Exploring Surface Tension in Milk and Its Impact on Dairy Products

Surface tension is one of those quiet but consequential physical properties that shapes how milk behaves at every stage – from the udder to your cup. It governs how milk foams, how it emulsifies, how it responds to heat, and even how reliably it can be tested for quality. Yet it rarely gets the attention it deserves in dairy science discussions. Understanding surface tension in milk is not just a matter of theoretical interest; it has direct, measurable consequences for dairy processing, product texture, and quality control.

What is surface tension and why does it exist? 🔗

Surface tension arises from molecular cohesion – the tendency of molecules in a liquid to attract one another. Inside a liquid, every molecule is pulled equally in all directions by its neighbours. But at the surface, molecules have neighbours only below and to the sides, not above. This imbalance creates a net inward pull, causing the surface layer to behave like a stretched, elastic film. The result is what we call surface tension, measured in dynes per centimetre (dyne/cm) or millinewtons per metre (mN/m).

Pure water has a relatively high surface tension of around 72 dynes/cm at 25°C. Milk, however, is not pure water – it contains fats, proteins, and other organic compounds that significantly disrupt this cohesive force at the surface.

Surface tension values in milk 🔗

The surface tension of fresh cow’s milk typically falls in the range of 40 to 60 dynes/cm, substantially lower than water. Measurements taken 4 to 6 hours after milking at 18°C using the DuNoüy method recorded values ranging from 46.6 to 55.3 dynes/cm, with most samples clustered between 49 and 52 dynes/cm.

This lower value compared to water reflects the presence of surface-active constituents in milk. The exact reading at any given time depends on temperature, fat content, processing history, and the age of the sample after milking.

What makes milk’s surface tension lower than water? 🔗

The key lies in milk’s composition. Free fatty acids and proteins are the primary surface-active constituents responsible for milk’s lower surface tension compared to water. These molecules are amphiphilic – they have both water-attracting (hydrophilic) and water-repelling (hydrophobic) portions. When they migrate to the liquid-air interface, they disrupt the tight cohesion between water molecules, reducing surface tension.

Role of milk proteins 🔗

Casein and whey proteins are natural surface tension reducers. Both casein and whey proteins migrate to the milk surface and interrupt cohesive forces between water molecules. Because these proteins have both hydrophilic and hydrophobic segments, they are drawn to the surface interface where they can orient themselves with one end in the water and the other facing outward. This arrangement weakens the surface film and lowers tension.

Role of milk fat 🔗

Milk fat globules, each enclosed in a protein-phospholipid membrane, also contribute to reducing surface tension. Milk fat, certain milk proteins, and free fatty acids are surface-active constituents that undoubtedly affect milk’s surface properties. Skim milk, which has had its fat removed, predictably shows higher surface tension than whole milk because it retains fewer surface-active lipid components.

Effect of fat content and temperature 🔗

As fat content increases, surface tension generally decreases. Temperature also plays a significant role. Cooling milk for 45 minutes or longer to below 10°C caused an average decrease in surface tension of about 2.12 dynes/cm, with an additional drop occurring upon prolonged cold storage – an effect that could be partially reversed by rewarming. This happens because cooling alters the distribution and orientation of surface-active molecules at the milk-air interface.

How is surface tension measured in dairy labs? 🔗

Several established methods are used to measure surface tension in milk. Each comes with its own operating principle, level of accuracy, and practical suitability for dairy applications.

The ring detachment method (Du Noüy ring method) 🔗

The Du Noüy ring method involves placing a platinum wire ring on the surface of the liquid, then gradually pulling it upward and measuring the maximum force required to detach it from the surface. This force, corrected for ring geometry, gives the surface tension value. The calculation is based on the maximum force recorded and the perimeter of the ring. It is widely used in dairy research due to its relative simplicity and good reproducibility.

The drop weight method 🔗

The drop weight method is based on Tate’s law, which approximates the balance between the gravitational force pulling a forming drop downward and the surface tension force holding it to the capillary tip. By measuring the weight of drops falling from a capillary of known diameter, and applying a correction factor, surface tension can be calculated. In practice, accuracy is improved by measuring the combined weight of several drops and dividing by their number. This method is particularly suited to routine quality control due to its low equipment cost and potential for automation.

The Wilhelmy plate method 🔗

The Wilhelmy plate method uses a roughened platinum plate as a probe. The calculations in this technique are based on the perimeter of the fully wetted plate in contact with the liquid, and it can be used to monitor changes in surface tension over time. This technique is quick, simple, and accurate, and is widely used to determine the surface tension of milk, including in studies linking surface tension to free fatty acid content and foaming behavior.

Surface tension and dairy processing 🔗

Surface tension is not just a laboratory parameter – it directly influences how milk behaves during industrial processing and how finished dairy products turn out.

