Why Do Some Coffees Have Bubbles on Top?

istock / istock

by Sarah Leffert

As you gaze down at your cup of black coffee in the morning, you might notice a small layer of foam floating atop it. This bubbly layer is the result of a chemical reaction often referred to as the "bloom."

It all starts with the roasting process. As coffee beans are roasted, gases become trapped inside—CO2 specifically. From that moment on, the beans continually release CO2, which is why you'll find a one-way valve on every coffee bag. This valve allows the CO2 to escape safely, while preventing additional oxygen coming into contact with the beans.

When the coffee grinds come into contact with hot water, the rate at which the gases are released increases. This rapid release of CO2 causes a layer of foam to appear on the surface of the coffee grinds. As the CO2 escapes from inside the grinds, it's replaced with water; thus begins the extraction.

So why is this layer of foam important? Simply put, it's an indication of how fresh and prominent the flavor of the coffee is. The majority of a coffee bean's flavor compounds are trapped in the CO2 gases. Over time—as the beans lose more and more CO2—the potency of the coffee's flavor notes will decrease, resulting in a stale coffee. Therefore, a coffee with less CO2 will produce a smaller amount of foam as it blooms, lessening the flavor compounds present.

However, this isn't always an exact measurement of how fresh the coffee is. Dark roasted beans contain more CO2 than a light roast because their roasting cycle is longer. As the beans continue to roast, additional CO2 is created. Due to their higher concentration of CO2 gases, dark roasted coffee grinds will bloom more than others. In addition to the type of roast, the amount of time the coffee is allowed to bloom will affect the potency as well. Who knew a congregation of coffee bubbles could be so delicious!