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Article

Effects of Oil Properties on Stability Behavior of High-Energy-Density Fat Emulsions

by
Xianmin Xu
1,
Wei Zeng
1,2,*,
Meijun Du
1,
Abdelaziz Elbarbary
1,3,
Jun Jin
1,4,* and
Xingguo Wang
1,4
1
State Key Laboratory of Food Science and Resources, School of Food Science and Technology, Jiangnan University, Wuxi 214122, China
2
School of Basic Medicine, Gannan Medical University, 1 Hexie Avenue, Ganzhou 341000, China
3
Dairy Science Department, Faculty of Agriculture, Benha University, Moshtohor 13736, Egypt
4
Food Laboratory of Zhongyuan, Luohe 462300, China
*
Authors to whom correspondence should be addressed.
Foods 2026, 15(4), 621; https://doi.org/10.3390/foods15040621
Submission received: 14 December 2025 / Revised: 28 January 2026 / Accepted: 3 February 2026 / Published: 9 February 2026
(This article belongs to the Special Issue Recent Advances in Lipid Delivery Systems for Food Applications)
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Abstract

Foods for special medical purposes play a critical role in clinical nutritional support, especially oil-in-water emulsions characterized as having high energy density, which could provide efficient energy for patients with insufficient intake or those requiring fluid restriction. The included oil types are the critical determinants of emulsion stability, which, in turn, governs digestive behavior, absorption efficiency, and ultimate bioavailability of the delivered nutrients. However, such emulsions face stability challenges during storage and application. In the present study, high-energy-density fat emulsions formulated with six typical oils, which contained 50% oil content, were prepared and systematically analyzed in terms of their particle size, zeta potential, microstructure, centrifugal stability, multiple light scattering, and rheological properties. The results indicated that oils with medium-chain fatty acids, due to their compact molecular structure and low viscosity, facilitated the formation of finer droplets and promoted the orderly arrangement of phospholipids at the interface of the emulsion system, leading to the formation of a dense, elastic interfacial layer and a gel network structure. Its marked shear-thinning characteristic and lowest frequency dependence contributed to desirable processing and storage stabilities. In contrast, long-chain triacylglycerols, especially those enriched with monounsaturated and saturated fatty acids, tended to form rigid but insufficiently elastic interfacial layers, which were unfavorable for resisting coalescence and phase separation induced by external forces. Highly unsaturated oils, on the contrary, exhibited medium levels for emulsion stability. Further analysis of the relationship between the physicochemical properties of oils and the characteristics of emulsions revealed that fatty acid species in the oil phase were the key determinants of emulsification behavior. It was therefore speculated that oils rich in medium-chain fatty acids with a moderate degree of unsaturation, especially including selected ω-3 and ω-6 fatty acids, could improve emulsion stability and fatty acid balance synchronously. This study provides a theoretical basis and technical support for the formulation design and stability control of high-energy-density fat emulsions.

1. Introduction

Cancer is a leading cause of mortality worldwide [1]. Patients often suffer from complications such as cachexia due to malnutrition, inflammation, and metabolic disorders during treatment, making effective nutritional support crucial [2,3]. For specific populations, including post-operative, gastrectomy, or fluid-restricted patients, high-energy-density nutritional emulsions serve as a key solution to deliver adequate energy under restricted fluid intake [4,5]. However, the clinical translation and broad application of such emulsions require overcoming a series of technical challenges related to formulation design and stability.
Oil-in-water (O/W) emulsions rich in multiple nutrients are widely used in foods for special medical purposes [6]. They address the demands for energy supply effectively. However, such O/W emulsions are thermodynamically unstable systems, especially those with high fat content, which are more prone to instability [7]. The stability of high energy density-fat emulsions is critical for nutrient retention and absorption efficiency. Enhanced emulsion stability contributes to the protection of encapsulated bioactive compounds against degradation induced by environmental stressors such as heat, light, and oxygen, which improves their retention rate and bioaccessibility [8]. Furthermore, the emulsions can safeguard bioactive substances from chemical and enzymatic degradation in the stomach and digestive tract, which enables them to promote transcellular absorption, reduces presystemic metabolism, and ultimately improves bioavailability [9,10]. The bioaccessibility of delivered nutrients in simulated in vitro digestion can be significantly influenced by regulating the type of oil phase, particle size, and emulsifier type [11]. In oil-in-water emulsions, the type of fat and oil significantly affects their digestion and absorption efficiency, as well as the included nutrients. For instance, long-chain fatty acids (LCFAs) facilitate the formation of mixed micelles with larger hydrophobic domains, which could solubilize a greater quantity of lipophilic substances and thereby enhance their bioaccessibility [12].
For high-energy-density fat emulsions, when compositions of the continuous phase are identical, the differences in the dispersed phase, also named the oil phase, become the primary factors leading to gravity-induced separation [13]. The stability of the emulsions, mainly the interfacial properties, rheological behavior, and droplet aggregation of their emulsions, is significantly influenced by compositional characteristics of the oil phase, particularly the chain length and saturation of the attached fatty acids, which could further affect both storage stability and digestive properties [14]. Long-chain and highly unsaturated fatty acids (UFAs) significantly affected the interfacial adsorption behaviors of proteins, facilitating the formation of a stiffer and less stretchable layer that resulted in enhanced short-term yet compromised long-term emulsion stability [15,16]. Compared with triglycerides, diglyceride crystals could strengthen interactions with interfacial proteins and modulate surface potential, thereby enhancing the long-term storage stability of emulsions [17].
To enhance emulsion stability, food emulsifiers are commonly incorporated during production to reduce oil–water interfacial tension and improve physical stability [18]. Among all the emulsifiers, synthetic emulsifiers such as Tween 80 demonstrate excellent emulsifying properties and are widely used in the food industry, including in nano-emulsion systems [19]. However, many studies indicate that synthetic emulsifiers may disrupt gut microbiota balance and impair intestinal barrier function, thereby potentially increasing the risk of chronic inflammatory and autoimmune diseases [20,21]. To address this challenge, the food industry is committed to replacing synthetic emulsifiers with natural and safe alternatives and accelerating the development of novel products that align with clean-label trends [22]. A phospholipid is a naturally occurring, small-molecule emulsifier with an amphiphilic structure, capable of stabilizing emulsions as the sole emulsifier [23,24]. Phospholipids play a crucial role in maintaining the physiological activity and metabolism of biological membranes in vivo [25]. Studies have shown that emulsion systems constructed using phospholipids as an emulsifier not only improve the oxidative stability of the oil phase but also significantly enhance the stability and biological delivery efficiency of lipophilic nutrients [26,27]. Lecithin, as emulsifier, enhances lipid digestion and absorption efficiency, thereby regulating plasma and tissue lipid metabolism and improving dyslipidemia and hepatic lipid accumulation [28]. For patients with impaired lipid metabolism, such as those with liver cancer, dietary supplementation with emulsions prepared using lecithin as the emulsifier may serve as a beneficial adjuvant approach.
The chain length and degree of unsaturation of both the emulsifier and oil phase are key factors influencing emulsion properties [29]. This study selected a typical commercial phospholipid as a fixed emulsifier to systematically investigate the stabilization of O/W emulsions formed using six selected oils with varying chain lengths and saturation levels. The physicochemical properties of the resulting emulsions were systematically analyzed to assess the impact of oil characteristics on emulsion stability, primarily particle size distribution, zeta potential, centrifugal stability, multiple light scattering, and rheological properties. The findings are expected to provide theoretical guidance and technical support for formulation design and stability control of lipid modules in specific medical applications.

