Interfacial engineering of rice bran wax–lard oleogels by polyglycerol fatty acid esters: Polymerization degree‐dependent modulation of β′‐crystal networks and bread quality

Abstract

BACKGROUND

The objective of this study was to evaluate the effects of polyglycerol fatty acid ester (PGFE) concentration and degree of polymerization on the structure of rice bran wax–lard composite oleogel (RLCO) and its application in bread. PGFE interfacial behavior was characterized by assessments of interfacial tension, adsorption kinetics, and dilatational rheology, while oleogel structure was evaluated using differential scanning calorimetry, Fourier transform infrared spectroscopy, X‐ray diffraction, and polarized light microscopy. Functional assessments included rheology and measurements of hardness, oil‐binding, and bread‐making application.

RESULTS

PGFEs were found to lower the interfacial tension and Gibbs free energy at oil–water interfaces while stabilizing interfacial films. Fourier transform infrared analysis suggested that PGFEs adsorbed at the rapeseed oil–lard interface through hydrogen bonding, promoting the orderly crystallization of rice bran wax. This markedly increased the proportion of β′ crystals, resulting in a denser, more stable, and cohesive RLCO network. In particular, 5 g kg⁻¹ polyglyceryl‐10 monostearate (PG‐10 MS) displayed the most significant enhancement, increasing bread moisture content by 7.07%, specific volume, and elasticity. However, excessive PGFE caused molecular overcrowding at the interface, increased interfacial tension, disrupted network integrity, and ultimately impaired bread quality.

CONCLUSION

The results indicate that the polymerization degree and concentration of PGFE can serve as predictors of interfacial activity to evaluate their effects on RLCO crystal morphology. This interfacial‐centric paradigm enables rational design of functional oleogels for premium baked goods, emphasizing the control of emulsifier‐guided fat crystallization in food structures. © 2026 Society of Chemical Industry.

INTRODUCTION

Excess body weight and obesity have become increasingly common around the world, leading to greater consumer interest in healthy diets. Fat forms an important part of the daily diet, not only contributing to the texture and flavor of food but also significantly affecting health. Among different fat sources, shortening is widely used as it gives the food a delicate, fluffy texture and unique flavor. However, the hydrogenation process used in its production can reduce the plasticity of the shortening, as well as leading to the formation of trans fatty acids, which pose potential health risks. ¹ Oleogels are a promising alternative to shortening, contributing to both food plasticity and nutritional value. ² The plasticity of an oleogel depends on the structure of its internal network, which derives from the self‐assembly and ordered aggregation of gelling factors in a specific environment. ³ Current strategies used to modulate this structure mainly involve the precise selection of gelling factors, controlling the kinetics of crystallization, and optimization of external environmental conditions. ³ For example, maintaining a beeswax to β‐phytosterol concentration of 150 g kg⁻¹ in an oleogel was found to result in both the densest network structure and maximum hardness (582.62 N) of the oleogel. ⁴ Control of nucleation, typically performed by the addition of templating agents such as lard to direct crystal growth, is a pivotal process in this context. ⁵ However, this approach ignores a key factor, namely the interfacial interaction effect.

Crystallization in oleogels involves complex interfacial interactions that are key contributors to both crystal formation and stability. ³ For instance, the low interfacial energy of rapeseed oil enhances gelling crystallization. ⁶ Similarly, rice bran wax (RBW) tends to aggregate and crystallize at the interface to form β‐crystalline networks. ⁵ A slower cooling rate promotes the adsorption of larger ceramide crystals on the oil droplets, resulting in stronger networks. ⁷ Heating has been found to enhance the structure and oil–water interfacial adsorption of soybean protein isolates, thereby promoting emulsion formation and the quality of the oleogel. ⁸ Moreover, the use of 50 g kg⁻¹ whey protein microgels was reported to be more effective in reducing interfacial tension and enhancing colloidal stability than 100 g kg⁻¹ microgels. ⁹ Therefore, reduction of the internal interfacial energy between the components of an oleogel will promote the formation of highly ordered crystals at the interface. ⁷ Although these findings reveal the reasons underlying the formation of different crystal networks in oleogels, few studies have investigated the effects of varying concentrations and polymerization of polyglycerol fatty acid esters (PGFEs) on the internal network structure of oleogels from the perspective of interfacial adsorption. As non‐ionic emulsifiers, PGFEs can arrange themselves rapidly at the oil–water interface, thereby inhibiting droplet coalescence and Ostwald maturation to enhance emulsion stability, and also have flexible structural properties for customized applications. ¹⁰ Therefore, we speculated that the introduction of PGFEs with varying hydrophilic–lipophilic balance (HLB) values into an oleogel system may elucidate the effects of these PGFEs on interfacial adsorption and subsequent oleogel crystallization.

