Enhancing Skimmed Milk Powder Functionalities Through Optimized Spray-Drying Conditions

Skimmed milk powder (SMP) plays a vital role in global food systems, providing a lightweight, shelf-stable, and nutritionally rich product used across multiple industries. From food manufacturing to pharmaceuticals and cosmetics, SMP contributes as a versatile ingredient capable of enhancing texture, stability, emulsification, and gelation. However, the functional performance of SMP is not fixed; it depends strongly on the technological parameters applied during its production, particularly spray-drying conditions.

Spray-drying is recognized as the most efficient and cost-effective method to transform liquid milk into powder. The process involves atomizing liquid skimmed milk into fine droplets and rapidly drying them with hot air. While this method ensures homogeneity, it also presents challenges. Heat-sensitive components such as whey proteins can be denatured, while powder quality issues like caking, deliquescence, or poor solubility may occur. This research carefully examined how two critical parameters—outlet drying air temperature and spraying air pressure—affect the physicochemical and functional properties of SMP.

The study emphasized the importance of a multicriteria optimization strategy, applying advanced modeling tools such as Response Surface Methodology (RSM), Principal Component Analysis (PCA), and a genetic evolutionary algorithm. These tools allowed researchers to predict interactions, balance conflicting outcomes, and identify ideal compromises for producing powders with enhanced industrial applicability.

Background and Industrial Relevance

The functional properties of SMP determine its end-use potential. For example, flowability influences handling, processing, and dosing accuracy in bakeries or pharmaceutical industries. Gelation capacity is critical in dairy products to produce stable gels with desirable textures. Reconstitution properties impact beverage manufacturing, requiring fast solubility, stability, and nutritional retention. Additionally, SMP can serve as a carrier for bioactive compounds in medical and nutritional applications. Therefore, tailoring spray-drying conditions is essential to meet these diverse industrial needs.

The complexity of SMP production arises from interactions among factors. Seasonal variations in milk composition, pre-treatment steps (such as homogenization or membrane filtration), and post-processing (like agglomeration or storage) all influence powder quality. Among these, the spray-drying stage stands out as a critical determinant. Outlet air temperature and atomizing pressure not only control moisture removal but also dictate particle morphology, protein stability, and color development.

Study Objectives and Approach

This research had three primary objectives:

• To analyze the impact of outlet drying air temperature and spraying air pressure on SMP’s physicochemical properties such as moisture, protein stability, and particle size.
• To evaluate functional outcomes, including flowability, gelation, color, solubility, and water interactions.
• To apply multicriteria optimization techniques in order to achieve an ideal balance between preservation of heat-sensitive proteins, flow behavior, storage stability, and gelation ability.

The experiments were conducted on two sets of fresh skimmed cow milk collected during different seasons to account for natural variability. Spray-drying trials were carried out using a pilot-scale MicraSpray MS 150 dryer equipped with a bi-fluid nozzle. Temperature ranges were set between 75–95 °C, while air pressures varied from 1–3 bar. Eleven trials were performed, generating detailed datasets on chemical composition, flow properties, gelation kinetics, water interactions, and color parameters. Statistical analyses, including one-way ANOVA and Tukey’s HSD tests, ensured significance and accuracy of results.

Key Findings on Physicochemical Properties

Dry matter content and water activity: Higher outlet air temperatures significantly increased dry matter levels (up to 96.55%), resulting in drier powders with lower water activity. This enhanced preservation and reduced microbial risks but also increased hygroscopicity, making powders more moisture-sensitive during storage.

Protein content and stability: While casein levels remained relatively stable, whey proteins were highly sensitive to heat. Denaturation increased at higher outlet temperatures, leading to reduced solubility but improved gelation. Moderate drying conditions better preserved whey proteins, suggesting a trade-off between nutritional preservation and functional textural benefits.

Particle size distribution: Spray pressure played a more decisive role than temperature in controlling particle size. Higher pressures produced smaller particles, which in turn increased powder cohesion and decreased flowability. Larger particles from lower pressures showed improved handling characteristics but reduced solubility.

