Surface Modification of Premium Whole Milk Powder: Leveraging the Low Viscosity of ACETEM and the High Hydrophilicity of PGE on Powder Particle Surfaces

Whole milk powder has long presented a technical bottleneck in instant dissolution, particularly in cold water, due to its high fat content and pronounced surface hydrophobicity. Although the conventional lecithin spraying process has been industrialized for decades, it still falls short in wetting speed, storage stability, and flavor preservation. This paper proposes a novel surface modification strategy based on the synergistic combination of ACETEM (acetylated mono- and diglycerides of fatty acids, E472a) and PGE (polyglycerol esters of fatty acids, E475). The low HLB value (2–3) and liquid-like low-viscosity characteristics of ACETEM enable it to efficiently infiltrate and wet the free fat layer on the surface of milk powder particles, thereby constructing a primary hydrophobic affinity interface. Meanwhile, the broad-range HLB value (3–13) and high hydrophilicity of PGE confer the requisite hydrophilic channels on the particle surface, allowing water to rapidly penetrate the porous structure of agglomerates. Compared with the traditional lecithin spraying process, the ACETEM/PGE gradient coating system demonstrates significant advantages in cold water (8–10°C) wetting time, dispersibility, and shelf-life oxidative protection, with full compliance with European Union and international food additive standards. This paper provides a systematic demonstration from both interfacial chemistry mechanisms and process implementation perspectives, offering a new technical pathway for the instantization improvement of premium whole milk powder.

Whole milk powder is a powdered dairy product manufactured from fresh cow's milk through processes including standardization, pasteurization, concentration, and spray drying. In contrast to skim milk powder, whole milk powder retains the natural flavor of milk fat and fat-soluble nutrients, making it widely demanded in premium infant formula, adult nutritional powders, coffee creamers, and bakery premixes.

However, the fat content of whole milk powder typically exceeds 26%–28%, and during spray drying and subsequent handling, approximately 0.5%–3% of free fat migrates to the particle surface, forming a coating over the milk powder granules. This free fat layer exhibits strong hydrophobicity, making it difficult for water molecules to breach the surface and penetrate into the interior of the particles, thereby severely compromising the wettability and dispersibility of the powder. The water contact angle of whole milk powder is generally around 50°, and in some cases exceeds 90°, at which point water infiltration is virtually completely obstructed. This surface hydrophobic characteristic represents the fundamental reason for the difficulty in reconstituting whole milk powder in cold water.

To address this issue, the industry has widely adopted the agglomeration–lecithin spraying process since the 1960s, in which a lecithin solution in vegetable oil is sprayed onto agglomerated milk powder particles in a fluidized bed following spray drying. Studies have demonstrated that lecithin spraying can reduce the wetting time of skim milk powder from over 300 seconds to approximately 8.9 seconds, a remarkable improvement. Nevertheless, lecithin itself possesses several inherent limitations, including a high polyunsaturated fatty acid content (approximately 50%–60% linoleic acid in commercial soy lecithin), susceptibility to oxidation leading to off-flavors, and limited thermal stability-shortcomings that are becoming increasingly apparent under the extended shelf-life requirements of premium products.

In recent years, the rapid development of synthetic non-ionic emulsifiers has provided new tool options for the surface modification of dairy powders. Among them, ACETEM (acetylated mono- and diglycerides of fatty acids) and PGE (polyglycerol esters of fatty acids), with their complementary molecular structures, differentiated HLB values, and tunable surface activity, demonstrate significant potential as partial or complete alternatives to traditional lecithin. This paper aims to systematically analyze the interfacial behavior of these two emulsifiers, investigate the physicochemical mechanisms underlying their synergistic action on whole milk powder particle surfaces, and provide reference guidelines for the optimization of industrial formulations.

1 Particle Surface Composition and Wetting Mechanism

The wetting behavior of milk powder particles is governed by two factors: surface composition and microstructure. From the perspective of surface composition, during the spray drying process, a pronounced compositional gradient forms on the particle surface due to the diffusive migration of solutes within the droplet and differential surface activities-hydrophobic milk fat and free fatty acids tend to accumulate in the outermost particle layer, while hydrophilic lactose and proteins are encapsulated in the interior. This "hydrophobic exterior, hydrophilic interior" compositional distribution causes the water contact angle of whole milk powder particles to be substantially larger than that of skim milk powder (approximately 20°), directly impeding water penetration. From the microstructural perspective, conventional spray-dried milk powder particles are fine (D90 typically in the tens of micrometers), with narrow interstices; even if water manages to break through the surface, it is readily obstructed by capillary effects between small particles, resulting in lump formation characterized by a "wet exterior, dry interior."

Thus, achieving instant dissolution of whole milk powder requires simultaneous resolution of two aspects: altering the chemical composition of the particle surface to reduce its hydrophobicity, and increasing particle size while constructing a porous structure to provide channels for water infiltration.

