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.

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.

Purging & Cleaning

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

Practicing

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