Does boiling milk destroy proteins? The science of heat and milk's nutritional value

October 17, 2025 •

4 min read

Milk contains two primary types of protein, casein and whey, which constitute approximately 80% and 20% of its total protein content, respectively. Exposing milk to high temperatures, such as boiling, significantly affects these proteins by causing denaturation, but not total destruction.

Quick Summary

Boiling milk causes protein denaturation, a structural change that impacts the heat-sensitive whey proteins more than stable casein proteins. The process alters the milk's nutritional profile by affecting some vitamins and minerals, influencing digestibility, and modifying the taste and texture. It does not completely destroy the milk's protein content.

Key Points

Denaturation, Not Destruction: Boiling milk causes proteins to denature (change shape), but does not break them down into non-nutritious components.
Casein is Heat-Stable: The majority of milk's protein, casein, is largely unaffected by boiling temperatures and remains stable.
Whey Protein is Heat-Sensitive: The minority whey protein is heat-sensitive and denatures at boiling temperatures, causing changes in texture and digestibility.
Some Vitamins are Lost: Boiling leads to the degradation of heat-sensitive vitamins, especially B-vitamins like riboflavin and folic acid.
Potential for Improved Digestibility: For some individuals, the denaturation of proteins in boiled milk can make it easier to digest.
Essential for Raw Milk: Boiling is crucial for unpasteurized raw milk to eliminate potentially harmful bacteria and ensure it's safe to consume.

Understanding the Milk Proteins

Before delving into the effects of boiling, it's crucial to understand the two main proteins in milk: casein and whey.

Casein: Making up about 80% of milk protein, casein exists in stable structures called micelles. It is relatively heat-stable, meaning it can withstand high temperatures without significant structural changes.
Whey: Comprising the remaining 20%, whey proteins like beta-lactoglobulin ($\beta$-lg) and alpha-lactalbumin ($\alpha$-la) are globular and heat-sensitive.

The Effect of Heat on Milk Proteins: Denaturation, Not Destruction

When milk is heated, the proteins undergo a process called denaturation. Denaturation involves the unfolding of a protein's complex three-dimensional structure due to external factors like heat. This is a crucial distinction: denaturation changes the protein's shape and properties but does not break its fundamental amino acid chains, meaning the protein is not 'destroyed' in the way it might be by burning.

Denaturation of Whey Proteins

Whey proteins are the first to be affected when milk is heated, with denaturation beginning at temperatures as low as 68°C (154°F). When milk is boiled at 100°C (212°F), a significant portion of the whey proteins denatures. This process has several noticeable effects:

Formation of a 'skin': As whey proteins unfold, they interact with each other and with milk fat globules, forming a visible film on the surface of the milk as it cools.
Altered functional properties: The change in whey protein structure can be beneficial, such as improving the texture of yogurt by increasing its water-holding capacity. However, for some applications like cheesemaking, this interaction with casein can interfere with the coagulation process.

Impact on Casein Proteins

Casein is far more heat-resistant than whey. While boiling doesn't cause casein to denature significantly, the denatured whey proteins can aggregate and interact with the casein micelles. This aggregation can slightly impact the overall structure of the milk but does not compromise the core casein protein.

Nutritional and Digestibility Changes

Boiling milk alters its nutritional profile, though not as drastically as some may believe. The effects are mixed, with some losses and some potential benefits.

Loss of Heat-Sensitive Vitamins

Heat is particularly damaging to water-soluble vitamins. Studies have shown that boiling milk can reduce the content of B vitamins, with folic acid levels dropping by as much as 36% and riboflavin by 27%. While most fortified commercial milk has some of these nutrients added back, the loss is still notable. Other vitamins like vitamin C are also negatively affected.

Impact on Minerals

Minerals like calcium are relatively heat-stable, but their bioavailability can be affected by boiling. Studies have found a slight reduction in bioavailable calcium due to the formation of calcium phosphate precipitates. However, the effect is typically minor.

Changes in Digestibility and Allergens

For some people, boiling milk can improve digestibility. The heat-induced changes in protein structure can make the milk's proteins easier for the body's enzymes to break down. Furthermore, since some of the major allergens in milk are heat-sensitive whey proteins, individuals with mild cow's milk protein allergies may find they can tolerate boiled milk better than raw or pasteurized milk.

Comparison: Boiled Milk vs. Pasteurized Milk


Feature	Pasteurized Milk	Boiled Milk
Temperature	Heated to 63-72°C for a short time.	Heated to 100°C until bubbling.
Protein Impact	Preserves most protein structure; minimal whey denaturation.	Significant whey denaturation and aggregation.
Nutrient Retention	Retains most vitamins and minerals.	Higher loss of heat-sensitive vitamins (B-vitamins, C).
Safety	Kills harmful pathogens, making it safe.	Also kills bacteria; vital for raw milk.
Taste & Texture	Maintains natural flavor and creamy texture.	Develops a 'cooked' flavor, can form a skin.
Overall Nutritional Profile	More balanced; best for maximizing nutrient intake.	Less nutritious due to higher vitamin loss.

Practical Tips for Heating Milk

To minimize nutrient loss and unwanted textural changes when boiling milk:

Heat gently: Use medium-low heat and stir continuously to prevent scorching and rapid denaturation of proteins.
Monitor closely: Stop heating as soon as small bubbles appear around the edges of the pot to avoid over-boiling.
Don't re-boil unnecessarily: If you are using commercially pasteurized milk, re-boiling is not required for safety and only leads to further nutrient degradation.
Know your milk: Boiling is only truly necessary for raw, unpasteurized milk to ensure safety.

Conclusion

While boiling milk does not 'destroy' its protein content, it causes significant denaturation, particularly affecting the heat-sensitive whey proteins. This structural change, along with the degradation of certain vitamins, means boiled milk is not nutritionally identical to its pasteurized or raw counterparts. The decision to boil milk should consider the trade-offs: the added food safety for raw milk versus the loss of some vitamins. For most consumers, gently heating commercially pasteurized milk is the optimal way to retain both its flavor and nutritional integrity.

Learn more about the science behind milk processing and nutrition by visiting the National Institutes of Health website.

Frequently Asked Questions

Yes, boiling milk can reduce its overall nutritional value by causing the degradation of heat-sensitive vitamins, particularly B-vitamins like riboflavin and folic acid. However, the total protein and mineral content are not destroyed.

In terms of nutrient retention, pasteurized milk is often considered better because it uses lower temperatures for a shorter time, which preserves more of the heat-sensitive vitamins. Boiling provides little additional safety benefit for already-pasteurized milk.

Denaturation is a structural change where a protein unfolds from its original shape due to heat. Destruction, in this context, would mean the complete breakdown of the protein's chemical bonds. Denaturation preserves the amino acid chains, so the nutritional content is largely intact, even if the protein's function and properties have changed.

Boiling denatures some of the whey proteins responsible for many milk allergies. Some individuals with mild cow's milk protein allergies might tolerate boiled milk better, but it is essential to consult a doctor before doing so, as it is not a guaranteed solution for all allergies.

The skin that forms is caused by the denaturation and aggregation of the heat-sensitive whey proteins, which then interact with milk fat to create a film on the surface as the milk cools.

Boiling milk does not destroy calcium, as it is a heat-stable mineral. However, some studies suggest that excessive heat can reduce its bioavailability due to the formation of precipitates, though this effect is generally minor.

Yes, it is safe to boil pasteurized milk, but it is not necessary for food safety. Boiling will further degrade some vitamins and change the milk's flavor and texture.