Functional Properties of SMP

Flowability: Flow behavior is essential for efficient processing in industries. Results indicated that higher temperatures improved flowability by limiting stickiness and promoting spherical particle formation. However, high spray pressures reduced flowability by creating small, cohesive particles with larger surface areas. The study recommended low spraying pressure combined with higher outlet temperatures to minimize cohesion and enhance handling.

Gelation ability: Reconstituted SMP demonstrated varied gelation behaviors depending on drying conditions. Powders produced at high temperatures and pressures formed stronger gels with shorter gelation times. Microscopic analysis (SEM) revealed that these gels had denser networks with fewer pores, correlating with superior water retention capacity. In contrast, powders dried at lower temperatures produced weaker gels with irregular pore structures and lower firmness.

Color and appearance: Temperature and pressure both influenced powder chromaticity. High temperatures increased browning reactions (via the Maillard reaction), reducing lightness and increasing yellowness. High pressures tended to maintain lighter colors by generating smaller particles. Industrially, this balance between visual quality and functional optimization is critical, particularly for consumer-facing products.

Water interactions: Water activity and hygroscopicity were key indicators of shelf stability. Low water activity extended preservation but excessive hygroscopicity led to caking issues during storage. Thus, powders produced at higher temperatures were drier yet more sensitive to moisture uptake, demanding controlled storage environments.

Advanced Insights Into Gelation and Protein Behavior

The gelation process of skimmed milk powder is strongly influenced by protein composition, thermal history, and particle reconstitution behavior. Caseins, which naturally form micelles, provide the backbone of gel networks. However, whey proteins such as β-lactoglobulin can interact with κ-casein during heat treatment, promoting stronger gel matrices. This study highlighted that higher outlet air temperatures accelerated whey protein denaturation, which in turn enhanced gel firmness and reduced gelation time. Although protein denaturation might appear undesirable from a nutritional perspective, the results demonstrated that this modification can actually improve textural and functional properties of reconstituted SMP gels.

Rheological measurements of acid gels confirmed this relationship. At elevated drying temperatures, the elastic and viscous moduli were significantly higher, indicating the formation of more stable protein networks. Conversely, samples dried at lower temperatures exhibited weaker gels with slower development. This effect was visible under scanning electron microscopy, which showed irregular pore structures and low water retention capacity in low-temperature powders compared to dense, uniform networks in high-temperature powders.

Furthermore, water retention capacity (WRC) was closely correlated with gel structure. Powders produced at high temperatures or under specific pressure–temperature combinations retained up to 98% water, providing superior textural properties for dairy applications. In contrast, gels formed from low-temperature powders retained only about 58–83% of water, leading to weaker and more fragile structures. These findings are crucial for industries such as yogurt, cheese, and dairy desserts, where SMP’s ability to form strong gels directly impacts consumer acceptance.

Color and Sensory Attributes

The visual appearance of skimmed milk powder is not only a matter of aesthetics but also a factor influencing consumer perception and product acceptability. The study demonstrated that drying parameters significantly altered powder chromaticity. Higher outlet air temperatures promoted the Maillard reaction, producing powders with darker tones and more yellowish shades. Increased pressure, however, counteracted this effect by generating smaller, lighter particles that reflected more light, resulting in powders with higher brightness values.

For manufacturers, balancing these outcomes is critical. While darker powders may indicate over-processing or nutrient loss to consumers, lighter powders are generally associated with freshness and quality. This duality underscores the importance of optimizing both temperature and pressure, particularly for applications where visual quality is paramount, such as infant nutrition products and premium dairy formulations.

Particle Size Distribution and Flowability

Particle size, a critical determinant of SMP performance, was shown to be primarily controlled by spray air pressure. Larger particles produced at low pressures enhanced flowability, minimized cohesion, and reduced stickiness—ideal for industrial processing and bulk handling. Conversely, small particles generated at high pressures increased cohesion and interparticle forces, reducing flowability and potentially complicating processing steps such as mixing or packaging.

