How to froth milk
From Whole Latte Love Support Library
Revision as of 05:40, 27 March 2017 by Mitch (added sentence about barista series non dairy milks)
Frothing milk is one of the more challenging aspects of creating espresso based drinks, but a microfoam can still be obtained, even on lower end equipment, by using the right technique. By understanding the theory of how milk frothing technique and what chemical, flavor, and texture changes the milk goes through you will be able to get the best results. Getting a good froth requires practice and patience, but the results are worth the effort.
- 1 Milk
- 2 Alternative Milks
- 3 Temperature
- 4 Equipment
- 4.1 Machine Features
- 4.2 Pitchers
- 4.3 Frothing Aids
- 5 Purging & Cleaning
- 6 Technique
- 7 Types Of Drinks
- 8 Latte Art
The velvety sweet texture of properly frothed milk is based off of three main components in the milk: lactose, proteins, fats. As the milk is heated these components change and interact. Lactose is responsible for the sweet flavors in milk. Proteins create the stability in the frothed milk to allow the formation of the foam. Fats create richness in the milk, imparting the body and velvety texture to the mouthfeel.
Lactose is a type of sugar contained in milk. When the milk is heated these sugars begin to caramelize. A good target range for milk froth is somewhere between 120-140 °F. Going past this range causes this point causes the milk to take on a flat burned flavor. Just like how heating sugar for caramel too long turns it from a decadent candy to a sticky block of charcoal.
Proteins are complex molecular structures within the milk that help provide stabilization and affect the surface tension of the milk. The more proteins the milk contains the more stable/firm the foam will be.
Fats are what give the milk a smooth velvety texture, as well as body. The greater the fat content the richer the feel of the foam/milk will be, so that the latte doesn't feel or taste watery.
The ratio of proteins to fats is what determines the final texture of the milk. Lower percentage milks such as skim milk or 1% milk will have stiffer foam that holds together longer. The foam also doesn't mix as well with the steamed milk because of the low fat content, causing quick separation of the froth from the steamed milk. These milks will tend to give a drier mouthfeel for the foam, and a watery mouthfeel for the steamed milk.
High fat milks, such as 2% or whole milk are typically more desirable. These milks have a higher fat to protein ratio. This means the foam isn't as stable, but it also means that the foam distributes more readily. The milk foam will not separate as easily, will taste more velvety and have a wetter mouthfeel, and will taste and feel richer. Because higher fat milks mix more easily they are going to be the easiest for beginners to work with.
The ratios of these three key components, as well as other flavor components also depends on the type of cows the milk is coming from, what the cows were fed, and how the milk is treated and pasteurized. While these difference may be more subtle in cold milk, once the milk is heated the differences become more pronounced.
Alternative milks exhibit quite a different flavor and texture. This is because they lack the chemical components that give regular milk it's ability to become sweet and take on air and create froth. Soy and nut based milks do not contain lactose and don't have a high fat content. Alternative milks to contain a high protein content, but the proteins are different than those found in cow's milk. They don't hold surface tension well, so while a foam can be created during frothing, as soon as steam stops being injected into the milk the foam structure begins to collapse very quickly. Additionally since there is very little fat content the foam doesn't mix into the milk easily.
The proteins in alternative milks also don't handle higher temperatures as well, which means they should not be frothed as long as regular milk. Soy milk also has a tendency to curdle in the presence of coffee because of the high acid content of coffee.
Some companies, such as Califia Farms and Pacific Natural foods offer barista series milks, which are designed to be much more conducive to steaming for espresso drinks.
Frothing temperature has a large effect on how the milk tastes and the kind of texture it takes on. The main advantage of frothing milk is that it helps the milk taste sweeter, which also imparts a sweeter flavor on the coffee. When frothed to the correct temperature the milk will be sweeter and have a velvety froth. A common misconception about milk frothing for lattes is that the milk should be really hot. For the best results milk should be frothed to somewhere around 130-150 °F. Going past this point towards 150 °F the milk may remain sweet, but the froth will become firmer and take on too much air.