Foaming 🔗

Milk’s lower surface tension compared to water makes it easier for air bubbles to form and persist, enabling foam production. Research has established a positive relationship between surface tension and the volume of foam generated by steam foaming of milk – meaning that milk with lower surface tension tends to produce more foam. However, this relationship is complicated by the role of free fatty acids (FFAs). Lipolysis of milk fat releases surface-active monoglycerides, diglycerides, and free fatty acids, all of which are highly detrimental to the formation and stability of milk foams. So while lower surface tension from healthy protein and fat content supports foaming, elevated FFAs from rancidity can actually damage foam quality – a nuance critical for barista-style milk, cappuccinos, and foamed dairy products.

Emulsification and homogenization 🔗

Milk is naturally an oil-in-water emulsion, with fat globules dispersed in an aqueous continuous phase. Milk proteins behave as surfactants, lowering interfacial tension between dispersed fat and the continuous aqueous phase to create and maintain this emulsion structure. During homogenization, lower surface tension makes it easier to break fat globules into smaller, more uniformly distributed droplets, producing a smoother product. Processing conditions such as heat treatment, high-pressure treatment, and ultrasound can all alter emulsion structure and protein behavior in milk, affecting emulsifying properties in ways that influence the quality of downstream products like cheese and yogurt.

Effects of heat treatment 🔗

Pasteurization and other thermal processes affect surface tension by modifying the proteins responsible for it. Whey proteins are particularly susceptible to heat-induced denaturation, which causes unfolding, aggregation, and changes in their hydrophobicity and emulsifying properties. These structural changes alter how proteins position themselves at the liquid surface, shifting the surface tension of the processed milk. Understanding these shifts helps processors anticipate and control how heat-treated milk will behave in further manufacturing steps.

Surface tension as a milk quality indicator 🔗

Changes in surface tension can signal important quality issues in milk, making it a practical diagnostic tool in dairy quality control.

Detecting hydrolytic rancidity 🔗

When the enzyme lipoprotein lipase acts on milk fat, it releases free fatty acids (FFAs) – a process called lipolysis. These FFAs are strongly surface-active and cause a measurable depression in surface tension. This depression of surface tension by free fatty acids has been used as an indicator of hydrolytic rancidity in milk. Because surface tension measurement does not require chemicals and can be performed rapidly, it represents a practical, environmentally friendly alternative to conventional FFA testing methods for dairy processors.

Detecting adulteration 🔗

Surface tension values shift predictably when milk is diluted with water or supplemented with materials like skim milk powder. Studies on buffalo milk have shown that adding water to whole or skim milk decreases foam ability, while increasing proportions of skim milk powder significantly raise surface tension values across all milk samples. These characteristic shifts make surface tension measurement a useful, rapid screening tool for detecting certain forms of milk adulteration or dilution at collection points.

Lipid oxidation and protein denaturation signals 🔗

As stored dairy products age, milk fats can undergo oxidative degradation, generating compounds that alter surface-active properties. Monitoring surface tension during storage allows processors to detect early chemical deterioration before off-flavours become perceptible. Similarly, lipid oxidation during storage can impact emulsion stability, while protein denaturation induced by heat or pH changes alters interfacial properties and thus overall product quality. Tracking surface tension provides an early-warning signal for both types of compositional change, enabling timely intervention.

Practical significance for dairy products 🔗

Surface tension knowledge translates into concrete advantages across the dairy product range. For cheese manufacturing, it affects how whey separates from curds and the resulting texture of the final product. For ice cream, proper emulsification – facilitated by appropriate surface tension – is essential to achieve a smooth, stable structure during freezing and storage. For yogurt, emulsion stability governed partly by surface-active proteins determines the final gel texture and mouthfeel. In each case, knowing how surface tension responds to composition, temperature, and processing conditions empowers dairy technologists to make more precise, consistent products.

What do you think? Given that surface tension can signal both lipid oxidation and protein denaturation, could it eventually replace more resource-intensive tests in routine dairy quality control? And how might surface tension considerations change as the dairy industry increasingly develops plant-based milk alternatives with very different protein and fat profiles?

References

1. https://www.biolinscientific.com/blog/3-ways-to-measure-surface-tension
2. https://www.journalofdairyscience.org/article/S0022-0302(59)90760-X/pdf
3. https://www.sciencedirect.com/science/article/pii/S002203025990760X
4. https://scialert.net/fulltext/?doi=ijds.2014.106.115
5. https://www.sciencedirect.com/topics/engineering/du-nouy-ring-method
6. http://www.surface-tension.org/articleshow_56.html
7. https://www.sciencedirect.com/science/article/abs/pii/S0963996908000677
8. https://pmc.ncbi.nlm.nih.gov/articles/PMC11507225/
9. https://www.researchgate.net/publication/353807654_Effect_of_Thermal_Denaturation_and_Calcium_on_Interfacial_tension_of_Whey_Protein_stabilised_Emulsions
10. https://pmc.ncbi.nlm.nih.gov/articles/PMC12311586/