2. Material and Methods

2.1. Materials

Medium-chain triglycerides (MCTs, synthesized from C8:0 and C12:0) were provided by Qingdao Seawit Life Science Co., Ltd. (Qingdao, China). Rapeseed oil was purchased from Daodaoquan Grain and Oil Co., Ltd. (Yueyang, China). Flaxseed oil was purchased from local supermarket. Sunflower seed oil, coconut oil, and palm olein were supplied by Yihai Kerry Arawana Holdings Co., Ltd. (Shanghai, China). Phospholipid (Tables S1 and S2) was purchased from Hisoya Biotechnology Co., Ltd. (Guangzhou, China). Potassium hydroxide, sodium sulfate anhydrous, and all the HPLC-grade solvents were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Supelco 37 FAME mixture and Nile Red were purchased from Sigma-Aldrich Chemical Company (Shanghai, China).

2.2. Determination of Fatty Acid Composition

The fatty acid composition of each oil was determined using a gas chromatograph system (7820A, Agilent, Santa Clara, CA, USA) equipped with a hydrogen flame ionization detector (FID). The specific detection conditions were based on our previous study [30]. The preparation procedure of fatty acid methyl esters is briefly described as follows: The oil (50 mg) was dissolved in 1 mL of 2 mol/L potassium hydroxide–methanol solution and 2 mL of hexane. After thorough mixing, the upper organic phase was collected, and an appropriate amount of Na2SO4 was added to remove water. The obtained organic phase was filtered with a filter (pore size, 0.22 μm) for GC analysis. Individual fatty acids were identified by comparing the relative retention times of the methyl ester peaks of the standards.

2.3. Preparation of the High-Energy-Density Fat Emulsions

The selected oil (50%, w/w) and the emulsifier phospholipid (3%, w/w) were thoroughly mixed. Deionized water was then added, and the mixture was blended using an Ultra-Turrax homogenizer (IKA, Staufen, Germany) at 15,000 rpm for 2 min. Finally, the premix was homogenized in a homogenizer (AH-2010, Antos Nano-technology Co., Ltd., Suzhou, China) at a pressure of 500 bar.

2.4. Measurement of Viscosity and Density

The viscosity of freshly prepared O/W emulsion was measured using a digital viscometer (NDJ-8S, Lichen Technology Co., Ltd., Shanghai, China). The measurement was performed using Rotor No. 1, and the revolution rate was controlled at 12 r/min. The relative density of each oil was determined using a pycnometer. Firstly, the masses of the dry pycnometer (m0) and the water-filled pycnometer (mw) were recorded. After redrying, the pycnometer was filled with the test oil (moil) and weighed again. The relative density was then calculated using the formula below. All measurements were conducted at 20 °C.
ρ o i l = ρ w × m o i l − m 0 m w − m 0

2.5. Analysis of Centrifugal Stability

Centrifugal stability of the emulsion was evaluated according to the method described by Latreille [31], with minor modifications. Briefly, by using a high-speed freeze centrifuge (Eppendorf, Hamburg, Germany), 10× g of the emulsion was centrifuged at 7000× g (rotor radius = 10 cm) for 60 min. The height of the lower whey phase was measured, and the creaming index was calculated using the following formula:
C r e a m i n g   i n d e x ( % ) = H s H t × 100 %
where HS represents the height of the lower separated serum phase, and Ht represents the total height of the emulsion.