Once past 160 °F the milk will lose its sweetness and start to taste flat, or even burned if it is heated further. The milk foam will also become very stiff and not mix will into the steamed milk. If you try pouring latte art with this kind of foam it will just blob on top of the coffee.
Whenever milk is heated some of the proteins and fats are destroyed. This is why you can't froth milk again once it has already been frothed. Once these compounds are destroyed they won't be able to do their job again. This means that the type of pasteurization that is used on the milk will have a result on the flavor and texture of the milk.
Pasteurization is a process of rapidly heating milk for a very short period of time to kill any bacteria in the milk. Pasteurization helps protect against harmful bacteria, as well as significantly increase the shelf life of the milk.
Milk will generally come in three different types of pasteurization: unpasteurized, pasteurized, and ultra pasteurized. The flavor differences between the milks come from the different temperatures. Unpasteurized milk is not heated, so none of the flavor compounds are destroyed. Normally pasteurized milk is heated to 160 °F briefly. This kind of temperature being to kill bacteria, but it is also destroying some of the proteins and fats. Ultra-high pasteurization is done at an even higher temperature for longer. The milk is heated to around 280 °F for several seconds. While this kills almost all of the bacteria in the milk, it also destroys a lot more of the fats/proteins.
This means that the unpasteurized milk will have the best starting flavor of the three milks, and will froth the best. These differences are subtle though compared to the temperature the milk is frothed to during steaming. Since the milk is heated for a much longer period of time and is not heated as rapidly during frothing, this means that flavor compounds of a lot more time to break down. This is why frothing to the proper temperature is so important regardless of the milk you start with.
Frothing thermometers help give a defined temperature of the frothed milk so that you're not scalding the milk. The temperature of the milk also affects the frothing texture. This is especially important for latte art, because different texture milks will be more suitable for certain types of pours. One thing to be aware of is that if you're not using an instant read digital thermometer, then the actual reading of the thermometer will not be the true temperature of the milk. Mechanical thermometers require time for the heat to transfer before they respond. You may notice the temperature continue to rise for a while after you stop injecting steam.
Since thermometers have a lag you won't frequently see an experienced Barista using one. What you may notice instead is that they will have one hand on the bottom or the side of the frothing pitcher. A thermometer is a good tool for understanding how hot your frothing pitching should feel to indicate when to stop steaming. Skilled Baristas already have a feel for this temperature and will instinctively stop injecting steam. As soon as the pitcher becomes uncomfortably hot to hold is usually a good indicator that you should stop frothing.
You should be able to achieve a decent microfoam with most espresso equipment, but certain machines will be better suited to frothing than others. Better equipment will typically feature the ability to control steam pressure and produce a larger volume of steam.
Just like with brewing espresso, the machine is important in getting the best results for frothing. Most espresso machines will be able to produce a good froth with the appropriate technique. To get truly phenomenal results though typically you will need higher end equipment paired with good frothing technique.
Steam Gauge Pressure
Steam pressure is a measurement of how much steam is built up in the machine, and at what pressure it is being stored at. Most semi-automatic and super-automatic style machines will not have a gauge to read steam pressure, but most prosumer and commercial level machines will. Machines with steam pressure gauges help you see what the exact boiler steam pressure is so that you can make sure you start steaming at the peak of the cycle, and know exactly when the boiler has recovered. Typically machines with steam gauges will also have a pressurestat which allows you to adjust the steam pressure. Most machines come factory set to around 1 bar of pressure, but can usually be safely raised to 1.2 bar pressure.
The extra steam pressure produces a drier steam that comes out more forcefully. This heats the milk more rapidly. This is an advantage because the milk heats more rapidly, results in less air being drawn into the milk, allowing for a smoother froth texture that has smaller bubbles. The force of the steam also circulates the milk better within the frothing pitcher as well, helping the froth and steamed milk mix together more easily. One disadvantage is that new users can be deterred by this amount of steam pressure - some new users will find that the milk heats too quickly to properly control the milk.
