Cappuccinos, lattes, and white coffees are all coffee beverages that contain milk, and if they lack milk froth, we cannot enjoy the smooth and full taste of these beverages.

Do you know what happens to milk when it is heated in a steamer? This article takes a look at what happens to milk when it is frothed and how it affects coffee.

The Compounds Contained In Milk

Milk is a key ingredient in many coffee shops and an important element in the production of many coffee drinks. Understanding more about its structure and the changes that occur when it is frothed can help to create and develop better tasting coffee.

Milk is a nutrient-rich, complex liquid that is mainly composed of water, but contains hundreds of chemicals. These can be divided into four main groups: protein (1-20%), fat (2-55%), carbohydrates and sugars (lactose 0-10%), and minerals.

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Protein In Milk

Protein is the most affected substance when heated and is also the key to the quality of the milk froth.

In general, protein can be defined as a molecule made up of one or more long chains of amino acids, which are held together by peptide bonds.

There may be too many scientific words here, but to put the above statement in perspective, the proteins in milk are distributed throughout the liquid in different structures and sizes.

Whey protein and casein are the two main proteins in milk. These two proteins have different structures and react differently under pressure, so when the milk is heated and froth, the two proteins change differently.

The structure of the proteins is actually a difference in the arrangement of the atoms. The casein in milk forms aggregates called micelles, which are composed of 𝞪-, 𝞫- and 𝞳-casein and are primary structured proteins.

In simple terms, casein has a simpler structure than whey, and this difference directly influences the behaviour of the two proteins when they are vapour-foamed.

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Casein is more stable than whey when heated, in other words, casein retains its structure better when heated.

Whey proteins have a more complex three-dimensional structure that unfolds when heated to 40°C. This process is called 'thermal denaturation' and whey proteins irreversibly lose their structure, resulting in a change in the way they behave when heated.

How Heating Affects Milk

Any heating action will affect the chemical structure of the milk proteins, but how it does so depends on the temperature and duration of heating.

Assuming you are using pasteurized milk, this means that the milk has been pasteurized at 72-80°C for 15-30 seconds before you get it.

Pasteurization will cause some of the whey proteins to be denatured, but as the heating process is not very long, it will not affect all of the whey proteins.

Over-temperature treatment or over-heating is responsible for the altered taste of the milk as a result of the sulphur flavour that occurs during the heat treatment process.

This brings us back to these proteins, which are the key to the success of milk frothing.

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In its natural state, milk has reactive chemical groups hidden in the complex structure of the whey proteins, which are released when the whey proteins unfold during the heating process...

Because these chemical groups are active, they form new chemical chains with other whey components in these unfolded structures and this can have an impact on the frothing of the milk.

How Does This Affect The Frothing Of Milk?

How do the chemical changes described above affect your coffee?

When we froth milk, we force water and air into the milk by heating it, which causes the proteins to form spheres in the air and then turn into stable bubbles.

The protein chain in milk is polarised: hydrophilic at one end and hydrophobic at the other. As the protein unfolds during the denaturation process, the hydrophobic end of the protein tries to move away from the water in the milk.

This means that each froth has a bubble structure with the hydrophobic end of the protein chain inside, while the hydrophilic end remains in the milk liquid.

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When the milk froths at 30-40°C, it becomes unstable and forms large bubbles quickly. Raising the temperature to 60°C stabilizes the milk froth and improves its texture and density. At higher temperatures, smaller and better dispersed froths are formed.

Fat plays an important role in stabilising the froth. This liquid fat helps to prevent froth build-up by forming a film on the surface of the froth (to avoid the formation of large froth).

However, it is important to avoid heating the milk to a temperature that will cause the milk to develop a sulphuric taste and cause it to fail to ferment.

In its natural state, the protein clings to the bubbles, preventing small bubbles from forming into large ones. If you continue to heat the milk at this point, you will denature more of the protein, thus destroying its original properties and making it impossible to stabilize the bubbles.

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This explains why you cannot re-foam milk, even if you reheat it, the proteins in the milk will not have enough natural structure to form stable bubbles.

Milk with a higher lipid content may help to form a more stable froth, but milk fat is the main fat in milk and it is a large, heavy fat.

Over 95% of milk fat is a small ball of 0.1-15 microns in diameter, and with fat this large and heavy, it can compress the air bubbles and cause them to burst. The fat also covers up other flavours, which means that some of the flavour characteristics of the coffee bean itself may be sacrificed.

But if you want to choose skimmed milk, remember that fat also affects the appealing taste of cappuccinos and lattes.

What Does This Mean For You?

When you want to choose the milk to add to your coffee, it's all about the protein content of the milk. If there is no protein, the milk will not ferment. This is the reason for the high protein content of products such as coffee milk, but you can also use regular milk if you can control the temperature effectively.

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The ideal temperature for frothing milk is between 60-63°C. Below this temperature, you may get unstable froth and large air bubbles; above this temperature, too much protein is denatured and the protein in the milk will not have enough natural structure to form stable bubbles.

Frothing with skimmed milk may result in stable froth, but it lacks the good taste of milk fat. With low-fat milk, you may be able to get both a stable froth and a full-bodied milk.

Understanding the chemistry of milk can help to make a better espresso-based drink, and by understanding how the proteins in milk change, you can avoid frothiness.

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Summary

• The protein in milk is the most affected substance when heated and is the key to the quality and success of the froth.
• Whey proteins have a complex three-dimensional structure that unfolds when heated to 40°C. This process is known as 'thermal denaturation'.
• High temperatures for sterilisation or excessive heating can alter the flavour of the milk, creating a sulphuric taste and causing frothing failure.
• Milk fat plays an important role in stabilising milk froth, and milk with a higher fat content may help to create a more stable froth. However, fat can also override other flavours, representing a possible sacrifice of some of the original flavour of the coffee beans.
• Even if reheated, the proteins in the milk will not have enough natural structure to form a stable froth.
• The ideal temperature for frothing milk is between 60-63°C. Below this temperature, you may get unstable froth and large air bubbles; above this temperature, too much protein may be denatured.

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