Therefore, a novel RBW–lard composite oleogel (RLCO) was prepared using rapeseed oil as the substrate, based on the crystallization characteristics of the RBW and the guiding effect of the lard template. The fatty acid composition of RLCO was systematically studied, and its fatty acid composition was studied. Three PGFE concentrations, 1, 5, and 10 g kg⁻¹, were selected to systematically examine the interfacial behavior in unsaturated, near‐saturated, and supersaturated states and the resultant effects on the RLCO crystallization network. PGFE polymers ranging from low (PG‐2 MS, HLB = 5.5) to high (PG‐10 MS, HLB = 12.5) degrees of polymerization were used to investigate the influence of hydrophilic chain length on interfacial activity and crystal regulation. In addition, the performance of the optimized RLCO as a fat substitute was evaluated in a model bread system. The study provides a prediction framework linking the interfacial properties of emulsifiers with oleogel properties, and it is expected that the findings will enhance the utilization efficiency of oleogels and promote their widespread application in the development of baked products.

MATERIALS AND METHODS

Materials

RBW was obtained from Hono Wax Products Dealership Co. Ltd (Dongguan, China), while PGFEs were supplied by Jinsheng New Material Technology Co. Ltd (Binzhou, China) and included polyglyceryl‐2 monostearate (PG‐2 MS), polyglyceryl‐3 monostearate (PG‐3 MS), polyglyceryl‐6 monostearate (PG‐6 MS) and polyglyceryl‐10 monostearate (PG‐10 MS). Rapeseed oil was provided by Yihai Kerry Arawana Holdings Co. Ltd (Shanghai, China), while lard was sourced from Jiangxi Dayouda Food Technology Co. Ltd (Ganzhou, China).

Determination of basic properties of oleogel components

The iodine value, ¹¹ acid value, ¹² and melting point ¹² of the RLCO components were determined according to a previously described method, with the results presented in Table S1. In this case, the ingredient content of RBW and the HLB value of the emulsifier were provided by Hono Wax Products Dealership Co. Ltd (Dongguan, China) and Jinsheng New Material Technology Co. Ltd (Binzhou, China), with the results presented in Tables S2 and S3.

Fatty acid composition of components of RLCO

Fatty acid methyl esters (FAMEs) were prepared using the AOCS method, ¹³ followed by analysis with a Shimadzu Nexis GC‐2030 gas chromatograph (Japan) equipped with a TR‐FAME column (60 m × 0.25 mm × 0.25 μm). The oven temperature program was set at 130 °C for 3 min, increased to 200 °C at a rate of 5 °C min⁻¹, held for 10 min, increased to 220 °C at a rate of 2 °C min⁻¹, and held for 3 min. The inlet and detector temperatures were 250 and 280 °C, respectively. The split ratio was 100, the injection volume was 1 μL, and the carrier gas (N₂ ≥ 99.999%) flow rate was 1.8 mL min⁻¹. FAMEs were identified by comparison of their retention times with a 37‐component FAME standard (Sigma‐Aldrich, USA). Quantification was performed using methyl heptadecanoate (Avanti Polar Lipids, USA) as the internal standard.

Characterization of interfacial properties of PGFE

Evaluation of interfacial tension

Dynamic interfacial tension (γ) was measured using an OSA100 surface analyzer (Ningbo New Boundary Scientific Instrument Co. Ltd, China) with a pendant drop bubble technique, as previously reported. ¹⁴ The control sample was imaged after 2000 s at 20 °C using a CCD camera after the formation of a 10 μL droplet at the needle tip. The dynamic interfacial tension was calculated from the image using the Young–Laplace equation.

Determination of interfacial adsorption kinetics

The interfacial tension was converted to interfacial pressure (π) using Eqn (1):

$$\pi ={\gamma }_{\mathrm{o}}-\gamma$$ (1)

where γ _o = 32.16 ± 0.4 mN m⁻¹, corresponding to the constant interfacial tension of ultrapure water–purified rapeseed oil.

