Milk Powder Adhesion: The Science of Stickiness & Fouling

Milk powders and infant formulas look simple on the shelf, yet the way those tiny particles interact with water and with surfaces is anything but simple. Adhesion is at the heart of many real‑world headaches: powders that cake in hoppers, sticky residues in spray dryers, stubborn films in baby bottles, and even the way bacteria attach to stainless steel in dairy facilities. As a smart hydration specialist, I see adhesion as the hidden force that links powder science, equipment hygiene, and safe everyday hydration.

In this article, we will unpack what scientists mean by adhesion in milk powder systems, how moisture and temperature drive stickiness, what really happens on stainless steel and drying surfaces, and how industry measures and mitigates these effects. Then we will translate these insights into practical guidance for both manufacturers and health‑conscious families preparing milk powder solutions at home.

What “Adhesion” Means In Milk Powder Systems

When you mix milk powder into water, two families of forces come into play: how particles stick to each other, and how they stick to whatever they touch.

Cohesion is the attraction between powder particles themselves. In a cohesive milk powder, particles cling together, form clumps, resist flow, and need more energy to break apart and disperse.

Adhesion is the attraction between the powder (or the resulting liquid film) and a surface such as stainless steel, plastic, or glass. In practice, adhesion shows up as powder sticking to hopper walls, dried films on spray dryer surfaces, or milk residues that cling to pipes, bottles, and mixing vessels.

Scientific work on milk powders increasingly connects these everyday observations to more fundamental quantities such as surface free energy and work of adhesion. In dairy science, “work of adhesion” describes the energy needed to separate a liquid from a solid surface. A higher work of adhesion means the liquid spreads more and holds on more tightly, which translates into more persistent residues and more difficult cleaning.

To make these concepts concrete, here is a compact reference.

Concept	Description	Relevance for milk powder solutions
Adhesion	Attraction between powder or liquid and a surface	Governs how strongly milk or formula films stick to stainless steel, glass, or plastic
Cohesion	Attraction between powder particles	Controls clumping, caking, and how easily powder disperses in water
Surface free energy	Tendency of a surface to minimize its area	Higher solid surface energy often leads to stronger adhesion with liquids
Work of adhesion	Energy needed to separate liquid from solid	Higher values mean more spreading and more stubborn residues
Glass transition temperature	Temperature where amorphous solids soften from glassy to rubbery	When lactose‑rich particles are above this point, they become sticky and prone to wall deposits
Water activity	Measure of available water, not just moisture content	Higher values encourage microbial growth in residues and caked zones

A recent Journal of Dairy Science study on milk protein ingredients pointed out that surface free energy and derived quantities like work of cohesion and work of adhesion are deeply connected to rehydration behavior and flowability of milk protein powders.

Diagram illustrating cohesion of particles and adhesion to a surface, relevant for milk powder solutions.

Because milk protein concentrates are inherently cohesive powders, these surface‑related properties often dominate gravity in determining whether a powder flows freely or bridges and cakes.

Moisture, Glass Transition, And Why Milk Powders Turn Sticky

The most powerful trigger for adhesion and cohesion in milk powders is moisture. Small changes in moisture content and humidity can flip a powder from free‑flowing to unmanageable.

A review on moisture in food powder handling emphasized that water does several things at once. It increases cohesion between particles, leading to clumping and poor flow. It modifies compressibility so that water‑softened particles can bind too strongly and form dense, hard masses. It also raises water activity, which encourages bacterial growth in high‑moisture niches such as caked powdered milk.

For milk powders and infant formulas, the glass transition of amorphous components, especially lactose, is the critical pivot point. In a mini‑review on spray drying of sugar‑ and acid‑rich foods, researchers described how amorphous sugars behave like hard, glassy solids at low temperature and low moisture, then become soft and rubbery as temperature and humidity rise. Above their glass transition temperature, the surface viscosity drops, particle surfaces become tacky, and both particle–particle and particle–wall adhesion increase sharply.

Dairy research on low‑protein spray‑dried emulsions shows the same story for lactose‑rich formulations. Proteins are large molecules with higher glass transition temperatures and are less sensitive to the plasticizing effects of heat and moisture. Lactose, by contrast, has a lower glass transition temperature and becomes sticky more easily, particularly when it contains impurities or has been partially hydrolyzed.