2.6. Determination of Particle Size, Particle Size Distribution, and Zeta Potential

Volume average particle size and particle size distribution of freshly prepared emulsions were determined using an S3500 laser particle size analyzer (Microtrac Inc.,Montgomery Ville, FL, USA). Deionized water was used as the dispersant, with the refractive index of the dispersed particles and dispersant being 1.590 and 1.330, respectively. The particle shape was assumed to be spherical. Each sample was measured in triplicate, and the results were averaged. For zeta potential measurement, the emulsions were diluted 200-fold with deionized water and then analyzed using an omni–Zeta potential and nanoparticle analyzer (Brookhaven Instruments, New York, NY, USA).

2.7. Observation of Microscopic Morphology

The microstructure of the emulsions was observed using a Zeiss LSM880 confocal laser scanning microscopy (CLSM; Carl Zeiss, Jena, Thuringia, Germany). The emulsion samples were stained with Nile Red (0.1 mg/mL in absolute ethanol) at a sample-to-dye volume ratio of 25:1. After incubation in the dark, 5 μL of the stained sample was placed on a glass slide for inverted microscopy observation at 100× objective. The excitation wavelength for Nile Red was set at 514 nm.

2.8. Stability Evaluation by Multiple Light Scattering

Physical stability of the emulsions was evaluated using a Turbiscan TOWER stability analyzer (Formulations, Toulouse, France) based on static multiple light scattering. First, 20 mL of the freshly prepared emulsion was transferred into a measurement cell and scanned from bottom to top at 25 °C. The backscattering light profile as a function of sample height and the Turbiscan Stability Index (TSI) as a function of time were recorded over a period of 12 h. Throughout the measurement, TSI values were recorded at 30 min intervals.

2.9. Measurement of Rheological Properties

The rheological properties of the emulsions were determined using a DHR-3 rheometer (Waters Corporation, Milford, MA, USA) equipped with a 40 mm parallel plate. All the measurements were conducted at 25 °C with a gap set to 1000 μm. A flow sweep was performed with a linearly increased shear rate from 0.1 to 100 s−1. For dynamic oscillatory test, a strain sweep (0.01% to 100%) was firstly conducted to determine the linear viscoelastic region (LVR), followed by a frequency sweep (0.1 to 10 Hz) conducted at a fixed strain of 0.1% within the determined LVR.
The flow behavior of the emulsions was mathematically described using the Ostwald de Waele model:
η = K × μ n − 1
where η, K, μ, and n represent the apparent viscosity, consistency coefficient, shear rate, and flow behavior index, respectively.
The frequency dependence of the storage modulus (G′) and loss modulus (G″) was described by the power law model:
G ′ = k ′ f m ′
G ″ = k ″ f m ″
where k′ and k″ are power law constants, m′ and m″ are frequency exponents, and f is the frequency.

2.10. Statistical Analyses

All the tests were conducted three times, and the experimental results are presented as the mean ± standard deviation. The experimental data was analyzed using one-way analysis of variance (ANOVA) by IBM SPSS Statistics 27.0., with a p-value < 0.05 considered statistically significant. Partial least squares regression (PLSR) analysis was conducted using SIMCA 14.1 software. All the graphs were plotted using Origin 2024.

3. Results and Discussion

3.1. Physicochemical Properties of Individual Oils

Individual oils with distinct fatty acid compositions were selected to prepare oil-in-water (O/W) emulsions, aiming to investigate the effects of oil chain length and unsaturation degree on their emulsion stabilities, which are presented in Table 1. The fatty acids in MCT were exclusively medium-chain fatty species, i.e., C8:0 and C10:0. Coconut oil was predominantly composed of medium-chain fatty acids (MCFAs = 57.03%) and exhibited a high level of long-chain saturated fatty acids (LC-SFAs = 33.16%). In palm olein, the contents of long-chain monounsaturated fatty acids and saturated fatty acids were comparable (MUFAs = 48.36%, and LC-SFAs = 38.41%). The total unsaturated fatty acid contents of rapeseed oil, sunflower seed oil, and flaxseed oil were 93.21%, 89.97%, and 90.11%, respectively, of which, rapeseed oil was rich in long-chain monounsaturated fatty acids (MUFAs = 62.91%), whereas both sunflower seed oil and flaxseed oil were abundant in long-chain polyunsaturated fatty acids (PUFAs = 68.58% and 69.60%, respectively). Specifically, sunflower seed oil had a higher content of C18:2, while flaxseed oil contained more C18:3. Among all these oils, MCTs showed the lowest viscosity (40.00 mPa·s at 20 °C) but the highest density, whereas palm olein exhibited the highest viscosity (104.70 mPa·s at 20 °C), followed by rapeseed oil (93.50 mPa·s at 20 °C). The selected oils exhibited significant differences in their fatty acid compositions, as well as chain length and degree of unsaturation, which were used to investigate the independent and combined effects of these structural variables on emulsion stabilities.

