Coffee lovers have many choices in their favourite beverages, such as latte, cappuccino and flat white. Milk froth is the key element that binds these drinks together, created through the steaming process, providing a velvety texture and creamy consistency, enhancing the intense flavours of the coffee.
While cow’s milk is traditional, plant-derived milk substitutes offer diversity for different dietary preferences. Whether almond, soy, oat or coconut milk, these alternatives bring unique flavours and textures to customised coffee experiences.
Baristas skilfully create the perfect milk froth, balancing microfoam and texture for exceptional taste and presentation. Milk froth is crucial to elevate your coffee experience.
Delving into the science of milk frothing
To truly understand the art of milk frothing, it is essential to understand the composition of the milk itself. By knowing the ins and outs of milk and how it behaves when heated, we can greatly improve our ability to create exceptional lattes, cappuccinos and a variety of other popular coffee drinks.
Cow’s milk, the most common type of milk used in coffee preparations, is a nutrient-rich liquid that has been part of human consumption for millennia, long before the advent of coffee. In essence, milk mainly comprises water, the fundamental element that harbours many chemical compounds.
Within cow’s milk, several components contribute to its distinctive characteristics:
- Fats, which account for 3-4% of the solid content, give the milk an exquisite creaminess.
- Proteins comprise about 3.5% of the composition and are crucial in creating structure and texture.
- Lactose, a type of sugar, accounts for approximately 5% of the milk composition.
Note: It is important to note that these figures serve as general indicators, as the exact chemical composition of cow’s milk can vary depending on factors such as the breed of cow. Each breed may have slight variations in the proportions of fat, protein and lactose, adding subtle nuances to the taste and texture of the milk.
The science behind milk frothing is an intricate dance between the heat of the steam and the components of the milk, resulting in a delicious sensory experience.
The Influence of milk proteins on milk froth quality
Proteins are molecules composed of long chains of amino acid residues known as polypeptides. They have different structures and sizes and are found in the liquid that makes up milk.
Two types of proteins are particularly important: caseins and whey proteins. Caseins form micelles aggregates, while whey proteins, including β-lactoglobulin and α-lactoalbumin, have well-defined secondary and tertiary structures.
The key difference lies in their response to heat:
- Caseins are highly thermostable, retaining their structure even when heated.
- Whey proteins, with their complex three-dimensional structures, unfold and undergo irreversible changes when exposed to heat, starting at around 40°C. This process, known as denaturation, is known as denaturation. This process, known as serum denaturation, alters their functionality.
Understanding these differences is vital, as they directly influence the behaviour of proteins when subjected to the heat of a steam lance during milk foaming.
The impact of heat on milk
When milk is subjected to heat, it undergoes changes that affect its chemical structure, mainly due to the temperature reached and the duration of heating.
In the industry, milk is often subjected to a heating process called pasteurisation, in which it is heated to 72-80°C for 15-30 seconds. This process helps to eliminate pathogens that could pose a health risk.
➡️ Pasteurisation denatures some whey proteins but does not affect all proteins, preserving the important caseins.
Proteins play an important role in milk, especially in frothing. Milk proteins are partially hydrophobic, meaning that one end of the protein chain attracts water while the other repels it. This property gives milk its opaque white colour and is crucial for building the foam structure.
Importantly, heat profoundly impacts milk, altering its proteins and ultimately influencing the taste and texture of the final product.
Milk changes with the steam lance
When heat is applied using a steam lance, fascinating transformations occur within the milk: The heat begins to break down the protein chains, causing them to fragment into smaller chains. In some cases, these chains even dissociate, separating into individual protein molecules.
Interestingly, these proteins exhibit a repelling nature towards the water. They instinctively wrap themselves around air bubbles since that’s the only entity they can adhere to. As this captivating interplay unfolds, a web-like structure of air bubbles emerges, giving birth to the much-desired milk froth.
The protein content influences the stability of the froth in the milk. Natural milk, as it occurs in its unaltered state, typically contains around 3.3% protein. However, additional proteins are added in many commercial kinds of milk to facilitate the homogenization process during heat treatment.
Therefore, the milk you utilize may likely contain a protein content ranging from 3.6% to 4.1%, resulting in a froth that exhibits enhanced stability and longevity.
The impact of vapour on milk foaming
As steam and heat are introduced, water vapour and air are incorporated into the milk. During this process, proteins play a key role, creating a structure that stabilises and forms bubbles.
Milk protein chains are polar in nature. One end of the chain is hydrophilic, which is attracted to water, while the other is hydrophobic, repelled by water.
During denaturation, when proteins unfold, their hydrophobic ends are exposed. These hydrophobic ends actively seek to move away from water within the milk. Consequently, within each bubble, all the hydrophobic ends face inwards, devoid of water.
Meanwhile, the hydrophilic ends are exposed to the surrounding milk, creating a structure that suspends the bubbles and helps maintain their integrity.
As the temperature rises above 30 to 40 degrees Celsius, the stability of the milk decreases, leading to the rapid formation of larger air bubbles.
However, when the temperature increases to about 60 degrees Celsius, the more stable and desirable milk foam is achieved. This foam has a superior texture and density, providing an enhanced sensory experience.
It should be noted that the smaller, finely dispersed air bubbles materialise within the temperature range of 60 to 63 degrees Celsius, never at higher temperatures.
How milk fat affects taste, froth and mouthfeel
When choosing milk for your coffee, three factors matter:
- Taste: Milk fat adds fantastic flavour.
- Foam stability: Fat can affect froth longevity.
- Mouthfeel: Fat contributes to a smooth texture.
Cow’s milk is an emulsion of fat and liquid. Butterfat, the main fat in milk, consists of large, heavy globules. These can destabilize foam and shorten its lifespan.
- Excessive fat prevents foam from holding, dissipating quickly. Raw milk can form a solid fat layer when cooled.
- High-fat content in milk hinders water-soluble coffee compounds from reaching your taste buds, creating a film that repels certain flavours.
- While skim milk avoids these issues, it sacrifices the smooth mouthfeel enjoyed in cappuccinos and lattes.
Finding the right balance is key, considering your coffee preferences for taste, froth, and mouthfeel.
What is the right temperature for frothing milk?
It is essential to avoid overheating the milk during the frothing process. In addition to the unpleasant taste of burnt milk, excessive heat can hinder the frothing process.
The natural milk proteins act as a protective layer around the air bubbles, keeping them intact. However, when the milk is overheated, the proteins are denatured, and there is not enough left to stabilise the bubbles.
Note: Reheating the milk will not produce the desired froth, as the structured state of the proteins is compromised.
The optimum temperature for steaming milk is 60-63°C.
Lower temperatures result in milk with too much foam, and lower temperatures result in milk with larger and more unstable bubbles. In addition, higher temperatures denature too much protein, resulting in insufficient stabilisation of the bubbles.
Remember to start with milk cooled in the refrigerator (1-4°C) to facilitate the steaming process and get better results.
Selecting the right milk for frothing
Skimmed milk produces the most stable froth but may lack the desired creamy texture. A compromise can be found with semi-skimmed or semi-fat milk, offering reliable foam and a satisfying mouthfeel.
Understanding the behaviour of milk’s structure when heated is crucial in creating exceptional espresso and milk-based beverages. By avoiding excessive temperatures and having knowledge of milk proteins, you can ensure that your foam remains intact.
When deciding on milk for espresso drinks, protein content is key. Without sufficient protein, the milk will not froth properly.
Barista milk is often specifically formulated with high protein content. However, regular milk can also be used successfully with careful temperature control.