One study on spray‑dried dairy emulsions noted several key points.

When the protein‑to‑lactose ratio decreases, the glass transition temperature of the powder decreases under dry conditions. That makes the powder more likely to enter a rubbery, sticky state during spray drying or warm transport.

Impurities in lactose significantly increase moisture sorption and susceptibility to caking. Hydrolysis of lactose in skim milk powder has been shown to increase stickiness in the spray dryer and lower the glass transition temperature.

Glass transition visualization of adhesion in milk powder solutions, showing solid to viscous phase change.

Replacing lactose with higher molecular weight carbohydrates such as maltodextrins or glucose syrups raises the glass transition temperature and reduces stickiness. However, when maltodextrins are heavily hydrolyzed, the improvement in glass transition temperature is smaller.

From a hydration perspective, this explains why some instant milks and formulas seem forgiving while others clump or cake even when you handle them similarly. Powders designed with higher‑Tg carriers and adequate protein can tolerate small moisture fluctuations, whereas lactose‑rich powders sit closer to their sticky threshold.

Adhesion Inside Dairy Equipment: From Protein Films To Mineral Scale

Adhesion is not only about clumps in the canister or shaker bottle. Inside processing lines, it is central to fouling, cleanability, and microbiological safety.

Protein And Calcium Phosphate Deposits

A detailed fouling study on heated dairy fluids used a polished stainless steel device to track deposit growth under well‑defined flow and temperature conditions. Across sweet whey, milk, model phosphate–calcium solutions, and protein solutions, three components dominated the deposits: proteins, phosphates, and lipids, associated with calcium, phosphorus, nitrogen, and carbon in surface analyses.

The deposits formed in stages. Proteins were always the major component in the earliest fouling layers, even at short exposure times, indicating that whey proteins such as beta‑lactoglobulin drive initial adsorption and early film growth. At moderate temperatures, phosphates were present mainly through complexation among phosphate, calcium ions, and proteins.

As temperatures rose toward about 147°F and above, protein denaturation reduced their ability to complex calcium and phosphate. This opened the door to a sharp increase in calcium phosphate content in the deposit. Under strong supersaturation, quasi‑amorphous calcium phosphate solids nucleated on the pre‑adsorbed protein layer. The resulting mixed organic–mineral phase was substantially more cohesive than a purely protein film.

At even higher temperatures around 190°F, researchers observed a decrease in phosphate content in the deposits, consistent with additional nucleation happening in the bulk fluid rather than solely on the surface. However, by that stage, the surface was already covered by a strongly adhering, protein–calcium phosphate composite layer.

This work led to a clear practical recommendation: in dairy heating and drying, completely avoiding protein adsorption is unrealistic, but severe, cohesive fouling is strongly linked to calcium phosphate precipitation. Controlling calcium and phosphate supersaturation, pH, and temperature profiles to suppress this mineral phase is often more effective at limiting adhesion than focusing on proteins alone.

Surface Fat And Wall Deposits In Spray Dryers

In spray‑dried emulsions such as powdered milk replacers, the lipid phase plays a second, critical role in adhesion and caking. Studies on spray‑dried dairy emulsions show that powder particles are not homogeneous. They tend to have a fat‑enriched surface and a protein‑enriched core.

One investigation found that powders containing 5% fat could have more than 45% surface coverage by fat, while powders with 20% fat could exceed 90% surface coverage. Higher drying temperatures and mechanical disruption in cyclones were associated with more surface and free fat.

Milk powder particle cross-section: protein-rich core and fat-enriched shell for adhesion.

Single‑droplet drying experiments confirmed that fat accumulates at the droplet surface during early and mid drying, and that the chemical nature of the fat (its fatty acid profile) influences how strongly it migrates.

Excess surface fat makes particles more prone to adhesion with metal surfaces and each other. During production, surface fat contributes to deposits in spray dryers, cyclones, and conveying equipment. During storage and transport, elevated temperatures can melt surface fat and promote uncontrolled agglomeration through fat bridging.

