Tackling Fat Crystal Polymorphism Challenges in Long-Haul Logistics: Anti-Crystallization and Oil Separation Inhibition Technology of PGMS and DMG in Soft Frying Oils and Margarine-Based Fat Products

Dramatic temperature fluctuations during long-haul logistics readily induce uncontrolled polymorphic transitions of fat crystals and liquid oil separation in soft frying oils and margarine-based fat products, leading to quality defects such as graininess, oil exudation, and phase delamination. Propylene glycol monostearate (PGMS) and distilled monoglycerides (DMG), as two critical lipophilic emulsifiers, exert differential crystallization regulatory mechanisms-PGMS induces heterogeneous nucleation and suppresses the growth of large crystals, while DMG aligns directionally on fat crystal surfaces to stabilize the β'-form crystal polymorph-thereby generating a synergistic crystalline network modulation effect in soft frying oil/margarine-type fat systems. This article systematically elucidates the molecular structural characteristics of PGMS and DMG and their mechanistic influences on fat crystallization behavior, analyzes the synergistic advantages of their combined use in inhibiting polymorphic transitions and preventing liquid oil separation, and proposes formulation design principles and application strategies tailored to long-haul logistics scenarios, providing theoretical foundations and technical references for enhancing the storage and transport stability of soft frying oils and margarine-based fat products.

Specialty fats for the food industry are oil-and-fat products manufactured from animal or vegetable base oils through specific processes, primarily encompassing shortening, margarine, non-dairy cream, powdered fat, and frying oils. Their core function lies in imparting specific processing properties and sensory qualities to foods. Among them, soft frying oils and plastic margarine-based fat products have experienced continuously growing demand across industrial baking, foodservice chains, and pre-prepared meal scenarios due to their excellent processing convenience and mouthfeel performance.

Nevertheless, these products face severe technical challenges during long-haul logistics transportation. The physical properties of fat products largely depend on the crystallization state of fats; the solid fat content (SFC), rheological properties, and microstructure of crystalline fats directly influence application characteristics such as plasticity, extensibility, spreadability, and mouth-melting behavior. Inevitable temperature fluctuations during logistics trigger polymorphic transitions of fat crystals-specifically, the transformation of metastable β'-form crystals into coarse, needle-like β-form crystals-leading to the collapse of the crystalline network structure and the expulsion of liquid oil from the solid matrix. Macroscopically, this manifests as severe quality defects including surface oil exudation, sandy texture, and structural delamination. According to industry estimates, the global specialty fats sector incurs substantial annual economic losses due to quality deterioration during storage and transportation, a contradiction that is particularly acute in high-temperature regions and long-distance transport scenarios with inadequate e-commerce cold chain infrastructure.

Traditional solutions have predominantly relied on hydrogenated fats as base oils to enhance crystallization stability. However, the trans-fatty acids generated during hydrogenation pose unequivocal health risks, and the global trend toward "hydrogenation reduction" and "clean label" has become irreversible. Against this backdrop, the precise regulation of fat crystallization behavior through the judicious blending of emulsifiers has emerged as a key pathway that is both technically viable and economically feasible. Propylene glycol monostearate (PGMS) and distilled monoglycerides (DMG), as the two most commonly used lipophilic emulsifiers in fat systems, have garnered considerable attention due to their differentiated functional characteristics in crystallization regulation. PGMS exhibits a pronounced α-crystal form tendency, whereas DMG stabilizes the β'-form crystal polymorph and prevents oil-water separation; the combined use of these two emulsifiers holds promise for constructing a robust crystalline network from multiple dimensions, thereby systematically addressing the crystal polymorphism challenges encountered during long-haul logistics. This article proceeds from molecular structure and mechanistic analysis, systematically examining the anti-crystallization and oil separation inhibition technology principles of PGMS and DMG in soft frying oils and margarine-based fat products, and proposes application-oriented blending strategies.

1 Molecular Structure and Mechanism of PGMS

Propylene glycol monostearate (PGMS) is synthesized through the esterification of propylene glycol with edible-grade fatty acids. It is a typical non-ionic lipophilic emulsifier with an extremely low HLB value (approximately 1.5–3.5), exhibiting excellent oil solubility in fat systems. In its molecular structure, the propylene glycol hydrophilic head group is small in size, endowing PGMS with the capacity to be highly soluble in the oil phase, a property that determines its core function-not broad-spectrum emulsification, but deep, specialized regulation of the fat crystallization process.

The most prominent characteristic of PGMS is its α-crystal form tendency. Studies have shown that PGMS has a low onset crystallization temperature; it can adsorb onto the surface of the fat blend and induce heterogeneous nucleation, with the heterogeneous crystals thus formed inhibiting crystal growth of the fat blend and reducing the interaction between crystals. During the crystallization process, PGMS serves as a crystallization modifier, inducing the formation of small, stable β'-prime crystals while inhibiting the growth of undesirable large crystals. However, when PGMS is used alone, the weakened inter-crystal interactions result in the formation of a fine but loose crystalline network structure. Although this delays the emergence of large crystals, the network lacks sufficient strength to completely prevent liquid oil separation during extended storage and transport. This characteristic reveals PGMS's role in fat crystallization regulation as being "strong in inhibition but weak in construction."