The adsorption rate (K _p, mN m⁻¹ s^−0.5) of the emulsifier was calculated using Eqn (2):

$$\pi =2C_0\mathit{KT}{\left(\mathit{Dt}/3.14\right)}^{1/2}$$ (2)

where C ₀, K, T, D, and t represent the emulsifier concentration (g mL⁻¹), Boltzmann constant, temperature (°C), diffusion coefficient, and adsorption time of the sample solution (s), respectively.

In addition, the penetration and rearrangement rates (K _d, s⁻¹ and K _r, s⁻¹) of emulsifier molecules at the water–oil interface were calculated using the following formula:

$$\ln \left[\left({\pi }_{2400}-{\pi }_{\mathrm{t}}\right)/\left({\pi }_{2400}-{\pi }_0\right)\right]=-K_{\mathrm{i}}t$$ (3)

where π ₂₄₀₀, π _t, and π ₀ are the interface pressures at 2400 s, at time t s, and at 0 s, respectively, and K _i is the first‐order rate constant.

Determination of interfacial dilatational properties

Interfacial dilatational properties were determined according to previous methods, with some modifications. ¹⁵ In the linear region, samples were subjected to a periodic sinusoidal oscillation using an OSA100 surface analyzer (OSA100, Ningbo New Boundary Scientific Instrument Co. Ltd, China). In this case, the droplet volume, oscillation frequency, amplitude, and test duration were set to 10 μL, 0.1 Hz, 1.34 × 10⁻⁴ m (10%), and 2400 s, respectively. The interfacial expansion modulus (E) was then calculated using Eqns (4) and (5):

$$E=\mathrm{d}\gamma /\left(\mathrm{d}A/A\right)=\mathrm{d}\pi /\mathrm{d}\ln A$$ (4)

$$\pi =\left[{\gamma }_0/\left(A_{\mathrm{a}}/A_0\right)\right]\left(\cos \theta +\sin \theta \right)$$ (5)

where γ ₀ and A ₀ are the interfacial tension and surface area at equilibrium, A _a is the expansion stress, and θ is the phase angle between the expansion stress and strain. The interfacial expansion modulus (E) consists of the interfacial elastic modulus (E _d) as well as the interfacial viscosity modulus (E _v), which can also be expressed as:

$$E=E_{\mathrm{d}}+{\mathit{iE}}_{\mathrm{v}}$$ (6)

Oleogel preparation

To a weighed mass of plant‐based oil in a round‐bottom flask, specific proportions of the gelling agent and emulsifier (based on total mass) were added. The mixture was blended with the lard and heated at 90 °C in an oil bath (XMTD‐702A, Hunan Qianyan Technology Co. Ltd, China), followed by homogenization using a high‐shear homogenizer at 1034 × g for 10 min, yielding a transparent, homogeneous solution. The solution was then cooled to 20 °C at a rate of 5 °C min⁻¹ and equilibrated at room temperature for 24 h before use. The specific formula is shown in Table 1.

Table 1.

Oleogel at different ratios

	Rapeseed oil (g kg−1)	Lard (g kg−1)	Gelling agent, RBW (g kg−1)	Emulsifier, PGFE (g kg−1)
1	520	400	79	1
2	520	400	75	5
3	520	400	70	10
4	520	400	80

Determination of oleogel functionality

Determination of hardness

Hardness was determined using a texture analyzer (TA.XT Plus, SMS, UK) as previously described. ¹⁶ Specifically, a P/0.5 probe was used under the following test conditions: a pre‐test speed of 1 mm s⁻¹, a test speed of 1 mm s⁻¹, a post‐test speed of 1 mm s⁻¹, a compression depth of 1 cm, and a trigger force of 0.1 g. Each sample was tested three times and the average value was reported.

Rheological properties

Rheological properties were assessed according to previous methods, with appropriate modifications. ¹⁷ Notably, a rotational rheometer (Discovery DHR‐1, TA Instruments‐Waters LLC, USA) was used to determine the viscoelastic behavior of the oleogels. The linear viscoelastic region was identified by prior strain scanning, with the constant strain determined to be 1 × 10⁻⁴. Frequency sweep tests were performed at 25 °C and in the frequency range 1–100 Hz. The temperature scan was performed at a fixed scan frequency of 1 Hz, a constant strain of 1 × 10⁻⁴, and a heating rate of 2 °C min⁻¹ from 25 to 80 °C.