From a formulation point of view, the same study defined its aim as lowering the susceptibility of low‑protein spray‑dried emulsions to stickiness caused either by amorphous lactose softening or by high surface fat content. The approach was threefold: maintain enough protein to stabilize emulsions and minimize free fat, partially replace lactose with higher‑Tg carbohydrates, and explore how fatty acid profile shapes fat migration toward droplet surfaces.

Microbial Adhesion, Conditioning Films, And Food Safety

Adhesion also sets the stage for microbial attachment and biofilm formation on stainless steel. Two complementary lines of research help explain this.

A study on work of adhesion of dairy products on stainless steel surfaces compared ultra‑high‑temperature whole milk, chocolate‑based milk, and infant formula. Using surface tension and contact angle measurements, the researchers calculated work of adhesion values and linked them to how strongly these liquids wet and adhere to stainless steel.

Chocolate milk showed the highest work of adhesion and lowest contact angle, meaning it spread best and formed the most strongly bound films. UHT milk and infant formula also adhered strongly, and all three supported much higher numbers of attached Enterobacter sakazakii (now Cronobacter sakazakii) compared with a nutrient broth control. The study emphasized that films from these dairy products are difficult to clean and provide favorable matrices for bacterial colonization and biofilm formation.

Food safety diagram: bacterial adhesion & biofilm growth on milk film on stainless steel.

Another PubMed Central paper looked at how preadsorbed skim milk and individual milk proteins influence bacterial adhesion to stainless steel. Cleaned AISI 304 samples were coated with skim milk or with single proteins such as alpha‑casein, beta‑casein, kappa‑casein, and alpha‑lactalbumin. These conditioned surfaces were then exposed to suspensions of several food‑relevant bacteria including Staphylococcus aureus and Listeria monocytogenes.

Surprisingly, skim‑milk conditioning substantially reduced attachment of S. aureus, L. monocytogenes, and Serratia marcescens compared with bare steel. Individual caseins and alpha‑lactalbumin also reduced adhesion of S. aureus, though to varying degrees. Surface analysis showed that even very dilute milk coatings formed measurable protein films, and that bacterial counts were inversely related to the nitrogen signal from proteins on the surface.

The authors interpreted this as evidence that a mobile, hydrated protein layer can act as a steric and hydration barrier, making it physically harder for bacteria to come into close contact with the underlying steel.

Anti-fouling mechanism diagram: hydrated protein layer repels bacteria, preventing adhesion in milk solutions.

When the proteins were cross‑linked with glutaraldehyde, this mobility decreased and bacterial adhesion partially returned, supporting the idea that protein chain mobility is key.

Taken together, these findings remind us that “conditioning films” from milk can both help and hurt. On one hand, dried high‑fat or chocolate‑based films have high work of adhesion, are difficult to remove, and favor pathogens. On the other hand, certain fresh protein films can temporarily reduce adhesion of some bacteria. For safe operation, dairy facilities must treat any persistent milk film as a risk, but understanding its physicochemical nature is crucial for choosing cleaning and sanitizing strategies.

Adhesion Between Milk Powder Particles: Blocks, Cakes, And Flow

Adhesion is also central to how milk powders pack, compress, and then dissolve again.

A study on block infant formula examined how particle morphology, moisture content, and particle size affect the compressive strength and solubility of compressed milk powder tablets. Four commercial formulas with different microstructures were tested. One powder had a compact, grape‑like agglomerate structure of relatively uniform, smooth particles, whereas others showed more onion‑like polymeric structures with heterogeneous sizes and many fines attached.

Using powder rheometry and texture analysis, the research team showed that the grape‑like morphology promoted a higher compression ratio and yielded blocks with higher compressive strength. The open, well‑organized structure formed robust mechanical interlocking during pressing but still left pores that allowed water to penetrate and dissolve the block efficiently.

Moisture content was tuned by storing powders at controlled relative humidity until they reached constant weight. The best overall performance occurred at an intermediate moisture level around 4.75% in the starting powder.

Adhesion of porous white particles, like milk powder, linked by a blue viscous substance.

