Test:Techniques

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.

Milk

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

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.

Temperature

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.

Pasteurization

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.

Thermometers

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.

Equipment

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.

Machine Features

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 machine 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.

Tub/Small Volume

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

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/Dual Boiler

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.

Wand Style

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.

Steam Tips

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.

Purging & Cleaning

https://www.youtube.com/watch?v=AE-tIbaJcI8

Practicing

https://www.youtube.com/watch?v=y17PenLLTdA