January 22, 2021

Why does milk foam disintegrate?

It happens to every barista at some point. A cappuccino or latte will sit on the counter for just a little too long, and its perfectly-textured microfoam will start to disintegrate and bubble. 

As a result, the beverage no longer looks “fresh” and appealing to the customer. This often means the barista has to make another, which wastes coffee, milk, and service time. But why does this happen? And how can baristas stop it?

To learn more, I spoke to Sam Pinkerton of Clive Coffee and Professor Thom Huppertz. Read on to find out what they said.

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The science behind quality microfoam

To make the best possible microfoam, baristas start with cold milk, as the proteins within are fully intact. You should use a pitcher that is a size bigger than the beverage you are preparing (for example, a 6oz/170ml drink requires an 8oz/220ml pitcher).

This provides more room for aeration as you swirl the milk. Additionally, baristas often recommend making sure that the pitcher is between one-third and one-half full to allow for enough space when steaming.

Swirling is a crucial part of creating microfoam. It allows the barista more control when incorporating air and even heating the milk, as it breaks down the larger bubbles into smaller ones. This improves the consistency of the foam’s texture. Experts recommend reaching full aeration before 38°C/100°F, otherwise it will be more difficult to break down the larger bubbles.

During steaming, the jets of air from the wand actually act as makeshift whisks. These add more air throughout the milk, generating more bubbles.

If the tip of the steam wand is too far above the surface of the milk, its larger bursts of air will lead to larger bubbles, and an inconsistent texture. Conversely, if the tip of the wand is too far below the surface of the milk, it will just heat up the milk, rather than creating any foam or bubbles.

As the barista steams and swirls the milk, it’s important to also make sure the steam wand is in the right position to improve texture consistency.

Thom Huppertz is a professor of Dairy Science and Technology at Wageningen University in the Netherlands. “When air or steam gets introduced into the milk, the air bubbles that are formed rapidly become covered by the milk proteins,” he explains. “The surface covering of the milk provides the bubbles with the required stability.”

Per 240ml serving, cow’s milk contains 3.4g of protein and 3.9g of fat, which are both important for creating smooth and stable microfoam. Some 80% of the protein content in milk is made up of “caseins”, while the remaining 20% consists of whey proteins.

Both of these compounds are found on the surface of fat globules. One “side” of each protein is hydrophobic, meaning it repels water and attracts air. In comparison, the other side is hydrophilic, meaning that it clings to the water in the milk.

Ultimately, to create a smooth, stable, and consistent microfoam, there should be an even balance of air in the layer.

Why does disintegration happen?

However, no matter how stable or consistent your microfoam is, eventually all steamed milk will begin to disintegrate and bubble.

“All foams are inherently unstable,” Thom explains. “It’s only the timescale at which instability occurs that differs from material to material. For milk foams, several forms of instability can be observed.”

Sam Pinkerton is the Head of Education at Clive Coffee in Portland, Oregon. She says: “Once you steam milk, assuming it’s well-textured and the foam you’ve created is evenly incorporated, you’re working against time and gravity. 

“As the drink sits, the milk begins to drain from the foam, causing the lighter air bubbles to rise.”

Air is less dense than both milk and water, so it will gradually float to the surface, while the liquids in the foam will sink. 

Thom adds: “First, liquid will drain from the foam under the influence of gravity.

“However, over time, we also see that large air bubbles become larger, and small air bubbles disappear; this process, called Ostwald ripening, is difficult to prevent.”

Ostwald ripening is, very simply, the process by which bubbles in a foam increase in size. This occurs when the larger bubbles “pull” the smaller bubbles towards them by way of pressure differences. The smaller bubble then joins the larger bubble to increase its total area, effectively “consuming” it.

“These small bubbles combine into larger bubbles until the air pressure inside the bubble exceeds the strength of proteins holding it in, and the bubble pops,” Thom says. “This can happen to even the best microfoam.”