Boiler Type & Volume
Different machines utilize different styles of boilers for steaming. Boiler size is often determined by the available space within the machine. Smaller home units, such as semi-automatics and super-automatics have very limit internal space, which require smaller volume boilers.
Small volume boilers frequently heat for both the brew and steam functions, or two separate small boilers are used for each function. This type of boiler is typically used on semi-automatic machines because of their limited internal space. Small volume boilers are controlled by simple thermostats that turn the boiler on or off. Because of their small volume these boilers typically won't have a very high steam pressure and can only make a fairly limited amount of steam because of the size of the boiler. There is also a larger variance in the steam pressure because the thermostat uses a simple on/off function. These boilers will not be good for frothing lattes back to back because of the required recovery time.
Rapid Steam/Thermo Block
Rapid Steam and Thermo Block boilers both operate by passing water through a small channel which is surrounded by metal that is heated to a very high temperature by a heating element. This is advantageous because the steam function of the machine heats very rapidly, making shorter recovery times. Because this is not a true boiler the steam pressure will be limited since only a small amount of water is heated at a time.
Heat exchange boilers are frequently used in higher end equipment and commercial machines. They are usually large volume boilers, which means there is a greater head of steam that is formed. This means the steam comes out of the wand with more force, and the steam will last longer. This type of system requires less recovery time. Machines that utilize these boilers need little to no recovery time between drinks. Heat exchange boilers are typically controlled by a pressurestat, which allows for the steam pressure to be adjusted lower or higher based on user preference. Pressurestat systems usually have less variance than thermostat controlled boilers as well. Some heat exchange machines will control the steam boiler through a PID unit which is set to an exact temperature. As a result the steam pressure and temperature is kept very consistent and has the least variance.
Twin and Dual boilers systems are advantageous for several reasons. The main reason is that there is an entire boiler dedicated to the steam and hot water functions specifically, while another boiler handles the coffee. This reduces recovery times. Additionally on single heat exchange machines the brew and steam temperatures are interrelated, because the set steam pressure determines the resulting brew temperature. One dual boilers systems the brew and steam temperatures are controlled separately. Many dual boilers systems allows the service (steam) boiler to be shut off completely, which saves energy if you don't plan on frothing.
Different wand styles affect how the user can interact with the wand during frothing. Some basic things to consider are the wand length, how the wand is angled, and how much space there is for the pitcher to fit beneath the wand. Certain machines are also meant to be used with a special frothing attachment, such as a pannarello. User convenient machines may not include a wand at all, but instead will use an automatic frothing carafe.
Wands are generally adjustable in three different ways. Pivoting wands simply swing from side to side. Rotating wands can are able to turn in place, but don't have any vertical motion component. Articulating wands will both swing and rotate, making this wand style much easier to position. High end home units and commercial machines will typically use an articulating style wand because of the easier control.
Most wands use a rubber or silicone grip to grasp for adjustment. If a wand is described as being no burn style, it means the wand has an internal insulating tube. This tube contains most of the steam so that it is not in direct contact with the metal tubing of the wand. This does not mean that the wand does not get hot. These wands simply take a longer time before they get hot, and they cool off much faster than a traditional steam wand.
Different steam tips will affect how the steam is directed and distributed from the end of the wand. The size of the hole in the steam tip, and the number of holes in the tip determine how quickly pressurized steam can exit the end of the wand. Larger or more holes allow for the pressurized steam to exit from the wand faster, while smaller or less holes may limit the amount of steam coming out. Limiting the amount of steam coming out is no necessarily a bad thing. As demonstrated in the video below, limiting the number of holes, or using a smaller hole diameter it will help the machine maintain steam pressure for longer.
The angle of the holes also effects how the steam distributes through the milk. Multi-hole tips, or single hole tips where the hole is off-center will cause more turbulence in the milk, causing it to start moving in a whirlpool motion more easily.
