Does Heating Milk Change the Protein? What You Need to Know

January 6, 2026 •

4 min read

Milk is a complex emulsion containing about 3.3% protein, primarily casein and whey. When subjected to heat, the proteins in milk undergo significant structural alterations, a process known as denaturation, which influences everything from taste and texture to nutritional profile.

Quick Summary

Heating milk causes protein denaturation, particularly affecting heat-sensitive whey proteins, which unfold and aggregate. Casein is more heat-stable, but can interact with denatured whey. The extent of change depends on temperature and time, impacting texture, taste, and allergenicity.

Key Points

Denaturation of Whey Protein: Whey proteins (beta-lactoglobulin and alpha-lactalbumin) are highly heat-sensitive and irreversibly unfold, or denature, when heated above 60-70°C.
Casein is Heat-Stable: Casein, the most abundant milk protein, is much more resistant to heat-induced denaturation and remains largely stable during heating.
Aggregation and Interaction: As whey proteins denature, they form aggregates and bind to the more stable casein micelles, which can alter the milk's texture and viscosity.
Altered Digestibility and Allergies: The structural changes caused by heating can make milk proteins easier to digest for some people and can reduce the allergenicity of whey proteins for those with milk protein allergies.
Nutrient Retention: Standard pasteurization retains nearly all protein quality, but intense heating (like UHT or boiling) can cause a minor loss of some heat-sensitive amino acids and vitamins.
Taste and Texture Changes: Severe heating triggers the Maillard reaction between proteins and sugars, leading to a 'cooked' flavor, a darker color, and the formation of a skin on the surface.

The Science of Milk Protein

Milk's protein content consists of two primary types: casein and whey protein. Casein makes up about 80% of the protein and exists as large molecular clusters called micelles. Whey protein, which is about 20% of milk's protein, comprises several smaller, globular proteins like beta-lactoglobulin (β-Lg) and alpha-lactalbumin (α-La).

The fundamental change that occurs when milk is heated is protein denaturation. This process involves the unfolding of a protein's complex, three-dimensional structure. While this sounds drastic, it's a common and natural part of cooking and is not necessarily a negative outcome. The consequences of denaturation depend on the protein type and the intensity of the heat treatment.

The Fate of Whey Protein

Whey proteins are significantly more heat-sensitive than casein. Their globular structure begins to unfold at temperatures above 60–70°C. This unfolding exposes reactive chemical groups, such as the thiol groups in β-Lg, which cause the proteins to interact with each other and with casein micelles. The specific effects on whey protein include:

Aggregation: Denatured whey proteins form aggregates with each other through disulfide bonds.
Binding to Casein: β-Lg, and to a lesser extent α-La, bind to the surface of casein micelles, particularly with κ-casein. This changes the micelles' properties and can increase viscosity.
Altered Digestion: The change in structure can affect how the protein is broken down by the body's enzymes, potentially improving digestibility for some and reducing it for others.
Reduced Allergenicity: For individuals with a cow's milk allergy triggered by heat-sensitive whey proteins, the denaturation caused by cooking can make the milk tolerable.

The Stability of Casein

In its natural state, casein is very heat-stable and does not denature easily, even at boiling temperatures. The changes observed in casein during heating are often a consequence of its interaction with the denaturing whey proteins. As whey protein binds to the casein micelles, it can increase their size and cause them to aggregate, which can lead to a change in the milk's texture.

How Various Heating Methods Affect Protein

The intensity and duration of heating have a direct correlation with the extent of protein denaturation and its resulting effects. Below are some common heat treatments and their impact on milk protein.

Pasteurization

This process is designed to kill harmful pathogens while minimizing changes to milk's sensory and nutritional quality. The most common method, High-Temperature Short-Time (HTST) pasteurization, heats milk to around 72°C for 15 seconds. This causes a minor, but not insignificant, amount of whey protein denaturation. Studies show that HTST can cause about 1% denaturation of immunoglobulins (a type of whey protein). The effect on overall protein and calcium content is minimal.

