Understanding Coffee Milk Bubbles and How to Avoid Them
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Specialty Batch
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27.05.24

Understanding Coffee Milk Bubbles and How to Avoid Them

For many coffee enthusiasts, bubbles on the surface of a latte or flat white can be frustrating. Understanding the science behind milk foam and bubble formation is essential for both baristas and coffee lovers aiming for the perfect cup every time.

Do Coffee Milk Bubbles Affect the Sensory Experience?

Beyond their visual impact, coffee milk bubbles can also influence the sensory experience of the coffee. While bubbles are generally seen as undesirable due to their interference with the smooth appearance of latte art, they can have both positive and negative aspects in terms of taste and aroma.

Taste and Texture

Bubbles can affect the texture of the coffee. A smooth microfoam contributes to a creamy mouthfeel, which is highly desirable in milk-based coffee beverages. Larger bubbles can disrupt this texture, making the milk feel airy or even watery, which detracts from the expected creaminess. However, some bubbles help distribute the flavours more evenly across the palate, enhancing the tasting experience​.

Aroma

The presence of bubbles can influence the aroma of the coffee. As the bubbles burst, they release aromatic compounds into the air, which can enhance the sensory experience of the coffee. This can be particularly appealing when enjoying a coffee with a rich, complex aroma, as the bubbles help release these scents more effectively​.

Interestingly, while excessive bubbles are typically avoided, the right amount of bubbles can indicate the freshness and quality of the coffee. Fresh coffee releases CO2, and the presence of bubbles can be a sign that the coffee is freshly roasted and freshly brewed. Additionally, the slight agitation caused by bubbles can help blend the coffee and milk more thoroughly, ensuring a balanced flavour profile throughout the drink


Why Do Coffee Milk Bubbles Form?

Bubbles in coffee milk result from several interacting factors, including the coffee itself, the milk, and preparation techniques. Here’s a detailed breakdown of these elements:

The Role of Coffee

Roast Freshness

Freshly roasted coffee beans contain a significant amount of carbon dioxide (CO2) that gets trapped during the roasting process. This CO2 is released gradually over time, a process known as degassing. When coffee is very fresh (less than a week old), it releases CO2 more rapidly, which can introduce bubbles into the milk when the coffee is brewed and steamed. The presence of CO2 is essential for the blooming process in coffee, where hot water causes the ground coffee to release gases and expand. This is why you often see bubbles forming during the initial stages of brewing fresh coffee​

As coffee ages, the rate of CO2 release decreases significantly. Beans that are about a week old have released enough CO2 to reduce excessive bubble formation but still retain enough to maintain optimal flavour and aroma​ ​. This balance is crucial for achieving a smooth, bubble-free microfoam in milk-based drinks.

Roast Level

The roast level of coffee also impacts the amount of CO2 present in the beans and, consequently, the degree of bubble formation in milk. Roasts that are more developed tend to have more CO2 compared to those that are less developed. This is because the longer roasting process in developed roasts causes more gases to be trapped within the beans. When these beans are ground and brewed, they release more CO2, leading to more bubble formation​.

Moreover, developed roasts have a higher concentration of compounds that can act as surfactants. Surfactants help stabilise foam by reducing the surface tension of liquids, which can enhance the formation of crema in espresso but might not have the same stabilising effect on milk foam​. The presence of these compounds can also contribute to the difference in foam stability and texture between different roast levels.


The Impact of Milk

  1. Fat Content: The fat content in milk plays a significant role in bubble formation. Skimmed milk tends to produce more bubbles compared to full-cream milk. Fat droplets in milk can destabilise foam. When milk proteins (mainly caseins) surround air bubbles during steaming, they prevent the bubbles from merging or bursting. Higher fat content can interfere with this process, leading to a more stable microfoam in full-cream milk.
  2. Homogenisation: Homogenised milk, where fat globules are broken down into smaller droplets, tends to create a less stable foam compared to unhomogenised milk. The smaller fat droplets absorb more proteins, leaving fewer proteins available to stabilise the air bubbles, resulting in more bubbles on the surface

Preparation Techniques

  1. Use Slightly Aged Coffee: Avoid using coffee that is too fresh. Coffee that has rested for about a week will have less CO2, reducing the likelihood of bubble formation​
  2. Choose the Right Milk: Opt for full-cream milk or unhomogenised milk for a more stable microfoam. If you prefer plant-based alternatives, soy milk generally produces fewer bubbles compared to almond milk
  3. Reduce Steam Wand Pressure: If possible, reduce the pressure on your steam wand to avoid injecting too much air too quickly. Introducing air slowly allows the milk to stretch rather than creating excess foam, resulting in a smoother texture
  4. Incorporate Air Effectively: Ensure you are effectively incorporating the air into the milk. Try aerating less and maintaining the swirling motion for longer to thoroughly mix the air and milk. If your milk heats too quickly, consider lowering the steam wand pressure
  5. Swirl Espresso and Milk: Before pouring, try swirling or stirring your espresso vigorously, add a small amount of milk, and swirl vigorously again. This can help elevate any excess trapped gas while also incorporating the milk and espresso smoothly, reducing the formation of bubbles

Final Words..

Understanding the factors that contribute to bubble formation in coffee milk takes you one step closer to achieving the perfect cup. By considering the freshness and roast level of your coffee, selecting appropriate milk, and refining your steaming and pouring techniques, you can minimise bubbles and enjoy a silky, smooth microfoam every time.

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Specialty Batch Coffee is a Dubai-based, homegrown brand backed by a wealth of industry knowledge. With roots in Melbourne, Australia, we bring a solid 30+ years of combined specialty coffee experience & expertise to the table.

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