon dioxide is depleted, the foam we can brew becomes scarce.

Therefore, the roast degree and freshness of coffee beans, as well as the extraction process, are the information that these foam can convey to us. We can determine the roast degree and freshness by observing the blooming state along with the quantity and color of foam, then understand the extraction process through the color changes of the foam. This can enhance our further understanding of coffee extraction and help us brew more delicious pour-over coffee!

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FrontStreet Coffee

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