2 Molecular Structure and Mechanism of DMG

Distilled monoglycerides (DMG) are produced through molecular distillation purification technology, achieving a monoester content exceeding 90%. They have an HLB value of 3–4 and exhibit pronounced lipophilicity in fat systems. Unlike PGMS, DMG possesses a glycerol-based hydrophilic head group of relatively larger size, enabling it to form more ordered directional alignment at oil-water interfaces and on fat crystal surfaces.

The crystallization regulatory mechanism of DMG operates at three levels. First, serving as a nucleation template: DMG has a relatively high onset crystallization temperature, crystallizing first during the cooling process and providing abundant nucleation sites for subsequent triglyceride crystallization, thereby inducing the formation of fine, uniform crystals. Second, interfacial directional alignment: DMG can arrange in an ordered manner on the surface of grease, controlling and stabilizing fat crystallization. Particularly for margarine, shortening, and similar fat products, it improves plasticity and extensibility while preventing oil separation and delamination. Third, polymorphic transition regulation: emulsifiers can serve as seed crystals to accelerate crystal nucleation, enter the fat crystalline network, act on crystallization sites to retard crystal growth, and hinder the β'-to-β polymorphic transition. Taken together, DMG possesses advantages that PGMS lacks in stabilizing the crystalline network-it not only modulates crystallization but also strengthens inter-crystal connectivity, constructing a solid matrix with greater shear resistance.

3 Physicochemical Essence of Crystallization-Induced Oil Separation

Fat crystallization is a multi-step kinetic process encompassing nucleation, crystal growth, polymorphic transition, and network formation. During cooling, triglyceride molecules initially form the metastable α-form crystals, then transform into the β'-form, and ultimately tend toward the thermodynamically most stable β-form. The β'-form crystals are small and needle-like, capable of forming a dense three-dimensional network that effectively entraps liquid oil within, imparting good plasticity and appearance to the product. In contrast, β-form crystals are coarse and plate-like, forming a loose network structure that cannot effectively retain liquid oil, leading to oil exudation from the solid matrix.

In long-haul logistics scenarios, frequent temperature fluctuations serve as the "driving force" for polymorphic transitions. Each time the temperature rises, a portion of the fine β'-form crystals dissolve; when the temperature drops again, the dissolved triglycerides preferentially deposit and grow on existing β-crystal surfaces rather than undergoing re-nucleation. This "dissolution-recrystallization" cycle accelerates the β'→β transition process, ultimately resulting in macroscopically visible oil exudation and graininess. The role of emulsifiers lies precisely in intervening at critical nodes of this process: either by altering the nucleation pathway (inducing heterogeneous nucleation), retarding crystal growth rates, or raising the energy barrier for the β'→β polymorphic transition.

1 Synergistic Nucleation and Polymorph Regulation

The synergistic effect of PGMS and DMG at the nucleation stage arises from their differentiated crystallization temperature windows. DMG, with its higher onset crystallization temperature, crystallizes first during the initial cooling phase of the system and serves as a nucleation template, providing abundant heterogeneous nucleation sites for triglycerides and promoting the rapid generation of numerous fine crystal nuclei. Although PGMS has a lower onset crystallization temperature, its excellent compatibility with fat molecules and its α-crystal form tendency enable it to adsorb onto the surfaces of already-formed crystal nuclei, inhibiting continued crystal growth in certain directions through a steric hindrance effect, thereby controlling crystal size within the fine range required for the β'-form.

PGMS's "crystal growth inhibition" effect and DMG's "nucleation promotion" effect form a favorable complementary relationship. DMG ensures a sufficient number of crystal nuclei-a prerequisite for forming a dense crystalline network-while PGMS prevents these nuclei from overgrowing into coarse β-form crystals-a key to maintaining network stability. Studies have indicated that, compared with single-emulsifier systems, blended emulsifier systems exhibit higher solid fat content (SFC), and no β-form crystals appear across various blending ratios. This implies that the combined use of PGMS and DMG can effectively inhibit the β'→β polymorphic transition, thereby fundamentally blocking the crystallographic basis for liquid oil separation.

2 Dual Network Construction and Liquid Oil Entrapment

The strength of the crystalline network structure directly determines the oil-holding capacity of fat products. Through directional alignment on fat crystal surfaces, DMG can form "molecular bridges" between adjacent crystals, enhancing inter-crystal interaction forces and thereby rendering the network structure more compact and robust. Although PGMS weakens direct inter-crystal interactions, the large quantity of fine crystals it induces can fill network interstices, increasing the migration resistance of liquid oil from the perspective of steric hindrance.

