How to make froth in coffee: why milk foam forms and separates

Milk froth forms when steam strikes cold milk and pulls air into it; milk proteins then hold that air as many tiny bubbles, so the liquid expands into foam. For a latte, the foam should stay thin and glossy with no visible large bubbles. Foam that fails to integrate, separates, or floats usually traces back to steam pressure and bubble size.

Temperature also matters. As milk warms, lactose dissolves and milk fat stabilizes the bubbles, so bubbles burst in the mouth and release aroma. Manual and steam methods both work, and the same principle explains why overheating, coarse bubbles, or a weak steam wand produce flat, separated foam instead of integrated microfoam. A tulip latte art pattern depends on getting this right.

Steaming milk is a fundamental skill that baristas must master; the quality of the milk foam directly affects the taste of a latte and the result of latte art. Drawing on the practical experience of Front Street Coffee, this article explains in detail the physical principles of milk foaming, answers common problems such as foam not integrating, separating, and floating froth, and explains how steam pressure and foam fineness determine the degree of integration. It also compares the stability of milk foam at different temperatures, provides two methods of frothing—manual and steam—summarizes five major misconceptions, and finally includes a step-by-step tutorial on tulip latte art. Whether you are a beginner or a coffee enthusiast looking to refine your skills, you will find practical guidance here.

To achieve ideal milk foam, you must first understand why milk foams. Front Street Coffee believes that steaming milk is a core foundational skill for every barista, and whether one can steam perfect milk is one of the factors used to judge whether a barista's level is up to standard. For example, the milk foam for a latte is on the thinner side, making it smooth and delicate to drink. So when steaming milk for a latte, it should not be too thick; the surface of the steamed milk should be glossy, with no obvious large or small bubbles, and overall delicate and beautiful.

The basic principle of milk foaming is to use steam to strike the milk, introducing air into the liquid milk, and relying on the surface tension of milk proteins to form many tiny bubbles, causing the liquid milk to expand in volume and become foamy milk froth.

During the foaming process, lactose dissolves into the milk as the temperature rises, and the foaming action seals the lactose within the milk, while the role of milk fat is to give these tiny bubbles a stable state, so that when these milk bubbles are drunk, the tiny bubbles burst in the mouth, allowing the flavor and aromatic substances to be better released and amplified, giving the milk a sweet, rich taste and mouthfeel. Moreover, when integrating with coffee, the bonding force between molecules is relatively strong, allowing the coffee and milk to fully combine, so that the characteristics of both the coffee and the milk stand out individually while also being completely integrated, achieving a mutually complementary effect.

Front Street Coffee is here to answer some questions you may commonly encounter when frothing milk!

Why does the foam not integrate?

The milk foam immediately integrates with the espresso as soon as it is poured in—forget about making a pattern, there isn't even a line, it just floats away; or else there's no problem during the initial integration, but when it comes time for latte art, the foam clumps up and floats directly on top. Front Street Coffee will first explain the first situation to everyone: this is because the amount of foaming is small, the steamed foam is thin, and it is mainly hot milk, so how could it hold a shape? So those of you who encounter this situation, please be bolder and foam a bit more!

So why is it that even though the integration is fine, the foam floats on top during latte art?

This is mainly because the degree of integration between the milk and the milk foam is relatively poor, so the milk and foam separate, and in the end the foam floats on top of the coffee. In this case, you can only make adjustments during the stage of texturing the milk foam!

What determines the degree of integration between milk foam and milk? The degree of integration mainly depends on the following two points:

1. The size of the steam wand's air pressure. (The size of the steam wand's air pressure causes the milk foam to roll to different degrees during the frothing process; the greater the pressure, the higher the degree of integration.)

2. The fineness of the milk foam. (The finer the milk foam produced, the less air each individual bubble contains, the higher the density, and the smaller the buoyancy; the coarser the milk foam produced, the more air each individual bubble contains, the lower the density, and the greater the buoyancy. If the milk foam produced is relatively coarse, the foam will quickly float to the surface of the milk, resulting in a lower degree of integration; if the milk foam produced is relatively fine, the foam will float to the surface of the milk more slowly, resulting in a higher degree of integration.)

Why is a high degree of integration between milk and foam required?

The higher the degree of integration between the milk foam and the milk, the clearer and more delicate the lines of the latte art pattern will be; conversely, the lines of the latte art pattern will be more blurred and coarse.