Using a frothing pitcher is important for getting good results. Frothing pitchers are designed to assist with the frothing and steaming of milk. Pitchers are shaped to help the milk move and roll properly. Some pitchers even have specially shaped bottoms to further direct the flow of milk.
One thing you may notice across almost all frothing pitchers is that they are made from a thin layer of stainless steel. Have you tried frothing in a glass, or ceramic mug before? The results are unwieldy, and frequently disastrous. The containers aren't shaped properly to help the milk move in the desired whirlpool motion. More importantly though is that they retain too much heat. Stainless steel has a high coefficient of thermal conductivity, where as glass/ceramic have relatively low coefficients. What does this mean? Simply put stainless steel is more receptive to heat. As a result the pitcher can quickly wick off more of the heat from the milk allowing for a longer and more controlled frothing time, and less risk of overheating the milk. The stainless steel is thin so that less of the heat is stored before is transfers out to your hands/surrounding air. This is also an advantage, because it more quickly lets your hands feel how hot the milk is during frothing, allowing you to react more quickly to turning the steam off when the milk has reached the correct temperature.
Another way of maintaining temperature control and preventing the milk from heating too quickly during frothing is to store the frothing pitcher in a freezer.
Using the right sized pitcher is important. Matching the pitcher to the size of the drink being made and the espresso equipment is an important consideration. Frothing too little milk in a large pitcher typically results in milk that is easily overheated and has large bubbles. This happens because the steam tip is positioned too close to the bottom of the pitcher, causing the steam to shoot up the side of the pitch. This causes the surface of the milk to bubble instead of roll, pulling too much air in. Alternatively you can also have too much milk in the pitcher. This can cause the milk to take an unusually long time to froth, or leave you with not enough room in the pitches, causing the milk to overflow. For smaller units trying to froth too much milk can be an issue because they will run out of steam before the milk is finished frothing.
The shape of the spout on the frothing pitcher does have an effect on how the froth pours. The shape of the spout is not too important unless you plan on pouring latter art. Pitchers with wider mouths on the end of the spout will be harder to use for latte art because they don't provide enough definition. Wide mouth spouts are better suited for simpler pours like hearts or tulips. The traditional narrow mouth spout tip is better suited to pouring latter art because it helps the milk pour in a more defined stream, which will assist when trying to create latte art that requires more detail, such as rosettas.
Some machines utilize additional accessories on their steam wands, or use a different frothing system altogether. The key feature of these frothing aids are that they simplify the frothing process for the user.
Pannarellos are a special wand attachment that many Gaggia and Saeco machines use to assist with manual frothing. Their main purpose is to assist in directing the flow of steam as well assist in properly aerating the milk. Using a pannarello does not completely remove the necessity of proper frothing technique. Pannarellos can be thought of as training wheels - they are a learning tool that helps you get good results even if technique is not perfect.
Pannarello wands do require a slightly different technique than a traditional frothing wand because of their air injection system. Proper frothing technique for the basic style of pannarello wand can be found in the video below.
The pannarello can also be use to do an automatic frothing. The milk foam may not be as high in quality, but it is a convenience factor that this wand attachment offers.
A special version of the pannarello style wand is available for many Gaggia/Sacco machines called a Latte Art Pannarello. As mentioned previously pannarello wands operate slightly differently and require a different technique to be used. The advantage of this wand is that it has a removable sleeve which transforms the pannarello into a manually operated wand, which allows for a much tighter microfoam to be produced.
To better see the advantage of the Latte Art style pannarello the video below shows
Cappuccinatore systems are a simpler auto-frothing system, but don't require any assistance from the user during frothing. These are commonly featured on semi-automatic and super-automatic machines. Cappuccinatores operate by drawing milk through a tube into a turbine system where steam is injected. The force of the steam causes the turbine to spin, which creates a pressure differential that draws the milk in. There is also a small air hole where air is also drawn into the turbine. As the air milk and steam move through the system the milk becomes frothed. This type of system will not create a tight microfoam. The bubbles will be larger, and there isn't any temperature control, because the milk only comes into contact with the steam while it is moving through the turbine. Some cappuccinatores have an adjustable air intake. This allows for more or less air to be let into the turbine. Less air will result in a foam that is closer to latte foam and more air will result in a cappuccino style foam.
