In the world of coffee, there is a wide variety beyond black coffee: coffee with milk offers a range of irresistible possibilities. Milk plays a fundamental role in these delicious creations, from the classic latte to the elegant cappuccino or the delicate flat white.
The milk froth, obtained through steaming, provides a velvety texture and creamy consistency that completely transforms the coffee experience. It is no longer just coffee but a perfect combination of coffee and milk, where the froth plays an essential role in highlighting and enhancing the intense flavors of the coffee.
A delicate balance between microfoam and texture is required to achieve the perfect milk froth, guaranteeing an impeccable presentation and unparalleled flavor in each cup. This art is based on good technique and minimal knowledge of how milk reacts to temperature and water vapor.
Relationship between the composition of milk and perfect froth
Cow’s milk consists mainly of water (90%), a content very similar to that of vegetable milk. However, in this article, we will focus on cow’s milk, although what is mentioned also applies similarly to milk such as soy, oat, rice, and other types.
In addition to water, milk contains a variety of compounds that make it very special and nutritious:
Fats present in cow’s milk
They represent approximately 3-4% of the solid content, provide a creamy texture and contribute to the flavor and mouthfeel of milk-based coffee beverages, such as lattes and cappuccinos. These fats are responsible for the creaminess and richness sought when milk is properly frothed.
Proteins present in cow’s milk
They represent about 3.5% of the milk composition and are crucial in forming structure and texture in coffee beverages. Milk proteins denature and intertwine during the frothing process to form a network that traps air bubbles, thus creating a stable and smooth froth.
Lactose present in cow’s milk
Representing approximately 5% of the composition of milk, lactose is a type of sugar present in cow’s milk. Lactose provides a slightly sweet taste to coffee drinks made with milk, and its content can also affect frothing capacity and froth stability.
The science behind milk froth is an intricate dance between the heat of the steam and the components of the milk, resulting in a delicious sensory experience.
The key to perfect froth is in the proteins.
Of the compounds mentioned above, proteins are crucial for frothing milk. Among the various proteins in milk, caseins, and whey proteins are two of particular importance.
These proteins play a key role in the composition and characteristics of milk but also greatly affect froth formation when heat is applied with a steam wand.
Caseins in milk
Caseins, present in milk, clump together to form micelles, which help maintain the milk liquid’s stability. These highly thermostable proteins retain their structure even when subjected to high temperatures.
Caseins contribute to the formation and retention of froth when milk is heated.
Whey proteins from milk
On the other hand, whey proteins such as Beta-lactoglobulin and Alpha-lactalbumin exhibit well-defined secondary and tertiary structures, which give them specific functions in milk, such as their special reaction to heat.
Above 40°C, these proteins with complex three-dimensional structures unfold and undergo irreversible denaturation.
Whey proteins decrease the air-holding capacity of the froth, resulting in a less dense but more stable froth. In addition, the alteration of whey proteins can affect the overall taste and texture of the froth.
How heat impacts milk
When milk is subjected to heat, two fundamental factors directly impact it: the heating temperature and the duration of the heating process.
Pasteurization is a prime example of the aforementioned impacts, as it significantly destroys microorganisms present in milk through heat. This process considerably reduces the risk of infections and diseases associated with its consumption.
However, heat also affects milk proteins. When subjected to heat, some proteins are denatured, except for caseins, which have a unique structure that allows them to resist thermal denaturation.
So how does heat affect milk, and how does it influence frothing? When heat interacts with the caseins in milk, they form a stable froth.
These caseins have hydrophobic properties and a compact structure, which allows them to adsorb at the air-water interface. In doing so, they create an elastic film that envelops the air bubbles and contributes to foam formation.
Changes in milk with the steam lance
As milk is heated, physical and chemical changes occur in its composition. Here are some of the transformations that occur:
- Transformation of proteins: the proteins in milk transform due to heat. This means the protein structures are modified, fragmenting into smaller chains and even individual molecules.
- Foaming: The protein content influences the formation of froth. This is due to the heat and pressure generated by the steam lance when stirring and incorporating air into the liquid.
- Caramelization of sugars: Milk contains natural sugars, such as lactose. When heat is applied, these sugars can caramelize, which involves a chemical reaction that results in characteristic flavor and color compounds.
- Water removal: Heat accelerates the evaporation of the water present in milk. As the milk is heated, water vapor is released, which can reduce the milk volume and concentration of the remaining components.
- Pasteurization: If heat with the steam lance is applied at a suitable temperature and for sufficient time, pasteurization of the milk can occur. Helping to destroy pathogenic bacteria and reduce the microbial load.
When steam and heat are introduced into the milk, water vapor, and air are incorporated. During this process, the proteins create a structure that stabilizes the milk and forms air bubbles.
How milk fat influences flavor, froth, and mouthfeel
When choosing milk for coffee, there are three important factors:
- Flavor: The presence of fat in milk improves flavor perception by providing a creamier texture and mouthfeel. In addition, fat can retain and enhance flavors, contributing to a richer and fuller flavor.
- Froth stability: Fat particles are distributed in small droplets trapped in the protein matrix when milk is agitated. The higher the fat in the milk, the less durable the froth.
- Mouthfeel: The presence of fat provides a smooth, unctuous mouthfeel.
The milk with the highest fat content is whole milk since it has not undergone any skimming process to reduce its fat content and contains approximately 3.5% to 4% fat.
On the other hand, there are other types of milk with lower fat content, such as semi-skimmed milk and skimmed milk, used in cappuccinos and lattes.
What is the right temperature for frothing milk?
The proper temperature for frothing milk ranges from 140 to 145 degrees Fahrenheit.
⬇️ When temperatures are below 140 to 145 degrees Fahrenheit, the milk may froth excessively and thinly. This means that the froth formed at low temperatures may have difficulty retaining air and, as a result, may collapse more easily.
⬆️ On the other hand, the resulting froth may have different characteristics if you use a temperature exceeding 140 to 145 degrees Fahrenheit. This is because excessive heat can affect the ability of the milk proteins to coagulate and stabilize the air bubbles.
⚠️ A very important factor to consider is the initial temperature of the milk. Cold milk, generally in the 34 to 39 degrees Fahrenheit, improves emulsification and steam distribution, resulting in a more consistent foam.
Selecting the right milk for frothing
Whole milk is the most suitable milk for frothing and obtaining a high-quality froth. The fat it contains contributes to a smoother, creamier, more stable texture in the froth. It acts as a natural emulsifier, helping to retain air in the foam bubbles and providing a richer consistency.
However, obtaining good results with lower-fat milk, such as semi-skimmed or skimmed milk, is also possible. This milk can generate acceptable froth, but the lower fat content may compromise texture and stability.
In general, fresh, quality milk is essential for good froth. Be sure to use preferably refrigerated milk, as it tends to froth better than milk stored for a long time.