At this moisture, capillary and solid‑bridge bonding between particles were strong enough to create durable blocks without excessively compromising solubility.

Particle size also mattered. Sieved fractions with coarser particles, predominantly larger than 80 mesh, produced blocks that were both strong and adequately soluble. Excessively fine fractions or broad size distributions increased cohesiveness and reduced dissolution.

These results show how interparticle adhesion is not inherently good or bad. For block infant formula, you want enough adhesion to form a strong, non‑crumbly tablet but not so much that the block resists water and dissolves slowly. Moisture content, morphology, and particle size distribution all tune that balance.

In bulk powders for blending and packaging, however, too much cohesion leads to arching, ratholing, and inconsistent dosing. A review on food powder flowability explained that milk protein powders are typically classified as cohesive, which means cohesion dominates gravity in controlling flow. In such powders, adhesion between particles must be carefully managed through formulation, moisture control, and sometimes the use of flow aids.

How Industry Measures Adhesion And Flow Behavior

Because adhesion and cohesion are invisible forces, food scientists rely on measurement tools to quantify them and connect them to real processing problems.

A review of food powder flowability described how dynamic powder rheometers, especially the FT4 instrument, have become central in this work. Unlike simple tests such as angle of repose or Hausner ratio, FT4 rheometry measures dynamic flow energy, bulk density, permeability, compressibility, and wall friction in a single integrated system.

For milk powders, one particularly useful FT4 parameter is Specific Energy, defined as the energy per unit mass needed to move a blade upward through a conditioned powder bed.

FT4 Powder Rheometer schematic illustrating powder mixing and energy for milk powder adhesion.

This metric is sensitive to cohesion, particle shape, surface roughness, and size distribution. The review proposed simple cohesion classes based on Specific Energy. Powders with values below about 5 mJ per gram behave as low‑cohesion materials. Values between about 5 and 10 mJ per gram indicate moderate cohesion, while values above 10 mJ per gram signal high cohesion.

These thresholds give formulators a practical way to compare batches, track the effect of moisture or formulation changes, and identify when a milk powder is shifting into a problematic regime.

The same review highlighted a Stability Index, defined as the ratio of Basic Flowability Energy in later cycles to that in the first cycle. Values close to 1 indicate that the powder maintains its structure under repeated movement, whereas values greater or less than 1 suggest tendencies to expand or compact. For milk powders that are susceptible to caking or structural evolution under vibration and aeration, this index helps predict behavior in hoppers and pneumatic systems.

On the adhesion side, scientists use a range of complementary tools. The milk protein surface study mentioned earlier combined contact angle measurements with the Young–Dupré framework to calculate work of adhesion for whole milk, chocolate milk, and infant formula on stainless steel. Pendant drop methods provided interfacial tension values, and contact angles quantified how a drop spread on the steel surface. From these data, work of adhesion could be estimated and compared across products.

A separate line of work in powder technology has developed a so‑called drop test method to measure powder adhesion on substrates. Although demonstrated on pharmaceutical powders like ibuprofen and on spherical aluminum alloy particles, the method is conceptually relevant for milk powders. A substrate is coated with powder and then subjected to controlled impacts. By analyzing which particle sizes detach and which remain attached, researchers estimate an effective work of adhesion for the powder–surface pair. Upgraded rigs with piezoelectric rings and optical sensors have improved accuracy and enabled automated image analysis, including artificial intelligence tools for recognizing particle clusters.

In parallel, contact‑angle‑based methods and models such as the Owens–Wendt–Rabel–Kaelble approach are used to derive surface free energy of powder beds from measured apparent contact angles. A recent Journal of Dairy Science article emphasized both the potential and the limitations of this route. Ideally, microscopic contact angles depend only on surface free energy. In practice, macroscopic measurements on compacted powder discs are influenced by compaction pressure, surface roughness, pore structure, and powder solubility. As a result, surface energy values derived from these models must be interpreted cautiously and validated against direct functional measurements such as wetting time and flow energy.

