The science behind milk steaming

The art of milk-based coffee, from lattes to cappuccinos, revolves around steamed milk. This technique creates a velvety foam that transforms the flavor and texture of coffee, resulting in a perfect harmony of coffee and milk. Achieving this foam requires balance and knowledge about how milk reacts to steam and temperature.

Note: Starting with quality coffee beans is essential to make good milk-based coffees.

The Relationship Between Milk Composition and the Perfect Foam

The quality of the foam in drinks like lattes and cappuccinos largely depends on the composition of the milk used, mainly cow’s milk. Although it shares a high water content with plant-based milks, its unique composition of fats, proteins, and lactose is crucial for achieving the ideal foam.

  1. Fats (3-4%): These contribute to creaminess and enhance both the flavor and mouthfeel in coffee drinks. They are key to achieving that rich, smooth texture desired in well-made foam.
  2. Proteins (3.5%): Vital for foam formation, proteins denature during steaming, forming a network that traps air. This process allows for a stable and smooth foam.
  3. Lactose (5%): The natural sugar in milk, which not only provides a slight sweetness but also influences the foaming ability and the stability of the foam.

The process of steaming milk involves a complex interaction between heat, fats, proteins, and lactose, which is fundamental for achieving the characteristic texture and flavor in these coffee drinks.

The Key to Perfect Foam Lies in Proteins

In the foam of milk for coffee drinks, proteins play an essential role. Among them, caseins and whey proteins, such as Beta-lactoglobulin and Alpha-lactalbumin, are particularly important.

  • Caseins: These proteins form micelles that contribute to the stability of milk. When exposed to high temperatures during steaming, caseins maintain their structure, which is fundamental for foam formation and retention.
  • Whey Proteins: When heated from 40°C upwards, whey proteins denature, changing their structure. This denaturation affects both the stability and texture of the foam, resulting in a less dense but more stable foam. Moreover, this transformation can influence the final flavor of the foam.

The balance between these proteins during the foaming process is key to achieving the desired texture and stability in the milk foam for lattes and cappuccinos.

The Impact of Heat on Milk

  1. Temperature and Duration of Heating: These two factors are crucial in altering the milk. Pasteurization is an example of how heat can destroy microorganisms, improving safety for consumption. However, heat also modifies the proteins in milk, though caseins resist thermal denaturation.
  2. Effect of Heat on Foam Formation: Caseins, due to their hydrophobic properties and compact structure, are fundamental in the stability of the foam. When heated, these proteins adsorb at the air-water interface, forming an elastic film around the air bubbles, which is essential for foam formation.

Changes in Milk with the Steam Wand

  1. Protein Transformation: Heat modifies the protein structures, unfolding them into smaller fragments.
  2. Foam Formation: The agitation and air incorporated by the steam wand, along with the protein content, are key in foam formation.
  3. Sugar Caramelization: Heat can caramelize natural sugars like lactose, affecting the flavor and color of the milk.
  4. Water Evaporation: Heating accelerates the evaporation of water in milk. As the milk heats up, water vapor is released, reducing the volume of milk and concentrating the remaining components.
  5. Pasteurization: Controlled application of heat can result in pasteurization, reducing the microbial load.
Steaming Milk in the Lelit Anna PID
Steaming Milk

Steaming Milk: The process of introducing steam and heat into the milk generates the incorporation of air, creating bubbles stabilized by proteins. This action transforms the milk, creating the texture and structure desired for the foam in coffee drinks.

Influence of milk fat on flavor, foam, and mouthfeel

  1. Flavor: Fat enhances flavor perception by adding texture and a creamier mouthfeel. It also retains and enhances flavors, contributing to a richer overall taste.
  2. Foam Stability: When milk is agitated, fat particles are distributed into small droplets trapped within the protein matrix. A higher fat content can reduce the durability of the foam.
  3. Mouthfeel: Fat adds smoothness and richness. Whole milk, with 3.5% to 4% fat, is richer, while lower-fat options like semi-skimmed or skimmed milk are also used in coffee beverages.

Ideal Temperature for Milk Foaming

  • Optimally, milk should be foamed between 60 and 63 degrees Celsius (140 to 145 degrees Fahrenheit).
  • Below 60°C, foam may be excessive and less dense, easily collapsing.
  • Above 63°C, excessive heat can alter the proteins’ ability to stabilize the foam.
  • Using cold milk, between 1 and 4°C (34 to 39°F), improves emulsion and vapor distribution, resulting in a more consistent foam.

Selecting the Right Milk for Foaming

  • Whole milk is ideal for foaming, thanks to its fat content which adds smoothness, creaminess, and stability to the foam.
  • Semi-skimmed or skimmed milk can also produce acceptable foam, though with slightly different texture and stability.
  • It’s important to use fresh and refrigerated milk for the best results.


Creating the ideal foam for lattes and cappuccinos is an art based on science. Every element of milk, from fats to proteins, along with the proper temperature, significantly contributes to the foam’s texture and flavor.

This knowledge is essential not only for baristas but also for coffee enthusiasts seeking perfection at home. Thus, behind every cup of frothy milk coffee, there’s a delicate balance of science and care that makes that unique and delicious experience possible.

Written by Pablo Barrantes Nevado
I am Pablo Barrantes, a coffee lover. I decided to start this website to solve all the doubts that arise every day when preparing our favorite drink: coffee. I am an industrial engineer by profession, but I have worked in coffee shops for many years, where I have learned all the secrets about coffee machines and coffee. My passion for coffee has led me to investigate and study beyond the obvious, and thanks to this, I can offer solutions and give news about coffee and coffee makers. I hope you enjoy reading as much as I research, document and write here.

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