Automatic frothing carafes are frequently featured on super-automatic style machines in addition to, or in lieu of a steam wand. Automatic frothing carafes automatic steam, froth, and sometimes also dispense the frothed milk. Frothing carafes that automatically dispense instead of being dispensed by the user are referred to as One Touch Cappuccino systems. The quality of froth produces by these systems will vary. For the most part they will produce a foam that is better than what a cappuccinatore can produce, but will still not be able to produce as good of a microfoam as manually frothed milk.
Purging & Cleaning
A frequently overlooked, but crucial aspect of proper frothing technique is cleanliness. Built up milk residues in the wand and the steam tip can significantly change how well a machine froths, is unsanitary, and in some cases can cause damage or premature wear to the machine.
Before an after steaming the steam wand should be purged. Purging before steaming bring the wand up to temperature and clears out and residual water that is sitting in the wand. You don't want this water to go into your frothed milk because it will change the consistency of the milk. Purging after you finish steaming is also important, because it blows out any milk residues that may still be in the tip. Failing to do this can eventually cause the steam tip/wand to clog, or may even pull milk residues back into the machine's boiler.
Also make sure to keep the steam wand very clean. After every single froth the wand and tip should be thoroughly wiped down to prevent residues from building up. Milk will bake onto the wand if not cleaned in between each use. This will lead to clogging, prevent the steam wand from operating properly, and will also impart undesirable flavors to the frothed milk.
Check your individual machine's support page or contact the manufacturer to see what cleanings should be done to the steaming portion of your machine and how frequently they need to be done.
Even with the best equipment frothing technique is the single most important determining factor in how the milk will froth. You can compare it to driving a car. Even if you have a luxury vehicle, if you do not know how to drive then you will not be able to get anywhere in the car. Similarly, even if you have high quality espresso equipment, if your technique is off it will be very hard to get a good microfoam and frothed milk that tastes good and pours well. The technique discussed in this section is for traditional commercial style frothing wands; other specialty type wands, such as pannarellos, will have their own specific technique that applies to them.
Select a milk based on the type of drink you are trying to make. High fat content milks will have sweeter flavor and a wetter froth, more suited to lattes and latte art. Lower fat milks will not be as sweet, but will have a drier more stable foam, which is better suited for cappuccino. Select a pitcher that will hold the appropriate amount of milk for the drink you are trying to make. For more details on proper milk to espresso ratios for each drink please refer to the drink types section of this article.
Preparing The Equipment
For smaller home equipment you may need to activate steaming mode for the machine to let it preheat. On larger equipment you may need to allow for time for the machine to recover if you frothed or brewed a shot recently. Optimally you will want to start frothing when the machine indicates it is at the maximum temperature. Make sure the wand and tip are clean, and be sure to purge the steam line before beginning to froth.
After you've purged then it is time to position the tip in the milk correctly. The video below shows how to properly position the tip for a latte vs. a cappuccino.
On very high end prosumer units and commercial units the machine may produce a lot of steam at a very high pressure. The technique in the video may not work as well, because so much steam is injected so quickly. On machines like you will have more control by positioning the tip and the wand differently. Position the wand so that the tip is pointed at the center of the pitcher. The tip should be slightly below the surface, just enough that the tip is covered without submerging it so far that the threading where the tip attached to the wand is submerged. The pitcher should be angled slightly towards you, just so you can see what is happening in the pitcher during frothing. Since these machines produce so much steam, enough air will be injected at the start the the milk will rise on its own, and you don't have to submerge the wand further to switch from frothing to steaming.