3.2. Particle Sizes and Zeta Potentials

The particle size of emulsion droplets influences the extent of gravitational separation during storage significantly, which is one of the core parameters for evaluating its stability. Generally, smaller particle size and a more normal (Gaussian) distribution indicate better emulsion stability [32]. The mean particle sizes of the high-energy-density emulsions prepared from individual oils are shown in Table 2, and the corresponding particle size distributions are presented in Figure 1. All the emulsions exhibited typical monomodal distribution. As the fatty acid chain length increased, the emulsion particle size increased significantly (450.00–770.50 nm, p < 0.05), and the distribution broadened (span: 1.37–1.87). In contrast, higher levels of UFAs led to a significant reduction in droplet size, i.e., from 743.50 to 526.00 nm (p < 0.05) and a narrower distribution (span: from 1.88 to 1.42). The differences in particle size could be attributed to variations in emulsification outcomes caused by the composition and properties of the oils. MCTs, compared to the other oils, had a lower viscosity, which facilitated the breakdown of droplets into smaller sizes during emulsification [33]. Additionally, oil molecules tend to accumulate between the tails of emulsifier molecules or form a separate hydrophobic core within micelles, and the ability of emulsifiers to incorporate oil molecules depends on the size and shape of their hydrophobic interior relative to the non-polar molecules [34,35]. McClements proposed that the size of the hydrophobic domain increases with chain length, and saturated (straight) chains are larger than unsaturated (bent) chains [36]. Therefore, the differences in relative size (e.g., medium and long chain) and shape (e.g., degree of unsaturation) of the dispersed oil molecules may affect their incorporation into emulsifier micelles, consequently resulting in the formation of droplets of different sizes.
The surface charge of emulsion droplets is a critical determinant of emulsion stabilities: higher surface charge generates stronger electrostatic repulsion, thereby inhibiting droplet flocculation [37]. As shown in Table 2, the emulsion prepared with MCTs exhibited a relatively higher absolute value of zeta potential, 20.46 mV. This phenomenon may be attributed to differences in the organization of the phospholipid adsorption layer at the surface of MCT and long-chain triglyceride (LCT) droplets [33,38]. On the other hand, the distribution of phospholipids between the oil and aqueous phases depends not only on the type of oil but also on the droplet size [39]. Smaller droplets provided a larger specific surface area, which led to a higher proportion of phospholipids being adsorbed at the interface. Consequently, the MCT emulsion, with its smallest droplet size, exhibited higher interfacial phospholipid content, thereby showing a larger absolute zeta potential value.
In the case of emulsions formulated with long-chain triacylglycerols, the higher saturation generally exhibited greater zeta potential values. For instance, the rapeseed oil emulsion had an absolute potential of 8.67 mV, while the flaxseed oil emulsion registered a lower value of merely 5.42 mV. This trend can be attributed to the fact that oils with low unsaturation degrees could facilitate a more ordered and densely packed arrangement of phospholipids at the interface, enabling more efficient distribution of negatively charged phosphate groups and resulting in higher surface charge density, which leads to a higher zeta potential value [40]. However, the absolute zeta potential values observed in most emulsions had been relatively low, indicating that electrostatic stabilization played a limited role. Therefore, in such emulsion systems, electrostatic repulsion was not the dominant stabilization mechanism, and the stability of the emulsion was likely dependent on other factors, such as the three-dimensional gel network formed by lecithin [41].

3.3. Microstructures

The CLSM observations further validated the droplet size distribution characteristics of emulsions prepared using different oil phases at the microscopic perspective. By adjusting the image contrast uniformly to highlight larger droplets, Figure 2 shows that all the fresh emulsions exhibited a relatively uniform lipid droplet distribution with no significant flocculation or coalescence, indicating good physical stability during the fresh stage. Only a small number of large droplet structures were observed in the MCT and flaxseed oil emulsions, while the rapeseed oil emulsion had the largest droplets (743.50 nm), and the palm olein emulsion also showed relatively larger droplets (770.50 nm). The above differences were primarily attributed to the distinct structures of the oil phases. Compared to MCFAs or PUFAs, long-chain saturated and monounsaturated fatty acids form larger hydrophobic domains and higher interfacial tension, resulting in the formation of larger emulsion droplets [36,42]. Such results were consistent with the particle size distribution results.