2 Advantages and Limitations of the Conventional Lecithin Spraying Process

Lecithin spraying represents the most mature surface modification technology in current industrial practice. Its mechanism of action can be described as follows: the hydrophilic groups of lecithin molecules (the quaternary ammonium headgroup of phosphatidylcholine) anchor on the exterior side of the particle surface, while the hydrophobic tail chains embed into the free fat layer, thereby forming a hydrophilic interfacial film that reduces the contact angle from above 50° to near zero.

However, the lecithin spraying process is subject to several inherent and difficult-to-overcome limitations. First, approximately 50%–60% of the fatty acids in commercial soy lecithin are linoleic acid (C18:2), a highly polyunsaturated fatty acid that readily undergoes autoxidation during storage, generating "beany" or "painty" off-flavors-a flavor risk that becomes significant over the 12–24 month shelf life of whole milk powder. Second, lecithin is thermally sensitive and may undergo partial degradation under the elevated temperature conditions of fluidized bed spraying. Third, the hydrophilic layer formed after lecithin spraying is a mono- or oligomolecular film that is susceptible to detachment through abrasion during transport vibration and long-term storage, leading to attenuation of wetting performance. These limitations provide a clear technological window for the introduction of novel synthetic emulsifiers.

1 ACETEM: Low-Viscosity Hydrophobic Interfacial Modifier

ACETEM (acetylated mono- and diglycerides of fatty acids, E472a) is a non-ionic emulsifier produced through the acetylation of mono- and diglycerides of fatty acids with acetic anhydride. It appears as a milky white powder or flake solid, with a melting point ranging from 25–40°C depending on the degree of acetylation, an HLB value of 2–3, and classification as a water-in-oil (W/O) emulsifier. It is insoluble in cold water, hot water, and cold oils, but soluble in hot oils.

The most prominent physical characteristic of ACETEM is its ability to significantly reduce the viscosity of fat-based systems. The introduction of acetyl groups alters the crystallization behavior of the glyceride molecule-non-acetylated monoglycerides are solid and waxy at ambient temperature, whereas ACETEM, with increasing degrees of acetylation, exhibits a decreasing melting point and can exist as a liquid or semi-solid at room temperature. The viscosity-reducing function of ACETEM has been industrially validated in applications such as caramel and chocolate coatings, where it "reduces viscosity and ensures even fat distribution". In coffee creamers and non-dairy creamers, ACETEM "promotes uniform fat dispersion, enhances emulsion stability, and contributes to a white, smooth mouthfeel".

In the context of milk powder surface modification, the liquid-like, low-viscosity characteristic of ACETEM enables efficient spreading across the free fat layer on particle surfaces. Unlike solid, waxy conventional monoglycerides, liquid ACETEM at spraying temperatures possesses excellent fluidity, allowing it to rapidly infiltrate the irregular fissures and pores on particle surfaces, forming a uniform hydrophobic interfacial film. Although this interfacial film is itself hydrophobic, its critical function is to seal the exposed sites of free fat, preventing the subsequent hydrophilic coating (PGE) from being "swallowed" and rendered ineffective by the underlying hydrophobic fat layer. This can be conceptualized as an "interfacial primer"-creating a stable surface condition for the firm anchoring of the hydrophilic layer.

2 PGE: Hydrophilic Surface Modifier with Tunable HLB Value

PGE (polyglycerol esters of fatty acids, E475) is a non-ionic surfactant produced by esterification of polyglycerol with varying degrees of polymerization and natural fatty acids. It appears as a pale yellow powder or granular solid, is readily soluble in oils, fats, and organic solvents such as ethanol, and can disperse in hot water. PGE is characterized by excellent high-temperature resistance and acid stability.

The core advantage of PGE lies in its tunable hydrophilicity. By varying the degree of polymerization of the polyglycerol moiety (from diglycerol to decaglycerol) and the type and degree of substitution of fatty acids, the HLB value of PGE can be precisely adjusted across a broad range of 3 to 13, with hydrophilicity and lipophilicity changing markedly depending on these structural parameters. Among them, PGE products with higher HLB values (>10) fall within the category of "hydrophilic monoglycerides," possessing strong emulsifying capacity and excellent interfacial activity. It has been well documented in the literature that "when added to dairy products, [PGE] can enhance their instant solubility".

The hydrophilicity of PGE originates from its multiple ether bonds (C-O-C) and terminal hydroxyl groups (-OH) within the polyglycerol headgroup, which are capable of forming an extensive hydrogen bond network with water molecules, thereby constructing a strongly hydrated interfacial layer on the particle surface. When PGE molecules anchor via their hydrophobic tail chains onto the ACETEM-pretreated fat layer, with the polyglycerol headgroups extending outward, a "hydrophilic outer garment" is draped over the milk powder particles. This structural design enables water molecules, upon contact with the particle, to rapidly penetrate the PGE hydrophilic layer and diffuse quickly into the particle interior via the ACETEM-primed smooth interface.