A clear sign that your tip or pitcher may be positioned incorrectly will be the sound the milk is making during frothing. If the milk sounds like it is screaming then the tip is submerged too far. The screaming is caused by steam hitting the bottom of the pitcher. Alternatively if the milk is making a loud spitting noise and large bubbles are forming across the surface then the tip is too close to the surface, or not submerged at all. When the tip is correctly positioned the milk will just make a very slight hissing noise during the air injection phase, and almost no noise when submerged for heating.
The optimal temperature for frothed milk is somewhere in the 130-150 °F range. There is some debate as to what the exact best temperature is, and it partially depends on the drink being made. What can be agreed upon is that once milk starts to go past 150 °F it starts to lose a little bit of the sweetness, and the velvety froth texture is lost to a drier, harder, gummier froth. After the milk goes past 160 °F the milk loses most of the sweetness, starts to taste flat, and after a while will start taking on a burned flavor.
The use of a frothing thermometers is a helpful learning tool, but a good barista will know when to stop the froth by feel alone. A good time to stop injecting steam is when the pitcher is just getting to the point of becoming too hot to hold. Since frothing thermometers lag, simply feeling how hot the pitcher is a more effective method of know how hot the milk is. By using this method you will reduce the risk of overheating your milk. You may notice that more skilled baristas don't use frothing thermometers for this reason.
For alternative milks it's preferable to froth at an even lower temperature than this, about 10-15 °F lower. Many alternative milks won't hold their structure as well at higher temperatures. Soy milk in particular will actually curdle if heated too far.
Don't forget to purge your wand after you stop frothing and wipe the wand/tip down thoroughly to remove any residues.
Before pouring your milk it's good practice to tap the pitcher to remove any large bubbles, and keep swirling the milk within the pitcher until you pour so that there is less separation between the time you stop frothing and begin your pour. Hold your cup at a slight angle, hold the pitcher high (4-5 in. from surface of espresso) and start pouring into the center of the cup. If any white spots appear you will want to fill them in by pouring milk in those spots. This pushes the frothed milk back down below the surface, letting the espresso come back to cover the top. Once your cup is about 2/3 of the way full lower the pitcher and start pour more slowly and closer to the surface of the espresso. More specific pouring technique for latte art will be covered later in the latte art section of this article.
If you're finding that your froth isn't coming out right then an adjustment typically needs to be made to technique, or the machine may require cleaning/maintenance. Common frothing mistakes are outlined in the video below. We suggest using whole milk if possible while getting used to frothing technique. The properties of this milk make it much easier to handle and get a good froth than other milks. Once you've mastered the process then switching to another milk will be easier.
When you make a mistake it is not a total loss. If you have frothed too long, injected too much air, or your froth has separated from the steamed milk there is a trick you can do to salvage the situation. Take a second frothing pitcher and pour the frothed milk from pitcher to pitcher several times. This will help incorporate the froth back into the steamed milk. It is not a perfect solution, but it definitely improves the quality some.
Wasting milk for frothing practice can get expensive. One trick to learning how to froth is to froth water with a small amount of dish soap in it. You won't be able to perfect your method this way, but it is very helpful in helping you learn where your more major mistakes are and how to correct them.
Types Of Drinks
Different drinks are going to have different ratios of coffee, steamed milk, and frothed milk. Certain drinks, such as a dry cappuccino, call for a specific type of froth. Knowing which drink you are trying to make will help you determine your frothing technique. Some of the more common milk based drinks are defined in the video below.
A controversial newcomer to the American espresso scene is the flat white. The drink originates from the down under, but has become more popular in the US. The definition of the proper ratios of this drink are a bit murky, and change from cafe to cafe, and country to country. We define our understanding of the flat white in the video below.
Once you have mastered creating the perfect froth the next step is to apply the skill to pouring latte art. Latte art requires a lot of practice and patience. Luckily if you are using home equipment you won't be rushed and can take the time to make mistakes and correct them. Again, you can use the trick of frothing water and soap for practice so you're not wasting expensive milk. A little bit of food coloring dissolved into a couple ounces of water can be used in lieu of espresso. To truly perfect your latte art though you will need to use real milk and espresso.