3.4. Viscosities and Centrifugal Creaming Indices

Viscosity, as a core physical property influencing emulsion stability, affects system stability by modulating destabilization processes such as droplet coalescence and Ostwald ripening [43]. As observed in Figure 3A, the MCT emulsion exhibited significantly higher viscosity than the other five emulsions, while the emulsion prepared with rapeseed oil showed the lowest viscosity. Generally, factors such as oil type and droplet size contribute to differences in viscosity among emulsions. Higher-density oil phases can increase the frictional force between emulsion droplets, reduce their fluidity, and thereby enhance the physical stability of the emulsion [44]. Similarly, smaller emulsion droplets possess a larger specific surface area, which strengthens intermolecular interactions and leads to a significant rise in emulsion viscosity [45]. Therefore, as can be seen from Table 2, the MCT emulsion, which had the smallest droplet size and highest oil density, displayed the highest viscosity, whereas the rapeseed oil emulsion exhibited the opposite result.
Centrifugation tests are used as an effective method to evaluate emulsion physical stability by accelerating destabilization and assessing resistance to phase separation [46]. All the emulsions exhibited phase separation after centrifugation, and their centrifugal stabilities were analyzed based on the height of the separated aqueous phase (Figure 3B). With the increase in chain length of the attached fatty acids, the creaming indices of the emulsions increased, and centrifugal stabilities declined. This may be because the longer hydrocarbon chains enhance molecular hydrophobicity and reduce the polarity of the oil phase, which promotes phase separation. Research has shown that emulsions containing LCFAs tend to form a rigid interfacial layer with insufficient elasticity, which cannot counteract centrifugation-induced phase separation [47]. Therefore, long-chain triglyceride emulsions (e.g., rapeseed oil and palm olein) showed lower centrifugal stability. Additionally, an increase in PUFAs improved emulsion centrifugal stability. This finding contradicted the results reported by Teh & Mah [13]. A possible explanation was that when the oil and emulsifier have similar fatty acid compositions, lecithin tends to dissolve in the oil phase rather than adsorb at the interface, reduces interfacial coverage, and weakens the deformation resistance of the interfacial film, making the emulsion more prone to coalescence under centrifugal force [48]. The fatty acid compositions of rapeseed oil and sunflower were closer to that of the emulsifier than flaxseed oil (Table S2). Thus, the flaxseed oil emulsion exhibited better centrifugal stability among the emulsions formulated with PUFA-rich oils.

3.5. Multiple Light Scattering

Emulsion instability is typically induced by various physical processes, such as coalescence, flocculation, and gravitational phase separation [49]. The Turbiscan analyzer, which is based on static multiple light scattering technology, enables the detection of alterations in physical phenomena associated with instability at an early stage. A decrease in BS intensity at the bottom suggests a local decrease in droplet concentration, indicating watery phase separation [50]. As observed in the spectra (Figure 4A), across all the emulsions, the BS intensity at the bottom declined as the measurement time increased, indicating that clarification and gravity-driven migration may be occurring in the bottom region. Further calculations yielded the TSI values, as shown in Figure 4B. Generally, a higher TSI reflects greater changes in droplet concentration and lower stability [51]. At the end of the scanning period, the TSI values of the MCT, rapeseed oil, sunflower oil, flaxseed oil, coconut oil, palm olein emulsions were 0.35, 0.70, 0.45, 0.42, 0.40, and 0.56, respectively. The results showed that a decrease in the content of MCFAs in the oil phase leads to a decline in emulsion stability, while an increase in the content of UFAs helps improve stability.
Among them, the rapeseed oil emulsion had the highest TSI value, indicating the poorest stability. This may be related to its larger droplet size and lower emulsion viscosity, making it more prone to sedimentation or creaming during storage. Conversely, although the palm olein emulsion also had relatively large droplet size and low viscosity, its TSI value was comparatively lower compared with the counterpart of rapeseed oil. This might be due to its higher palmitic acid content, which might effectively enhance emulsion stability through interactions with interfacial components [52].
Sunflower oil and flaxseed oil, rich in PUFAs, contributed to the formation of emulsions with smaller droplet sizes and relatively higher viscosity, thereby reducing the TSI values. Notably, both the MCT and coconut oil emulsions exhibited lower TSI values, which were closely associated with their high MCFA contents. These fatty acids might contribute to the formation of small droplet sizes and high-viscosity emulsion systems while also effectively penetrating the hydrophobic layer of the emulsifier, thereby reducing the spontaneous curvature of the interfacial film [53]. Together, these effects improved the macroscopic storage stability of the emulsion. Consistent with the findings of Wang et al., an increase in MCFA content can enhance emulsion stability [54].