Despite these challenges, smaller particles improved solubility, dispersibility, and lightness. Therefore, industries prioritizing instant solubility, such as beverage manufacturers, may prefer higher spray pressures despite reduced flowability. On the other hand, sectors demanding bulk powder handling and minimal caking, such as bakery or pharmaceutical industries, may benefit from lower spray pressures. This dual functionality highlights the need for tailored spray-drying strategies depending on the final application.

Water Interactions and Shelf Stability

Shelf stability of SMP is directly linked to water activity, moisture content, and hygroscopicity. The study confirmed that outlet air temperature was the most influential factor. Higher temperatures reduced water activity, enhancing microbial safety and chemical stability. However, these powders also exhibited higher hygroscopicity, meaning they more readily absorbed ambient moisture and were prone to caking. Thus, while high temperatures provide excellent short-term stability, they increase long-term storage challenges unless powders are stored in strictly controlled environments.

This trade-off is critical for global supply chains. In regions with high humidity, SMP must be packed in moisture-resistant packaging to prevent degradation. Moreover, powders destined for long-term storage require specific processing conditions that balance dryness with moderate hygroscopicity. The research suggests that combining intermediate spray pressures with optimized temperatures may yield powders that balance both immediate preservation and long-term storage stability.

Protein Denaturation and Nutritional Implications

The nutritional quality of skimmed milk powder depends heavily on protein preservation. Whey proteins, particularly α-lactalbumin and β-lactoglobulin, are highly sensitive to heat. The study revealed that their levels decreased significantly at higher drying temperatures, confirming denaturation and loss of native functionality. However, this denaturation enhanced gelation and rheological properties, providing functional benefits.

From a nutritional standpoint, partial protein denaturation can reduce biological activity but does not eliminate nutritional value, as amino acids remain intact. The balance between functional performance and nutritional preservation is therefore context-specific. For example, infant formulas require maximum preservation of whey proteins for their health benefits, while cheese-making applications benefit from enhanced gelation induced by protein denaturation. The findings thus emphasize that there is no universal “optimal” condition; instead, the optimal spray-drying setup depends on intended use.

Statistical Modeling and Optimization

The research applied advanced statistical tools to analyze the experimental data. Response Surface Methodology (RSM) provided quadratic models describing relationships between drying conditions and powder properties. These models showed high predictive accuracy, with coefficients of determination (R²) often exceeding 0.90 for critical parameters such as dry matter content and particle size. This accuracy confirmed that drying parameters could be reliably manipulated to predict powder outcomes.

Principal Component Analysis (PCA) further simplified interpretation by revealing underlying correlations. For example, high temperatures consistently correlated with improved gel firmness and reduced gelation time but also with increased whey protein denaturation and higher hygroscopicity. Pressure, by contrast, was strongly correlated with particle size and cohesion. These insights allowed the researchers to identify distinct clusters of powder functionalities based on drying conditions.

The study’s most innovative feature was the use of a genetic evolutionary algorithm for multicriteria optimization. Unlike single-variable optimization, this approach allowed simultaneous improvement of multiple powder functionalities. The algorithm generated thousands of possible drying scenarios, iteratively refining them to approach an optimal Pareto front—a set of conditions where no single outcome could be improved without compromising another.

Optimization Results

Two main optimization strategies were tested. The first focused on enhancing gelation ability, aiming for strong, firm gels with short gelation times. The second focused on protecting whey proteins, preserving their structure while maintaining fair flowability and solubility. The algorithm produced four optimal conditions, each offering a unique compromise:

• High temperature + high pressure: Produced strong gels with excellent firmness but greater whey protein denaturation.
• High temperature + low pressure: Produced firm gels with larger particle sizes and better flowability but darker colors.
• Moderate temperature + low pressure: Preserved whey proteins and reduced denaturation, but gel firmness was lower.
• Moderate temperature + moderate pressure: Achieved a balance between protein preservation, solubility, and storage stability.

These outcomes underscore the fact that optimization is not about finding a single “best” condition but about tailoring spray-drying parameters to meet specific industrial needs. For instance, pharmaceutical industries may favor protein preservation, while dairy dessert manufacturers may prioritize gelation.