Ultra-High-Temperature (UHT) Processing

UHT sterilization heats milk to temperatures of 135–150°C for only a few seconds, creating a shelf-stable product. This extreme heat treatment causes extensive and irreversible denaturation of whey proteins and more significant interactions with casein. This results in a distinctive, cooked flavor and can activate heat-stable enzymes like plasmin, which may cause age-gelation during storage. Some studies suggest UHT milk may result in slightly reduced protein retention during digestion compared to regularly pasteurized milk.

Boiling Milk at Home

When milk is boiled at home (around 95°C), the whey proteins denature extensively, much like in UHT processing. This leads to the formation of a skin on the surface of the milk, which is a result of aggregated and dried protein and fat. Boiling also accelerates the Maillard reaction, a chemical reaction between milk proteins and lactose, which gives the milk a slightly darker color and a sweeter, more caramelized taste.

A Comparison of Heating Methods on Milk Protein


Feature	Raw Milk	Pasteurized Milk	Boiled Milk (Home)
Whey Protein Denaturation	None	Minor (~1-10%)	Extensive
Casein Micelle Interaction	None	Limited	Significant
Protein Structure	Native, globular	Mostly native, some unfolded whey	Extensive unfolding of whey protein
Effect on Digestibility	May be harder to digest for some	Generally not affected	Denatured whey may be easier to digest for some
Impact on Flavor	Mild, fresh taste	Mild, fresh taste	Distinctive cooked or caramelized flavor
Allergenicity of Whey	High for sensitive individuals	Potentially lower for sensitive individuals	Significantly reduced for sensitive individuals

Is Heated Milk Less Nutritious?

While heating milk causes protein denaturation, it does not typically destroy protein's overall nutritional value, especially in the context of commercial pasteurization. The amino acid profile remains largely intact. However, prolonged or severe heating can cause the loss of certain heat-sensitive vitamins, such as some B vitamins (B12, riboflavin, folate) and vitamin C. In most diets, milk is not the primary source of these vitamins, so this loss is generally not considered a major nutritional drawback. Furthermore, modern fortified milk often has these vitamins added back in after processing.

Conclusion

Heating milk fundamentally changes its protein content through denaturation. The degree and nature of these changes vary significantly depending on the temperature and duration of the heat treatment. While the heat-sensitive whey proteins are most affected, casein micelles also undergo alterations as they interact with denatured whey. For commercial products, pasteurization offers a safe middle ground, killing pathogens with minimal nutritional and sensory impact. Boiling at home, while safe, results in more extensive protein denaturation, causing a thicker texture, sweeter taste, and some reduction in heat-sensitive vitamins. Ultimately, the question of whether to heat milk depends on your preference for taste, texture, and whether you are consuming raw milk, which always requires heat treatment for safety. For most consumers, the protein in commercially processed milk remains a highly nutritious component of their diet.

Frequently Asked Questions

While boiling can cause a minor reduction in heat-sensitive vitamins like B12 and folate, the overall protein and mineral content remains largely unaffected. The impact on overall nutritional value is minimal, especially since many commercial milks are fortified.

Each time milk is heated, its proteins, particularly whey, undergo further denaturation and aggregation. This can lead to a grainy or unpleasant texture and a 'burnt' taste, but it does not completely destroy the nutritional value of the protein.

The skin on boiled milk is formed by a layer of denatured whey proteins and fats that have aggregated and dried on the surface. It is completely safe to eat or can be skimmed off if you prefer.

Yes, for some individuals with a cow's milk allergy triggered by heat-sensitive whey proteins, heating milk can denature these allergens, making the milk more tolerable. However, casein is heat-stable and can still trigger a reaction, so it is crucial to consult a doctor first.

Yes, heating milk, especially at high temperatures like boiling or UHT, deactivates many naturally occurring enzymes. While some enzymes in raw milk are considered beneficial, their inactivation in processed milk is not considered a significant nutritional loss.

No. Pasteurization uses lower heat for a shorter time, resulting in only minor denaturation of whey proteins. Boiling uses higher heat for longer, causing much more extensive and irreversible denaturation and aggregation of whey protein.

For most people, digestibility is not significantly altered. In fact, for some, the denaturation of protein can make it easier to break down. However, for a small number of people, the aggregation of denatured proteins might slightly decrease digestibility.