3.6. Rheological Properties

Rheological properties are the critical determinants of emulsion stability and textural quality, so precise modulation of these properties is essential for developing emulsion-based products with superior sensory characteristics [45]. The apparent viscosity of emulsions prepared from different oils under increasing shear rates is shown in Figure 5A. As the shear rate was raised from 0.1 to 100 s−1, the apparent viscosity of all the emulsions decreased, demonstrating typical shear-thinning behaviors [55]. At identical shear rates, higher apparent viscosity was observed in the MCT emulsion, indicating that emulsions made from MCTs showed better stability [56]. This is consistent with the results of the TSI values. Based on the fitting results of the Herschel–Bulkley model, except for the MCT emulsion (τ0 = 0.35 Pa), the yield stresses of the other emulsions were not significant (τ0 < 0.1 Pa). To facilitate consistent comparison and highlight their dominant shear-thinning behavior, the Ostwald de Waele model was employed to fit the flow behavior of all the emulsions in this study, with the corresponding parameters provided in Table 3. The model was found to provide excellent fits to the flow curves of all emulsions, with R2 values ranging from 0.96865 to 0.99868. All the emulsions exhibited a flow behavior index (n) less than 1, confirming pronounced non-Newtonian shear-thinning behavior. A lower n value indicates more marked shear-thinning characteristics [57]. It was found that both increased MCFA content and a higher unsaturation degree in the oils enhanced the shear-thinning behaviors of the emulsions. The consistency index (K), which reflects emulsion viscosity, followed a trend consistent with the apparent viscosity measurements. The highest K value was obtained for the MCT emulsion, while the lowest was recorded for the sunflower seed oil emulsion. Moreover, the chain length of the oils was found to exert a greater influence on K than the degree of unsaturation. This observation could be attributed to the improved diffusion and adsorption of phospholipids, e.g., lecithin, at the oil–water interface with increasing levels of MCFAs, leading to the formation of a reinforced internal structure and, consequently, enhanced viscosity [48,58].
Variations in the storage modulus (G′) and loss modulus (G″) of all the emulsions with the frequency are presented in Figure 5B. As observed, G′ values of all the emulsions were higher than the corresponding G″ values, and both moduli remained nearly parallel over the frequency range of 0.1–10 Hz, indicating that all the emulsions exhibited a weak gel-like network structure [59]. The frequency-dependent behaviors of the emulsions were quantitatively evaluated using the power law model, with the results summarized in Table 4. Emulsions containing MCFAs showed higher k′ and k″ values, while an increase in oil unsaturation also led to the elevation of these two values, suggesting that the incorporation of both MCFAs and UFAs enhanced the viscoelastic properties of the emulsions. Parameters m′ and m″ are related to the frequency sensitivity of the gel network structure, with higher values indicating stronger frequency dependence [55]. The results demonstrated that MCT and coconut oil emulsions had the lowest frequency dependence, and the rapeseed oil emulsion showed the highest frequency dependence. Therefore, compared to the other emulsions, the MCT and coconut oil emulsions exhibited the highest G′ and the weakest frequency dependence, indicating favorable stabilities. Consistent with the findings of Feng et al. [14], emulsions with the highest storage modulus and the lowest frequency dependence also displayed the best stability. The higher G′ values observed in the MCT and coconut oil emulsions may be attributed to the ability of MCFAs to integrate into the phospholipid interface, forming a densely packed and mechanically robust interfacial film that strengthened the emulsion network. In addition, the possible partial crystallization of coconut oil at the interface may contribute to a protective layer, further enhancing structural stability and increasing the storage modulus [60,61]. In contrast, the rapeseed oil emulsion showed a lower G′ than the other emulsions, which may be due to its larger droplet size and lower viscosity, resulting in a weaker network structure.

3.7. PLSR Analysis

This study determined the physicochemical properties of six oils and the characteristic indicators of their corresponding emulsions. The oil properties included MCFAs, MUFAs, SFAs, PUFAs, oil viscosity, and density. The emulsion indicators comprised mean particle size, zeta potential, emulsion viscosity, creaming index, and TSI value. To systematically explore the multivariate relationship between oil properties (particularly chain length and degree of unsaturation) and emulsion performance, a PLSR method was employed to construct an association and prediction model linking multiple independent variables X (oil properties) and multiple dependent variables Y (emulsion properties).
PLSR analysis revealed the relationship between oil properties (X variables, n = 6) and the emulsion characteristics (Y variables, n = 4), excluding zeta potential. The X variables (R2X = 0.998) effectively explained the variation in the Y variables (R2Y = 0.942) (Figure 6A). To validate the reliability of the model, 200 permutation tests were performed. The model was not overfitted and possessed statistically significant predictive capability (Figure S1). The regression model showed that the different oil-based emulsions clustered into three distinct groups according to their properties. The MCT and coconut oil emulsions exhibited stable characteristics with high viscosity and small particle size due to their rich MCFA contents. The rapeseed oil and palm olein emulsions shared similar properties owing to their high MUFA contents. The sunflower seed and flaxseed oil emulsions formed a separate cluster due to their PUFA contents. This classification aligns with the physicochemical property measurements obtained earlier.
The variable importance in projection (VIP) value is a key metric in PLSR models for screening influential variables. Generally, variables with a VIP value greater than 1 can be considered key drivers explaining the emulsion properties. As shown in Figure 6B, the VIP values for the contents of MCFAs, PUFAs, and SFAs in the oils were all greater than 1, specifically 1.545, 1.202, and 1.128, respectively. This indicated that they were key factors influencing emulsion particle size, viscosity, creaming index, and TSI value. In contrast, the VIP values for MUFA content, oil viscosity, and density were lower than 1, suggesting that their effects are relatively minor. To clarify the specific effects of oil properties on emulsion characteristics and identify key variables, regression coefficients and their uncertainties were estimated using the Jack-knife method to pinpoint predictor variables with significant impacts (Figure 6C). A positive coefficient value on the Y-axis indicated a positive effect, while a negative value indicated a negative effect, with the absolute value of the coefficient reflecting the magnitude of the influence [62]. Regression analysis demonstrated that the fatty acid compositions of the oils significantly affected the emulsion properties. Specifically, the contents of MCFAs, SFAs, and PUFAs in the oils showed a significant negative correlation with emulsion particle size, whereas the MUFA content and oil viscosity showed a significant positive correlation. Regarding rheological properties, MCFA and SFA contents significantly increased emulsion viscosity, but the oil’s own viscosity was negatively correlated with emulsion viscosity. Furthermore, PUFA content reduced centrifugal creaming index, and both SFA (including MCFA) and PUFA contents could lower TSI values, indicating their positive role in enhancing emulsion stability.