Altogether, Ostwald ripening means fewer bubbles in the foam over time. This causes the layer to eventually disintegrate, requiring the barista to steam a fresh pitcher of milk all over again.

What else affects foam disintegration?

Beyond the stability of the foam and its battle against time and gravity, there are a number of other factors which cause it to disintegrate.

“Competition with other compounds on the surface of the milk may prevent stable surface layers from being formed,” Thom says.

One such compound is fat. Milk fat globules typically range from 0.1 to 10 micrometres in size. The consistency of the liquid (in terms of milk fat globules) will depend on how the milk has been processed.

During processing, homogenised milk is forced through a small nozzle. This breaks up the fat globules and evenly disperses them throughout the liquid. In comparison, non-homogenised milk contains larger, more uneven fat globules.

“In the region of 10 to 30°C, unstable foams made from non-homogenised milk can form as the [uneven] fat globules can be damaged in some foaming processes,” Thom explains. “For homogenised milk, this is far less likely to happen.

“Milk fat, however, also plays a very important role. It provides the milk foam with the right sensory characteristics, the flavour and mouthfeel that consumers love.”

This is most evident in skimmed milk. The lack of fats allows the proteins to fully encase the bubbles, meaning vastly improved stability when the foam is created. However, this stability means that the milk is often too rigid for pouring good or precise latte art.

Thom also says it’s important to keep in mind the difference between dairy and plant milks when steaming. “For plant-based milk alternatives, different foaming properties are typically observed,” he says. “Some are very low in protein and it is therefore difficult to form good foams.”

Almond and oat milk, for instance, have considerably lower protein levels. However, the addition of fats through a barista “version” of these milks supports the creation of creamier, more desirable textures. In contrast, soy milk has a similar level of protein to dairy, meaning it can form a more stable microfoam. 

“[Plant milks] often contain natural or added surface-active compounds that compete with the proteins and interfere with foam formation,” Thom adds. “Often, larger air bubbles and quicker destabilisation are observed [more commonly] in plant-based alternatives than in cow’s milk.”

Making your foam more stable

When foam begins to disintegrate, bubbles usually form on the part of the beverage where the espresso and milk touch. Pouring latte art that takes up the majority of the surface in the cup (such as a simple, large heart design, for example) will help to reduce the amount of bubbles present, and help your foam remain more stable for longer.

Before pouring latte art, Sam recommends adding a small amount of milk to the espresso. He says you can gently swirl it for full incorporation – almost like the bloom phase when brewing filter coffee. This creates a stable base for your microfoam, and may slow down the disintegration process.

“Depending on the milk you are steaming, and the quality of foam you build, the time in which it takes for your foam to disintegrate will differ,” Sam explains. “You’ll usually see this take effect after just a few minutes.”

Research has found that, on average, bubbles begin to appear two to three minutes after the beverage has been poured. Serving all beverages before this point is best. It will mean that customers receive drinks that not only look appealing, but also have the best possible texture.

The roast profile of the coffee can also affect how quickly the microfoam disintegrates. Darker roasts contain more carbon dioxide, which is highly soluble in water; as a result, the CO2 diffuses quickly through foam and increases the rate at which bubbles converge. 

Additionally, fresher coffee just a few days past the roast date has higher levels of CO2, which means it produces even more bubbles and consequently causes the foam to disintegrate more quickly. For more stable microfoam, use coffee roasted a week or so ago. This will mean it has had an appropriate amount of time to degas.

While your microfoam will always disintegrate, there are of course ways to mitigate and delay the process. Ultimately, the best thing to do is create the smoothest microfoam in the first place, made stable with the smallest possible bubbles.

To achieve this, baristas should focus on their steaming technique and look for improvements there. However, it’s important to also consider the types of milk and coffee that you use, and keep an eye on how these affect the speed of disintegration.

Enjoyed this? You might also like A Guide To Working With Plant Milks

Photo credits: Clive Coffee, Tasmin Grant, Neil Soque

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