How should the degree of integration of milk and foam be judged?

Front Street Coffee teaches you how to judge the degree of integration of milk and foam! Pour the frothed and integrated milk into a glass and let it sit, observing the time it takes to separate. The length of the separation time indicates the degree of integration of the milk and foam!

If it separates within 30 seconds, it is poor; if within 30-60 seconds, it is medium; if after 60 seconds, it is excellent.

Also, by no means should you leave the frothed milk foam sitting aside to use later, because this will also cause separation!

Perhaps someone will ask, why is it required to control the temperature when steaming milk?

Generally, Front Street Coffee heats the milk to around 60 degrees, but it must not exceed 70 degrees, otherwise the protein structure in the milk will be destroyed.

Front Street Coffee previously conducted an experiment, steaming milk to different temperatures:

The situation at 90°C: when it exceeds around 70°C, even if you do not deliberately foam, the foam will rapidly expand, and at high temperature it keeps rolling; by the time it approached 90°C, the editor's milk had already overflowed the milk pitcher (as shown in the picture above), and it stopped at 94°C. At around 1 minute 30 seconds, obvious signs of foam dissipation had already appeared.

The situation at 60°C: this is the temperature we use more commonly. Frothing the milk foam according to standard operations, stopping when it feels hot to the touch, the temperature was 63°C. Obvious signs of foam dissipation appeared at around 10 minutes 50 seconds.

The situation at 30°C: frothing according to standard technique, the temperature felt warm to the touch, and the stopping temperature was 33°C. Obvious signs of foam dissipation appeared at around 4 minutes 20 seconds.

The situation at 12°C: here we used milk directly from the refrigerator and a manual frother; the temperature when poured into the frother was 8°C, and the temperature after frothing was 12°C. Obvious signs of foam dissipation appeared at around 5 minutes 15 seconds.

Milk foam in the 50-70°C range has the best durability whether in pure milk or after being added to coffee, for two reasons: first, after frothing the milk foam, there is relatively ample time for mixing and texturing between the foam and the liquid milk; second, it is in the period when the milk has begun to denature but has not yet reached irreversible denaturation, so stability is relatively high.

When the milk foam is frothed to around 80-90°C, irreversible denaturation occurs in the milk. There are two types of protein in milk: whey protein and casein; whey protein begins to denature above 60°C. So when we froth to around 90°C, the protein's elasticity is insufficient, so the foam dissipates relatively quickly.

When the milk foam is frothed to around 30-40°C, the reason the foam dissipates relatively quickly is insufficient mixing and texturing time between the milk foam and the liquid milk after foaming.

The reason cold milk foam dissipates quickly is that we use a manual frother, and its degree of mixing cannot compare with the high-speed mixing and integration of a steam wand. The degree of integration is not high, and in addition, the period from low temperature to room temperature is a warming process for the milk foam; subjected to external heat, the internal gas expands with heat, so the milk foam bursts relatively noticeably.

How can Front Street Coffee produce a large amount of fine-textured milk foam?

Regarding how to make a good milk foam, here is an idiot-proof milk foaming method: pour half a pitcher of milk into the stainless steel pitcher, position the steam wand perpendicular to the very center of the milk or at the edge of the pitcher, then let the steam tip just touch the surface of the milk, then open the steam to the halfway point, do not move the pitcher, slowly close it when the milk reaches 50 degrees, and close it exactly at 60 degrees; the final temperature will reach 66 degrees, and in this way you can produce a good pitcher of milk foam.

What type of espresso machine should be used for frothing milk?

Generally, machines have three-, four-, and five-hole steam tips, and as long as you become somewhat familiar with them, you can produce good milk foam. However, the machine's steam output, the wetness or dryness of the steam, and the size of the steam holes still affect milk frothing.

Why practice frothing milk with water?

First, as everyone knows, water is transparent, allowing you to see more clearly how the vortex forms.

Second, it allows you to better master frothing methods for different numbers of holes! Third, beginners must remember that before you can froth milk to a silky consistency, you should use a thermometer; if you cannot control the temperature well, then the coffee you make will be unstable—perhaps out of ten times you produce good milk foam once or twice, but that is not stable quality.