4. Conclusions

This study focused on high-energy-density fat emulsions and their regulation techniques on processing and storage stabilities. Chain length and unsaturation of oils significantly influenced interfacial properties, rheological behaviors, and droplet aggregations of the high-energy-density emulsions. MCTs, due to lower viscosity and higher density, could facilitate tighter packing of the phospholipids at the interface during the formation of the emulsion system, which might increase interfacial charge density and absolute zeta potential, thereby enhancing electrostatic repulsion and inhibiting droplet aggregation. Their marked shear-thinning characteristic and lowest frequency dependence contributed to slowing down the gravity-driven phase separation process, manifesting as lower creaming indices and TSIs. The desirable emulsion stability could also be found in the case of coconut oil. In contrast, MUFAs and LC-SFAs, especially in rapeseed oil and palm olein, owing to their strong hydrophobicity, might tend to form a rigid but insufficiently elastic interfacial layers, which was unfavorable for resisting coalescence and phase separation induced by external forces, therefore showing poor stabilities. Highly unsaturated oils, such as flaxseed oil and sunflower seed oil, although capable of forming smaller droplets in the emulsion systems, led to a reduced zeta potential and weakened stability of the interfacial layers, resulting in the middle levels for emulsion stabilities. PLSR analysis further highlighted that the contents of MCFAs, PUFAs, and SFAs in the oil phase were the key factors governing emulsion behaviors. These compositional traits collectively appear to regulate multiple stability-related parameters by regulating interfacial structure, droplet size, zeta potential, and rheological properties. In the formulation design of fat delivery systems, selecting oils enriched with MCFAs, as well as a moderate degree of unsaturation, could effectively enhance emulsion storage stability and nutrient delivery efficiency, providing a theoretical basis and technical support for the development of clinical nutritional preparations for patients with high metabolic demands.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/foods15040621/s1, Table S1. Concentration and composition of the phospholipid; Table S2. Fatty acid composition of the phospholipid; Figure S1. Permutation test (200 permutations) of the PLSR model for (A) mean particle size, (B) emulsion viscosity, (C) creaming index, and (D) TSI value.

Author Contributions

X.X.: Data curation, Investigation, Methodology, Software, and Writing—original draft. W.Z.: Data curation, Formal analysis, Resources, and Project administration. M.D.: Software and Writing—review and editing. A.E.: Resources and Writing—review and editing. J.J.: Project administration, Supervision, Writing—review and editing, and Funding acquisition. X.W.: Resources, Project administration, and Supervision. All authors have read and agreed to the published version of the manuscript.

Funding

The research was supported by the China Dairy Industry Association Dairy Science and Technology Innovation Fund-Mengniu Special Research Support Project (CDIAKCJJ-MN-2025-001).