Oil binding capacity (OBC)

OBC was measured as previously described, with some modifications. ¹⁸ After weighing, 5 g of each orange gum sample was transferred to a centrifuge tube. To ensure complete precipitation of the liquid oil, centrifugation was performed at 11 504 × g for 15 min at 20 °C (PK‐16A, Hunan Pingke Scientific Instrument Co. Ltd, China). The tubes were then inverted on filter paper for 12 h to enable complete drainage of the free oil, after which each sample was tested three times to obtain the average value. OBC was eventually calculated as follows:

$$\mathrm{OBC}\left(\%\right)=\frac{\left(b-c\right)}{\left(b-a\right)}\times 100$$ (7)

where a is the mass of the empty centrifuge tube, b is the mass of the centrifuge tube and oleogel sample before centrifugation, and c is the total mass of the centrifuge tube and oleogel sample after centrifugation.

However, this method has certain limitations, as high centrifugal force may partially disrupt the gel network, resulting in an OBC value that is slightly lower than the true oil‐holding capacity. Nevertheless, under strictly controlled experimental conditions, it was found to yield reliable comparative data for sample‐to‐sample analysis in this study.

Determination of oleogel structure

Differential scanning calorimetry (DSC)

Thermal properties of the samples were determined using a DSC instrument (DSC 214 Polyma, Netzsch, Germany) as previously reported. ¹⁹ Approximately 10–12 mg of each sample was sealed in an aluminium pan and heated from −20 to 55 °C at a heating rate of 5 °C min⁻¹. This temperature was held for 3 min, after which it was lowered from 55 to −20 °C at a cooling rate of 5 °C min⁻¹.

Fourier transform infrared (FTIR) spectroscopy

The FTIR spectra of RLCO were recorded using the potassium bromide pellet method (Nigolis Instruments, Madison, USA). ²⁰ Specifically, the structural characteristics of rapeseed oil, PGFE, gelling agent and oleogel samples were analyzed at a resolution of 2 cm⁻¹ over a spectral range of 500–4000 cm⁻¹. The experimental data were eventually processed using OMNIC8 (Nigolis Instruments, Madison, USA) and PeakFit v4.12 (SeaSolve Software Inc., USA).

X‐ray diffraction (XRD) analysis

XRD analysis was performed according to previously reported methods, with some modifications. ²¹ Measurements were recorded using a MAXima_X XRD‐7000 instrument (Shimadzu Corporation, Japan) under the following test conditions: a Cu‐Kα radiation source (wavelength λ = 1.542 Å), an operating voltage of 40 kV, an operating current of 30 mA, a scanning step length of 0.02°, a scanning rate of 2° min⁻¹, emission and anti‐reflection slits of 1.0 mm, a receiving slit of 0.15 mm, a test temperature of 25 °C, and a 2θ angle scanning range of 5.0°–55.0°. Finally, the relative crystallinity was calculated using the following equation:

$$\text{Relative crystallinity}\left(\%\right)=\begin{array}{l}\text{Area of crystal diffraction peaks}/\\ (\text{Area of crystal diffraction peaks}\\ +\text{Area of amorphous diffusion peaks})\end{array}$$ (8)

Polarized light microscopy (PLM) analysis

The microstructure and crystal morphology of the oleogels prepared with different emulsifiers were visualized at 20 °C using an upright bright‐field microscope (Leica LK‐40P, Wetzlar, Germany) equipped with a custom‐built polarizer; images were acquired using a CCD camera (E3ISPM08300KPC).

Preparation of bread and evaluation of bread properties

Bread making

Bread was prepared according to the American Association of Cereal Chemists method (AACC Method 10‐09) with appropriate modifications. The main processing steps were as follows: high‐gluten flour mixing → addition of ingredients → dough preparation → degassing and shaping → fermentation → baking → cooling → slicing → packaging → final product.

Bread volume

The bread specific volume was determined using the ‘rapeseed displacement method’ and calculated according to the following equation:

$$\text{Bread specific volume}\left(\mathrm{mL}{\mathrm{g}}^{-1}\right)=\text{Bread volume}\left(\mathrm{mL}\right)/\text{mass}\left(\mathrm{g}\right)$$ (9)

Moisture content

The moisture content of bread was measured using the direct drying method. After cooling the bread to room temperature, 2 g was weighed and transferred to a pre‐dried crucible. The samples were then dried to constant weight in a 105 °C drying oven, after which the moisture content was calculated based on the difference in mass before and after drying. Each sample was tested at least three times and the average value was reported.