The synergistic construction of these two emulsifiers can be summarized as a "framework reinforcement + void filling" dual network strategy: DMG constructs and reinforces the framework of the crystalline network, while PGMS regulates the secondary crystal units that fill the network interstices. This synergistic network can effectively enhance the system's binding capacity for liquid oil (i.e., the critical oil-holding capacity). Even during localized dissolution-recrystallization cycles caused by temperature fluctuations, the crystalline network can maintain sufficient structural integrity to prevent large-scale liquid oil exudation.

3 Special Adaptation Mechanisms for Long-Haul Logistics Scenarios

In addressing the specific requirements of long-haul logistics, the synergistic advantages of PGMS and DMG are also reflected in their tolerance to temperature fluctuations. Owing to its low onset crystallization temperature, PGMS does not prematurely precipitate and lose its activity during low-temperature phases of logistics (e.g., at night). DMG, with its higher onset crystallization temperature, can maintain the basic framework of the crystalline network intact during relatively high-temperature phases of logistics (e.g., daytime solar exposure). This "high-low temperature complementarity" of crystallization temperature windows enables the blended system to maintain effective crystallization regulatory function across a broad temperature range, significantly outperforming single-emulsifier systems.

Moreover, combined use can reduce the usage level of each individual emulsifier, thereby avoiding side effects such as off-flavors and color changes associated with excessive addition of a single component. A typical recommended protocol involves a PGMS-to-DMG blending ratio between 1:1 and 3:1, with a total addition level controlled within the range of 0.3%–0.8% of the total fat content. The specific ratio should be optimized according to the fatty acid composition of the base oil and the target plasticity range of the product.

1 Application in Soft Frying Oils

Soft frying oils require good fluidity and thermal stability while exhibiting moderate plasticity at ambient temperature to prevent excessive flow during packaging and transport. The role of PGMS in frying oils is primarily manifested in inducing the formation of fine β'-form crystals and inhibiting the growth of large crystals. DMG, through directional alignment, reinforces the stability of the solid fat crystalline network in frying oils, thereby preventing liquid oil from separating during the storage and transport stages preceding high-temperature frying. With the combined use of these two emulsifiers, frying oils can maintain a uniform semi-solid texture after experiencing temperature-fluctuating logistics transport, without exhibiting the stratification phenomenon of a liquid oil layer separating at the top and solid fat sediment settling at the bottom.

2 Application in Margarine and Shortening

Margarine and shortening represent the most mature application domains for the synergistic use of PGMS and DMG. PGMS, when used in shortening, can retard the staling of bread and pastries while improving their processing properties; when used in margarine, it enhances whippability and prevents oil-water separation. DMG controls and stabilizes fat crystallization, improves plasticity and extensibility, and prevents oil separation and delamination.

In margarine systems, the combined use of PGMS and DMG can systematically improve multiple quality indicators: solid fat content (SFC) becomes more stable, the range of hardness variation in products subjected to temperature fluctuations is reduced, and plasticity and spreadability are maintained throughout the entire storage and transport period. This is particularly important for commercial products that must endure long-haul logistics and multiple transfers.

3 Representative Case Studies and Supporting Data

Taking palm oil-based margarine as an example, researchers selected multiple emulsifiers including distilled monoglycerides (DMGS) and propylene glycol esters (PGMS) for comparative experiments. The results demonstrated that different emulsifiers significantly influence the crystalline morphology, solid fat content, and polymorphic transition behavior of the emulsion system. PGMS exhibited outstanding performance in inhibiting the crystal growth of high-melting-point fatty acids, reducing the crystallization peak area of high-melting-point fatty acids-a function complementary to DMG's role in promoting the crystallization of high-melting-point components. In ice cream melt-resistance studies, the blended system of PGMS and DMG also demonstrated favorable synergistic effects-emulsifiers with different molecular structures and HLB values work in concert to construct an interfacial film possessing both rigidity and flexibility, simultaneously enhancing foam stability and melt-resistance.

The synergistic blending of PGMS and DMG provides a precise and effective technical pathway for addressing the crystallization-induced oil separation problems encountered by soft frying oils and margarine-based fat products during long-haul logistics. Through the complementary mechanisms of "nucleation promotion and growth inhibition," the dual network strategy of "framework reinforcement and void filling," and the advantage of "high-low temperature complementary" temperature windows, these two emulsifiers safeguard the polymorphic stability and oil-holding capacity of products across multiple dimensions throughout the entire storage and transport chain.

Looking ahead, with the expansion of cold chain logistics networks and the penetration of e-commerce channels into lower-tier markets, specialty fat products will face increasingly complex and variable storage and transport environments. Achieving more precise regulation of fat crystallization still depends on further in-depth investigations into crystallization mechanisms. Furthermore, driven by the clean-label trend, the development of novel emulsifier systems that combine naturally derived origins with highly efficient crystallization regulatory functions will represent a crucial direction for the next generation of anti-crystallization and oil separation inhibition technology.