Validation of Optimized Conditions

The optimized drying conditions identified by the genetic algorithm were tested experimentally using fresh skimmed milk with slightly different seasonal compositions. The results confirmed the predictive power of the models but also revealed natural variability in milk composition as an important factor influencing powder properties. For example, higher protein content in the validation milk batches led to stronger gels than predicted, demonstrating that raw material quality remains a critical determinant of final powder functionality.

Dry matter contents achieved under optimized conditions consistently exceeded 96%, demonstrating the efficiency of selected drying strategies. Cohesion values were higher than expected, partly due to smaller particle sizes in pilot-scale production compared to industrial scale. Chromaticity values matched predictions, with powders produced at lower pressures being darker, while those produced at higher pressures maintained lighter tones. Importantly, gelation properties exceeded predicted values, with stronger firmness and shorter gelation times than modeled. These results validated the optimization framework while also highlighting the importance of seasonal milk variability.

Industrial Implications

The findings of this study carry significant implications for industries relying on skimmed milk powder. By carefully adjusting outlet air temperature and spray pressure, manufacturers can design powders with targeted properties to suit specific applications. For example:

• Dairy product manufacturers can maximize gelation ability by selecting high-temperature, high-pressure conditions to produce firmer gels with strong water retention capacity.
• Beverage industries can prioritize solubility and light color by opting for moderate temperature and higher spray pressure conditions, which generate smaller, easily dispersible particles.
• Pharmaceutical applications can benefit from moderate temperature and pressure combinations that minimize whey protein denaturation, ensuring maximum bioactivity preservation.
• Storage and logistics sectors must account for hygroscopicity by implementing strict moisture-control packaging and environments, especially when powders are produced under high-temperature conditions.

This tailored approach allows industries to move beyond the “one-size-fits-all” method of spray-drying, instead producing functional powders optimized for specific consumer and technical needs. Such strategies are crucial in an increasingly competitive global market where quality, performance, and cost-efficiency drive consumer trust and industry success.

Challenges and Future Directions

Although this study provided robust insights, certain challenges remain. First, variability in milk composition due to seasonal and regional factors continues to impact powder functionalities, suggesting the need for adaptive optimization frameworks that incorporate real-time milk composition analysis. Second, while high temperatures improve gelation, they also increase Maillard browning and reduce protein solubility, which may not be acceptable in sensitive markets such as infant nutrition. Third, scaling pilot-scale results to industrial levels may introduce new complexities, particularly in controlling particle size and minimizing cohesion.

Future research could integrate machine learning algorithms with real-time process monitoring to predict powder outcomes even more precisely. Additionally, exploring alternative spray-dryer designs, multi-stage drying, or the use of protective additives could further enhance functionality while minimizing undesirable effects. Expanding research into encapsulation capabilities, nutritional bioavailability, and applications in non-dairy industries such as cosmetics or nutraceuticals could also broaden SMP’s market value.

Conclusion

This study demonstrated the critical influence of outlet air temperature and spray pressure on the physicochemical and functional properties of skimmed milk powder. High temperatures improved dryness, gelation, and flowability but also increased hygroscopicity and whey protein denaturation. Spray pressure determined particle size and color, directly impacting flowability and solubility. By applying advanced statistical models and a genetic optimization algorithm, the research successfully identified optimal spray-drying conditions that balanced competing outcomes.

The results highlight the potential of multicriteria optimization to transform SMP production from a conventional, experience-based process into a rational, data-driven strategy. Such approaches enable industries to produce powders with tailored properties for diverse applications, ranging from dairy and bakery products to pharmaceuticals and infant nutrition. Ultimately, the integration of genetic algorithms into spray-drying optimization represents a significant advancement in food engineering, providing a roadmap for designing functional powders that meet the evolving demands of consumers and industries alike.

Reference

Original article: Enhancing skimmed milk powder functionalities through multicriteria optimization of outlet drying air temperature and spraying air pressure, Journal of Food Engineering, Elsevier, 2024.