No single test captures all aspects of adhesion and flow. Reviews on stickiness measurement conclude that combining glass transition or water‑activity mapping with at least one direct adhesion or process‑simulating test provides the most reliable insight. For milk powders, that might mean pairing differential scanning calorimetry and state diagrams with wall‑deposition tests or rotary drum experiments that mimic spray dryer and conveying conditions.

Adhesion, Moisture, And Safety: From Plant To Home

Adhesion phenomena do not stop when the product leaves the plant. They influence how easily powders disperse in the kitchen, how much residue remains in bottles and appliances, and how hospitable those residues are to microbes.

The moisture–adhesion connection remains central. High local moisture around caked powder or dried films raises water activity, creating mini‑environments where bacteria and molds can grow. A moisture‑focused review highlighted examples such as Salmonella or E. coli growth in high‑moisture powdered milk and the elevated sterility risks for powders that contact wounds or are injected. While those cases are more extreme than everyday home use, the principle is the same: sticky, hard‑to‑remove residues are also harder to sanitize.

From an equipment design point of view, engineering guides on powder adhesion in industry recommend several strategies: using smoother, polished stainless steel (such as SUS304 or SUS316L) to reduce mechanical interlocking; applying low‑friction coatings; managing static electricity through grounding and conductive materials; adding gentle vibration or air jets to keep powders moving; and controlling equipment temperature to avoid condensation and humidity spikes that amplify adhesion.

Moisture control strategies from the broader powder science literature are similarly applicable to dairy powders. These include maintaining low, well‑controlled humidity in processing and storage spaces, using moisture‑barrier packaging and desiccants when needed, and designing equipment so that it limits unwanted moisture exposure and can be cleaned thoroughly. Inline moisture sensing tools, such as near‑infrared systems, have been used in practice to stabilize dryer operation and avoid under‑ or over‑drying that would otherwise shift powders into sticky regimes.

For health‑focused consumers preparing milk powder solutions at home, the same physics translates into simple habits. Keeping powder containers tightly closed between uses helps minimize ambient moisture uptake and clumping. Avoiding steam exposure from kettles or dishwashers reduces local humidity spikes near the lid and scoop. Promptly rinsing and washing bottles and mixing jars limits the time that adhesive milk films have to dry and strengthen on surfaces. While high‑quality filtered water primarily improves taste, removes off‑flavors from chlorine or metals, and protects appliances from scale, it can also help maintain smoother internal surfaces over time, which indirectly lowers mechanical anchoring points for sticky milk residues.

Short FAQ

What is the single most important factor driving adhesion and stickiness in milk powder solutions?

Across dairy and food powder research, controlled moisture content is the dominant factor. Moisture shifts amorphous components like lactose through their glass transition, alters surface free energy, increases cohesion and adhesion, and raises water activity. Temperature and formulation then determine how close the system sits to a sticky, rubbery state.

Why do some milk powders dissolve cleanly while others leave stubborn films?

Studies show that powders with higher protein levels, grape‑like agglomerate structures, and coarser, more uniform particle sizes tend to compress and then redisperse well. In contrast, lactose‑rich powders with high surface fat and many fines are closer to their sticky threshold and more likely to form clumps and films, especially if their moisture content and storage conditions are not well controlled.

Does polishing stainless steel really make a difference for milk residues?

Evidence from dairy processing and general powder handling supports that smoother, polished stainless steel reduces mechanical interlocking and lowers adhesion. However, surface chemistry and product composition still matter. For example, chocolate milk with high work of adhesion can leave persistent films even on polished steel. Polishing is helpful but must be combined with suitable cleaning protocols and process control.

When we understand adhesion not as a nuisance but as a controllable interface between powder, water, and surfaces, smarter choices emerge. Dairy scientists can formulate powders that flow and dissolve more reliably. Plant engineers can design equipment and cleaning regimes that limit fouling and biofilms. And at home, families can handle milk powders in ways that respect the physics of moisture and surfaces while supporting safe, convenient hydration.

References

About the author

Dr. Elena Brooks

Certified Water Quality SpecialistEnvironmental ScientistSmart Home Wellness Advocate

Environmental scientist turned smart home expert. Elena decodes the science of hydration to help modern families choose technology that supports long-term health.

Understanding the Adhesion Phenomenon in Milk Powder Solutions