Why Does My Coffee Bubble When Poured?

If you stumbled across this article, you’ve probably experienced tiny bubbles in your coffee when pouring it and wondered why this is happening. Is there residual soap in your coffee pot or cup? Most likely, this is not the case.

The bubbles or foam you see when pouring your coffee are due to carbon dioxide. These gases get trapped within the coffee beans during the roasting process. They are gradually released over time, but the rate at which this occurs increases when the coffee beans are ground and brewed. But is this a good thing? 

Like the bubbles you see when you open a soda can, carbon dioxide is released, creating that great fizz in your cup. This is very similar to what is happening in your cup of coffee.

Where do the Bubbles in Coffee Come From?

We briefly described why your coffee sometimes has little bubbles on top, but let’s discuss why this happens a bit further. It all starts when the beans are in the coffee roaster. As they begin to roast, the heat causes a reaction between the amino acids and carbohydrates in the coffee beans, also known as the Maillard reaction.

Certain gases, such as Carbon Dioxide or CO2, get trapped inside the beans. Once the CO2 becomes trapped, the beans begin degassing (release the gases) quickly the first couple days after being roasted, and then more slowly for the following weeks. In fact, 40% of the gases leave the beans in the first twenty-four hours.

More often than not, you’ll see small valves on coffee bags. These are there for this exact reason. They allow the gases to escape while preventing oxygen from coming in.

When you’re ready for that cup of coffee, you grind your fresh coffee beans and begin brewing. When freshly ground coffee comes in contact with hot water, the speed at which the CO2 gases release increases. This is where the extraction of the coffee grounds occurs.

The fast rate at which this release occurs creates a layer of tiny air bubbles.

Bubbles in your coffee are the result of the release of Carbon Dioxide from the beans.

You may be wondering; Are these bubbles from the CO2 safe to drink? The answer is yes. You may notice the bubbles, or foam, tasting a bit more bitter than the rest of your drink. This is completely normal and safe to consume.

Are the Bubbles in Coffee a Good Thing?

Yes, it is good to see these bubbles, or foam, in your brewed coffee. The presence of bubbles is a sign of two things:

  1. How fresh your coffee beans are.
  2. How prominent the coffee’s flavor is.

Remember, once the coffee beans are roasted, they begin to release these gases. The longer they sit, the more gases escape from the beans. Since flavor compounds are inside these CO2 gases, as the gases dissipate, the flavor compounds are released as well.

This is one reason older coffee beans taste stale and contain less prominent flavors in your cup.

So sit back and enjoy a fresh, flavorful cup of hot coffee when you see bubbles in your coffee. If you pour your coffee and see a lack of bubbles, just be prepared that your coffee may taste a bit old and stale.

Does Roast Level Affect the Amount of Bubbles in Coffee?

Now that we’ve gone over that bubbles are an indicator of freshness, one thing to keep in mind; roast level also plays a role in the amount of bubbles you will see in your mug.

As discussed in another article, “Differences Between Light, Medium, and Dark Roast Coffee,” dark roast beans are roasted longer than medium and light roast coffees. Longer roast times lead to more gases being trapped in the beans. As a result, dark roasts contain greater amounts of carbon dioxide than lighter roasts.

Since dark roasts contain more CO2, they have more gases to release; thus creating more bubbles in your cup.

Why Are There Bubbles With Instant Coffee?

If you remember, instant coffee has already been brewed and then dehydrated for later use. So, in this case, any sign of bubbles or foam would not be the result of the release of carbon dioxide being from the coffee beans.

Any bubbles in your instant coffee are most likely due to air mixed in with the water creating tiny air pockets. As the air pockets make their way to the top of your cup, you’ll see small bubbles on the surface.

Pour-Over and Coffee Bloom

If you are using a pour-over to brew coffee, get your coffee grounds wet before brewing and leave them sitting for about 30 to 45 seconds to let the coffee bloom. This allows the CO2 gases to escape before making your coffee, leaving you with less bitterness and a more balanced cup of coffee.

This is one reason I love my pour-over coffee. I get to appreciate a full flavor, full body, and perfectly balanced cup of coffee every time, especially if I use coffee beans that are less than a couple of weeks old.

If you aren’t sure how long ago your beans were roasted, check out our other article, “How to Tell if Your Coffee Beans Have Gone Bad.”

What Does the Presence of Rainbow Foam Mean?

You understand why you often see those tiny bubbles on the surface of your coffee, but why are they sometimes rainbow color? This doesn’t quite seem right to you. Well, no need to fear; rainbow bubbles were also completely normal.

The presence of multi-colored bubbles results from carbon dioxide bubbles (just as we discussed in this article) coming through the oils of the coffee grounds. This is similar to when you see oil in puddles on the street or, sadly, in a nearby lake. Oil in water will often give off rainbow colors.

In your coffee, though, this oil is supposed to be there. If you read the differences between light, medium, and dark roast coffee, you know the longer the beans are roasted, the more oily their surface becomes. The increased temperature causes the beans’ skin to rupture, causing the natural oils to come to the surface.

Typically, these oils get trapped in paper filters but will pass through if using metal filters. Either way, it is perfectly fine if you see some rainbow bubbles in your coffee.

Final Thoughts

Hopefully, you can now appreciate the presence of bubbles in your next cup of coffee. They are not a sign to worry, but a sign to let you know your coffee beans are fresh and full of fantastic flavor. So sit back and enjoy!

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