Data Availability Statement

The original contributions presented in the study are included in the article; further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Particle size distribution of the high-energy-density fat emulsions.
Figure 1. Particle size distribution of the high-energy-density fat emulsions.
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Figure 2. Microstructures of the high-energy-density fat emulsions.
Figure 2. Microstructures of the high-energy-density fat emulsions.
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Figure 3. (A) Viscosities and (B) centrifugal emulsion separation rates of the high-energy-density fat emulsions. Note: Different letters indicate significant differences (p < 0.05).
Figure 3. (A) Viscosities and (B) centrifugal emulsion separation rates of the high-energy-density fat emulsions. Note: Different letters indicate significant differences (p < 0.05).
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Figure 4. (A) Turbiscan spectra and (B) TSI values of the high-energy-density fat emulsions.
Figure 4. (A) Turbiscan spectra and (B) TSI values of the high-energy-density fat emulsions.
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Figure 5. Rheological properties of the high-energy-density fat emulsions. (A) Apparent viscosity and (B) frequency dependence of storage modulus (G′) and loss modulus (G″). RO: rapeseed oil; SO: sunflower seed oil; FO: flaxseed oil; CO: coconut oil; PO: palm oil.
Figure 5. Rheological properties of the high-energy-density fat emulsions. (A) Apparent viscosity and (B) frequency dependence of storage modulus (G′) and loss modulus (G″). RO: rapeseed oil; SO: sunflower seed oil; FO: flaxseed oil; CO: coconut oil; PO: palm oil.
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Figure 6. PLSR analysis of the relationship between oil properties and emulsion properties: (A) biplot, (B) VIP plot, and (C) regression coefficients. 1–3: MCT, 4–6: rapeseed oil, 7–9: sunflower seed oil, 10–12: flaxseed oil, 13–15: coconut oil, and 16–18: palm olein.
Figure 6. PLSR analysis of the relationship between oil properties and emulsion properties: (A) biplot, (B) VIP plot, and (C) regression coefficients. 1–3: MCT, 4–6: rapeseed oil, 7–9: sunflower seed oil, 10–12: flaxseed oil, 13–15: coconut oil, and 16–18: palm olein.
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Table 1. Physical and chemical properties of individual oils.
Table 1. Physical and chemical properties of individual oils.
Fatty AcidsMCTsRapeseed OilSunflower Seed OilFlaxseed OilCoconut OilPalm Olein
C6:0--------0.41 ± 0.06--
C8:060.52 ± 0.48------6.03 ± 0.49--
C10:039.43 ± 0.42------5.26 ± 0.19--
C12:00.05 ± 0.01------45.33 ± 0.320.28 ± 0.03
C14:0--0.05 ± 0.010.06 ± 0.010.05 ± 0.0119.39 ± 0.310.97 ± 0.10
C16:0--4.61 ± 0.056.36 ± 0.305.65 ± 0.0810.52 ± 0.0433.51 ± 1.46
C16:1--0.22 ± 0.010.07 ± 0.010.07 ± 0.00--0.20 ± 0.00
C18:0--2.12 ± 0.063.60 ± 0.164.19 ± 0.083.25 ± 0.043.65 ± 0.05
C18:1--62.69 ± 0.3221.32 ± 1.4920.44 ± 0.167.80 ± 0.0448.16 ± 1.91
C18:2--20.57 ± 0.1168.19 ± 1.9216.77 ± 0.132.01 ± 0.2112.90 ± 0.70
C18:3--9.73 ± 0.090.39 ± 0.0052.83 ± 0.28--0.29 ± 0.01
∑ MCFA100.00------57.030.28
∑ SFA100.006.7810.029.8990.1938.41
∑ MUFA--62.9121.3920.517.8048.36
∑ PUFA--30.3068.5869.602.0113.19
Viscosity
(mPa·s, 20 °C)		40.00 ± 0.71 e93.50 ± 3.54 b75.75 ± 1.06 c66.25 ± 0.35 d65.75 ± 4.60 d104.70 ± 1.13 a
Density
(g/cm3, 20 °C)		0.947 ± 0.002 a0.910 ± 0.002 c0.922 ± 0.002 b0.925 ± 0.002 b0.922 ± 0.003 b0.908 ± 0.003 c
Different lowercase letters within the same row indicated significant differences (p < 0.05).  --, not detected.
Table 2. Average particle size and potential of the high-energy-density fat emulsions.
Table 2. Average particle size and potential of the high-energy-density fat emulsions.
Oil TypeParticle Size (nm)SpanZeta Potential (mV)
MCT450.00 ± 18.38 e1.37 ± 0.05 b−20.46 ± 3.57 b
Rapeseed oil743.50 ± 21.92 ab1.88 ± 0.13 a−8.67 ± 1.42 a
Sunflower seed oil642.50 ± 67.18 bc1.46 ± 0.24 ab−8.55 ± 1.53 a
Flaxseed oil526.00 ± 35.36 de1.42 ± 0.19 b−5.42 ± 0.67 a
Coconut oil558.00 ± 21.21 cd1.41 ± 0.15 b−6.81 ± 0.98 a
Palm olein770.50 ± 57.28 a1.87 ± 0.20 a−8.33 ± 1.19 a
Different lowercase letters within the same column indicated significant differences (p < 0.05).
Table 3. Fitting parameters of Ostwald de Waele model for the high-energy-density fat emulsions.
Table 3. Fitting parameters of Ostwald de Waele model for the high-energy-density fat emulsions.
Oil TypeKnR2
MCT0.61 ± 0.020.48 ± 0.020.99290
Rapeseed oil0.12 ± 0.000.72 ± 0.020.96865
Sunflower seed oil0.10 ± 0.000.66 ± 0.010.99701
Flaxseed oil0.17 ± 0.000.57 ± 0.010.99478
Coconut oil0.44 ± 0.000.59 ± 0.010.99868
Palm olein0.26 ± 0.010.41 ± 0.020.98867
Table 4. Frequency dependency of G′ and G″ by the power law model for the high-energy-density fat emulsions.
Table 4. Frequency dependency of G′ and G″ by the power law model for the high-energy-density fat emulsions.
Oil Typek′m′R2k″m″R2
MCT3.71 ± 0.680.81 ± 0.090.933322.20 ± 0.050.42 ± 0.010.99210
Rapeseed oil0.61 ± 0.201.30 ± 0.150.948170.60 ± 0.030.76 ± 0.020.99492
Sunflower seed oil1.36 ± 0.511.02 ± 0.180.852050.59 ± 0.030.63 ± 0.030.98777
Flaxseed oil2.23 ± 0.550.66 ± 0.130.779010.97 ± 0.060.55 ± 0.030.97850
Coconut oil4.63 ± 0.430.67 ± 0.050.970534.04 ± 0.120.54 ± 0.020.99428
Palm olein0.93 ± 0.361.16 ± 0.190.891780.60 ± 0.050.67 ± 0.040.97998
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Xu, X.; Zeng, W.; Du, M.; Elbarbary, A.; Jin, J.; Wang, X. Effects of Oil Properties on Stability Behavior of High-Energy-Density Fat Emulsions. Foods 2026, 15, 621. https://doi.org/10.3390/foods15040621

AMA Style

Xu X, Zeng W, Du M, Elbarbary A, Jin J, Wang X. Effects of Oil Properties on Stability Behavior of High-Energy-Density Fat Emulsions. Foods. 2026; 15(4):621. https://doi.org/10.3390/foods15040621

Chicago/Turabian Style

Xu, Xianmin, Wei Zeng, Meijun Du, Abdelaziz Elbarbary, Jun Jin, and Xingguo Wang. 2026. "Effects of Oil Properties on Stability Behavior of High-Energy-Density Fat Emulsions" Foods 15, no. 4: 621. https://doi.org/10.3390/foods15040621

APA Style

Xu, X., Zeng, W., Du, M., Elbarbary, A., Jin, J., & Wang, X. (2026). Effects of Oil Properties on Stability Behavior of High-Energy-Density Fat Emulsions. Foods, 15(4), 621. https://doi.org/10.3390/foods15040621

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