The difference between frothing milk with steam and frothing milk by hand

First of all, hand-frothed milk foam simply cannot compare with machine steam. The fineness of steam makes the milk foam it produces very fine and silky-smooth; steam has a wide radiating surface, so the milk foam produced is relatively even and mixes relatively well. Hand frothing mainly presses through the small holes of a filter mesh, so the milk foam produced is somewhat coarser.

How does Front Street Coffee make milk foam?

Method 1: Make handmade milk foam with a milk frother/French press

1. Pour the milk into the milk frother, and the amount should not exceed 1/2 of the frother, otherwise the milk will overflow due to expansion when making the foam.

2. Heat the milk to around 60 degrees, but it must not exceed 70 degrees, otherwise the protein structure in the milk will be destroyed. Note! The lid and filter mesh must not be heated directly. (If making iced milk foam, cool the milk to below 5 degrees; of course, do not over-chill it and let the milk freeze.)

3. Put on the lid and filter mesh, and quickly pump the filter mesh to press air into the milk. When pumping, you do not need to press all the way to the bottom, because the goal is to introduce air into the milk, so just move it at the surface of the milk; the number of times also does not need to be too many, about thirty gentle pumps will do.

4. Remove the lid and filter mesh, use a spoon to scrape off the large coarse foam on the surface, and what remains is dense hot (iced) milk foam.

Method 2: Make milk foam with a steam milk frother

First, do not put the steam wand into the milk, because there may be some condensed water in the steam wand, so first release some of the initial steam, and by the way expel the excess moisture.

Insert the thermometer into the container, then insert the steam wand diagonally into the milk, and turn on the steam switch.

Slowly adjust the position of the steam nozzle to a point just a little away from the surface of the milk, but never above the liquid surface, otherwise the milk will splash everywhere. When the position is correct, you will hear a steady "hissing" sound; otherwise it will be very loud or almost silent.

When there is enough milk foam, you can bury the steam wand a bit deeper, letting the steam continue to heat the milk. The best angle for burying the steam wand is just enough to make the milk rotate.

When the temperature reaches between 60 and 70 degrees, you can turn off the steam switch.

Use a wet cloth to wipe clean the milk adhering to the steam wand, and at the same time release some steam again, so that it will not be difficult to clean after the milk dries.

Tips for measuring milk foam temperature by hand:

1. The most suitable temperature for milk foam

When milk is frothed on a coffee machine, the most suitable temperature is controlled within the range of 55-65 degrees, for the following reasons:

1. If the milk temperature is below 60 degrees, it will lower the temperature of the coffee a great deal and affect the taste;

2. The lactose in milk between 55-65 degrees will be brought out, making the coffee more fragrant, sweeter, and smoother.

3. In milk above 70 degrees, the nutrients in it will be destroyed; after all, the milk will become more watery, and its smoothness and sweetness will deteriorate.

2. Tips for measuring milk foam temperature by hand

Now that we know the most suitable temperature for milk is between 55-65 degrees, how do we grasp this temperature? My personal suggestion is that you need to equip yourself with a thermometer; you can directly insert the thermometer into the frothed milk, and then the temperature of the milk will be clear at a glance. Of course, with accumulated experience, you can eventually do without the thermometer and measure the temperature of the milk foam directly by hand.

The techniques are as follows:

1. Hold the milk pitcher with your left hand, turn on the steam with your right hand, and begin foaming the milk;

2. After turning on the steam, place your right hand back against the wall of the milk pitcher to feel the temperature of the milk foam;

3. When your hand feels very hot and can no longer stay against it, you can stop foaming the milk, then insert the thermometer into the milk foam to measure the temperature.

4. If the temperature has not reached the range of 55-65 degrees, it is suggested that next time when it becomes hot to the touch, you move your palm away and begin counting silently in your head 1, 2, 3, 4, usually in seconds, to add extra time. For example, after you move your hand away from the milk pitcher, if after adding four seconds the measured temperature reaches 65 degrees, then congratulations, you have found the pattern, and from then on you can just add about four seconds when it becomes hot to the touch. By adding or subtracting time according to point 4 above, you can easily master the technique of measuring milk foam temperature by hand. The above techniques are somewhat trick-like; in fact, as long as you froth milk for a period of time, you will more or less find the pattern and the feel, and the temperature of the milk foam will also be relatively easy to control.

