Why Does Pour-Over Coffee Have Foam? Is Foam in Pour-Over Coffee Good or Bad?

Introduction

When brewing pour-over coffee, many find it "therapeutic" to create rich coffee foam. Have you ever wondered where coffee foam comes from and how it affects the brewing flavor?

Coffee Foam

Those who have brewed pour-over coffee have certainly experienced rich foam emerging during the first pour after the bloom. The coffee foam that appears in the early pouring stage shows a golden-yellow color, while the foam from later pours gradually turns white and becomes thinner.

To uncover the mystery of this foam, FrontStreet Coffee decided to scoop up the floating foam during brewing to investigate. FrontStreet Coffee normally uses a three-stage pour method to brew Yirgacheffe (medium-light roast) and Brazil Queen Estate Yellow Bourbon (medium-dark roast). After completing the second pour, the surface coffee foam was scooped up with a spoon.

During brewing, we could observe that the foam from the medium-light roast Yirgacheffe was noticeably thinner and lighter in color compared to the medium-dark roast Yellow Bourbon.

When we observed the scooped surface foam, we could see it consisted of three parts: coffee oils, extremely coarse grounds, and extremely fine grounds. The fine grounds were particularly evident in medium-dark roast coffee.

From these two comparisons, we can see that the initial foam appears dark brown or golden-yellow because extremely coarse grounds have low water absorption and are light in weight, so they initially float on the surface. As continuous pouring washes over them, the extremely coarse grounds gradually become saturated with water, become heavier, and sink to the bottom. Consequently, the foam on the liquid surface gradually becomes lighter and whiter. Some extremely fine grounds, being light in weight themselves, are easily absorbed by the coffee foam and float on the surface.

What Does Foam Taste Like?

After daring to taste a sip of coffee foam, the texture clearly felt rough with coffee particles. The flavor wasn't as negative as imagined - slight woody and roasted notes with other off-flavors. When tasted alone, it wasn't particularly unpleasant, but if these flavors appeared in a cup of coffee, they would certainly be considered negative flavors.

So if the flavor of coffee foam is so negative, wouldn't removing it during brewing result in better flavor? To test this, FrontStreet Coffee used Queen Estate Yellow Bourbon coffee beans to brew two versions: a normal version and another where the foam was scraped off after the second pour before the final pour. FrontStreet Coffee found that the coffee with scraped foam was noticeably thinner in texture and had more muddled flavors.

Coffee with Foam is Actually Better Than Without

If you've brewed with stale coffee beans, you've likely observed sparse or even no foam on the coffee surface. In such cases, the fine grounds that would normally float on the surface sink to the bottom and participate in more thorough extraction, extracting woody flavors and other off-flavors into the coffee. Additionally, fine grounds settling at the bottom can clog the drainage holes, creating a risk of over-extraction. This is also why stale coffee beans tend to produce woody and muddled flavors when brewed.

Although the foam absorbs fine grounds and exhibits undesirable flavors, they remain floating on the liquid surface in the filter cup. Since coffee liquid flows from top-right to bottom, these negative flavors have difficulty flowing into the coffee in the lower pot. Some traditional Japanese brewing schools believe foam is the source of bitterness and off-flavors, so during the final pour, they use a large water flow to push the coffee foam upward, then use a flow-cutting method to prevent the foam's bitter and off-flavors from entering the final brew.