3 Synergistic Surface Modification Mechanism of the Dual-Component System

The interfacial chemical complementarity between ACETEM and PGE arises from their pronounced differences in physical properties:

The synergistic mechanism of the two can be described as a gradient coating process. First, liquid ACETEM, by virtue of its low viscosity and strong affinity for fats and oils, uniformly spreads across the particle surface, wetting and partially sealing the hydrophobic sites of free fat, forming a primary "leveling interfacial layer." The function of this layer is to eliminate microscopic irregularities on the fat surface, creating favorable conditions for the uniform coverage of the subsequent hydrophilic coating. Subsequently, PGE molecules anchor via hydrophobic interactions between their tail chains and the ACETEM layer, with the hydrophilic polyglycerol headgroups orienting outward to form a secondary "hydrophilic outer layer." This "hydrophobic interior, hydrophilic exterior" gradient structure fully exploits the lipid content intrinsic to the milk powder particle as an anchoring matrix, while avoiding the issue of uneven coverage that may occur when hydrophilic emulsifiers are sprayed directly onto a fat-rejecting surface.

In terms of dispersibility and instant solubility, the low-viscosity characteristic of ACETEM can facilitate moderate inter-particle adhesion during the agglomeration process, contributing to the formation of a more stable porous agglomerate structure. The abundant capillary channels within porous agglomerates are essential for achieving rapid reconstitution, and the "moderate tackiness" provided by ACETEM precisely enhances the mechanical strength of agglomerates without causing excessive clumping. The strong hydrophilicity of PGE, in turn, ensures that once water makes contact with the particle surface, it rapidly slides along the hydrophilic layer into the internal pores of the agglomerate, completing wetting and sinking within seconds.

1 Spraying Process Design

The ACETEM/PGE dual-component surface modification spraying process can be implemented on the existing fluidized bed systems of whole milk powder production lines without the need for large-scale equipment modifications. The recommended process flow is as follows: the spray-dried milk powder base enters the first-stage fluidized bed for agglomeration and granulation; it then proceeds to the second-stage coating fluidized bed, where a mixed emulsifier solution of ACETEM and PGE (dissolved in food-grade vegetable oil, such as MCT or high-oleic sunflower oil, at a total concentration of 10%–20%) is uniformly sprayed onto the particle surface via a twin-fluid nozzle. The spraying temperature is controlled within the range of 50–65°C, which maintains the low-viscosity, liquid-like fluidity of ACETEM while ensuring adequate dissolution and dispersion of PGE.

2 Reference Formulation and Expected Dosage

Based on the interfacial synergistic mechanisms of ACETEM and PGE and prevailing food additive usage standards, the recommended reference dosage is as follows (as a percentage of total milk powder dry matter mass):

This dosage range is broadly comparable to that of the conventional lecithin spraying process (0.2%–0.3%), achieving functional upgrading within a manageable cost framework. Under China's GB 2760 standard, the maximum permitted use level of PGE in modified milk powder and modified cream powder is 10.0 g/kg; ACETEM may be used in accordance with Good Manufacturing Practice (GMP), thus fully satisfying regulatory requirements. The fatty acids used in the ACETEM/PGE combination are predominantly saturated (stearic acid) or of low unsaturation, conferring superior oxidative stability compared to the highly polyunsaturated fatty acid-rich soy lecithin, thereby substantially reducing the risk of fat oxidation in whole milk powder over a 12–24 month shelf life.

3 Expected Performance Indicators

It is worth noting that achieving rapid wetting and dispersion in ice-water mixtures at 8–10°C constitutes a key technical requirement for premium instant beverage products, such as cold-water instant protein shakes and outdoor sports nutritional powders. Lecithin-sprayed products exhibit markedly diminished surface activity at low temperatures, whereas the ACETEM/PGE combination, leveraging the low-temperature resilience of PGE and the low-melting-point leveling capability of ACETEM, is expected to maintain excellent wetting performance under low-temperature conditions.

This paper has systematically analyzed the synergistic mechanisms and application potential of ACETEM and PGE in the instantization improvement of whole milk powder. The study demonstrates that the gradient coating strategy of the two can effectively overcome the functional limitations of traditional single-emulsifier spraying processes: ACETEM exploits its low viscosity and excellent spreadability to construct a uniform primary interfacial layer, sealing the hydrophobic sites of free fat; PGE, relying on its tunable high hydrophilicity, constructs an outer hydration channel, endowing the particle surface with superior cold water wetting capability. This combination upgrades the interfacial design from a "monomolecular hydrophilic layer" to a "gradient functional interface," and is expected to achieve synergistic improvements in low-temperature instant dissolution performance, storage stability, and oxidative protection.

Future research directions may focus on: (1) the precise influence of different ACETEM acetylation degrees and PGE polymerization degrees on wetting kinetics, establishing a quantitative "structure–property" relationship model; (2) the applicability assessment of this composite system in the instantization of plant-based milk powder alternatives (e.g., oat milk powder, coconut milk powder); (3) the integration of ACETEM/PGE surface modification technology with the encapsulation and release of fat-soluble micronutrients (vitamins A, D, E) to develop dual-function whole milk powder products capable of "instant dissolution + nutrient delivery." As the global premium milk powder market increasingly demands cold-water instant solubility and clean-label credentials, synthetic emulsifier blend strategies based on precision interfacial design will exhibit broad application prospects.