Five major misconceptions about frothing milk

Regarding milk foam, there are several misconceptions we need to understand, so that we can better understand milk foam.

Misconception one: Using up-and-down shaking of the latte art pitcher to remove coarse surface foam. This is a very good method, but many friends have developed a strong dependence on it. If their own milk foam is not well frothed, they do not summarize why it was not well frothed, but instead use this method to compensate. Over time, dependence develops. Here Front Street must say that if you rely only on this method to remove coarse foam, your milk foam technique cannot improve, and you also cannot practice advanced latte art techniques.

Misconception two: Using a spoon to scoop off the coarse surface foam. Front Street does not recommend this method. Although some competitors use this method in competitions, Front Street believes this is because their milk foam technique is not up to standard, there is too much coarse surface foam, and it cannot be shaken off, so they can only scoop it. Anyone who uses similar methods to handle milk foam, please be sure to strengthen your steam control technique in milk frothing.

Misconception three: After frothing the milk foam, leaving it to sit for a short while. This method is very troublesome; it is a manifestation of milk foam technique not yet having entered the door. It will intensify the separation of the milk foam, and the integration of hot milk and foam will be very difficult to complete. If you are not careful, it becomes milk as milk and foam as foam. The whole cup of coffee has no balanced taste.

Misconception four: When pouring back and forth between two latte art pitchers, holding them at a very high position; the original purpose is integration, but instead it stirs up more coarse foam.

Misconception five: Blaming the steam when the milk foam is not well frothed. Front Street believes that a coffee machine steam pressure (that is, boiler pressure) of 0.8 BAR is a relatively easy milk-frothing pressure to master; with this pressure, milk frothing is very sustained, and it is also very easy to understand the principles of milk foaming, but making a pitcher of milk foam takes about 13 seconds—alas, not as fast as high pressure. If the pressure is 1.0 or 1.1 BAR, then the way and method of frothing milk will change somewhat, but the principle is the same. (This point excludes the phenomenon of "spitting" caused by impure steam, when the latte art pitcher also shakes badly. If this phenomenon occurs, please empty the steam, then fully heat it once and try again, and it should be fine.)

When making complex patterns, the most important thing is the need for a pitcher of high-quality milk foam, and the temperature of the milk foam is a key factor affecting the quality of the milk foam.

Front Street Coffee will share with everyone the method for making tulip latte art:

First, you still need to extract espresso. Front Street Coffee uses its own Front Street Sunflower Warm Sun espresso blend beans.

Espresso part

Coffee bean information

Extraction parameters

How Front Street makes tulip latte art

1: Integrate to the four-tenths full mark, choose a point at the one-third position to inject, and keep the latte art pitcher against the edge of the coffee cup.

2: Slowly move to the center position, and note that a slight left-right swaying is enough.

3: When the textured pattern flows back, reduce the flow rate, speed up the swaying, and pay attention to the amplitude of the swaying.

4: Raise the latte art pitcher; this step is similar to a rosetta.

5: Inject again at the one-third position, pay attention to the flow rate and the height of the latte art pitcher, then push forward, and note the pattern changes at the arrow position.

6: Push until the pattern is enclosed, raise the latte art pitcher, and draw in finely; a small gap will appear.

7: Inject again, grasp the flow rate well, keep the spout of the latte art pitcher on the surface of the coffee liquid, push forward, and pay attention to the small gap in the previously pushed pattern.

8: When the "petal" is enclosed by the previous "petal," raise the latte art pitcher and draw in finely, still leaving a small gap.

9: Inject again, sway slightly left and right, pause, and form a small heart; pay attention to the changes in the gap at the black line position.

10: Raise the latte art pitcher, control the flow rate well, and finish.

11: You can clearly see how the "petals" are formed; when finishing, be sure to pay attention to the flow rate.

The textured tulip is not especially difficult, and it is also relatively suitable for practice. Watch the video a few more times to understand the formation and changes of the pattern; this will be relatively helpful for latte art practice. During practice, pay attention to the amplitude of the swaying; it does not need to be too fast, and do not worry about too few texture lines. You can also just push forward twice. Once you are proficient, you can appropriately add some texture lines and push forward a few more times, which can make the pattern look better.

For more specialty coffee beans, please add the private WeChat of Front Street Coffee, WeChat ID: kaixinguoguo0925

The above content is compiled by CoffeeHunters, a coffee news website.