Texture profile analysis (TPA) of bread

TPA was performed according to a previously reported experimental method. ²² Bread that had been cooled to room temperature was cut into 2 × 2 × 2 cm³ pieces. Analysis was then performed using a TA‐XT2 texture analyzer in TPA mode with a P/36R probe. Double compression was applied and tests were conducted using a pre‐test speed, test speed, and trigger force of 1 mm s⁻¹, 1 mm s⁻¹, and 5 g, respectively, along with a compression length of 1 cm. Each sample was measured at least three times, and abnormal data were removed before calculating the mean.

Statistical analysis

Significant differences (P < 0.05) were analyzed using analysis of variance (ANOVA) in SPSS 22.0 (IBM Corp., Armonk, NY, USA), followed by Duncan's multiple range test for multiple comparisons, which is well suited for detecting ordered trends among treatment groups with moderate sample sizes. Normality (Shapiro–Wilk test) and homogeneity of variance were confirmed before ANOVA application using Shapiro–Wilk and Levene's tests, respectively. Mapping and Pearson correlation analyses were also performed using Origin 2022 (OriginLab Corporation, Northampton, USA).

RESULTS AND DISCUSSION

Analysis of fatty acid composition

The fatty acid composition data are presented in Table 2. Notably, oleic acid (C18:1) and linoleic acid (C18:2), representing the two predominant fatty acids in rapeseed oil, collectively accounted for over 800 g kg⁻¹ of the total fatty acids. In contrast, lard exhibited a broader fatty acid profile, which was dominated by palmitic (C16:0), palmitoleic (C16:1), stearic (C18:0), and oleic (C18:1) acids. Its relatively high content of medium‐ and long‐chain saturated fatty acids, particularly C16:0 and C18:0, favors the formation of the thermodynamically more stable β′ or β crystals. ²³ The carbon number distribution of fatty acids in RBW was found to be C16–C30, composed predominantly of C22:0 and C24:0; the linear structure of this saturated fatty acid enables tight accumulation, promoting the formation of β crystals. ²⁴

Table 2.

Fatty acid composition of different oils

	Rapeseed oil (g kg−1)	Lard (g kg−1)	RBW (g kg−1)
C10:0	—	102.2	—
C12:0	—	9.0	—
C14:0	—	10.9	—
C16:0	45.2	141.9	42.3
C16:1	—	32.2	—
C18:0	20.5	154.0	78.7
C18:1	648.6	322.2	—
C18:2	186.6	193.2	—
C18: 3N3	6.4	3.6	—
C18: 3N6	75.7	0.4	—
C20:0	7.6	2.8	112.5
C20:1	9.4	15.2	—
C20:4	—	3.5	—
C22:0	—	9.0	199.2
C24:0	—	—	280.1
C26:0	—	—	129.1
C28:0	—	—	42.2
C30:0	—	—	27.3

Evaluation of PGFE interfacial properties

In oleogels, a uniform dispersion of the liquid phase typically depends on the interfacial characteristics of the emulsifier. ²⁵ Emulsifiers can regulate the distribution of oil droplets in oleogels by adjusting the microstructure of the interface, which in turn affects the structural strength of the oleogel. ²⁶ Therefore, investigating the interfacial properties of PGFEs at the water–oil interface is essential for constructing a uniform gel network.

Interfacial tension of PGFEs

Changes in interfacial tension reflect the emulsifying capacity of PGFEs at the oil–water interface, with Fig. 1 showing variations in such tension over time. ²⁷ For all PGFE samples, the interfacial tension initially decreased rapidly. This was followed by a gradual decline until equilibrium was reached. Overall, this behavior suggested that the adsorption process consisted of three distinct stages, namely induction, rapid decline and equilibrium, which were consistent with the behavior reported for β‐lactoglobulin. ²⁸

Figure 1.

Interfacial tension diagrams of different PGFE concentrations.

Although all samples exhibited a similar trend, changes in interfacial tension were different across PGFEs of different concentrations. Specifically, compared with low‐concentration PGFEs, those with concentrations just above their critical micelle concentration (CMC) (Table 3) exhibited lower interfacial tension and a higher rate of decline of interfacial tension. However, at concentrations significantly exceeding the CMC, the interfacial tension of PGFEs increased. These results suggested that when the PGFE concentration is near the CMC, the interface is nearly saturated with PGFE molecules, hence minimizing oil–water interactions. ²⁹ Conversely, at concentrations exceeding the CMC, the rapid and excessive adsorption of PGFE molecules at the interface hinders structural rearrangement, thus resulting in a slight increase in interfacial tension. This was consistent with the molecular arrangement mechanism proposed by Rout et al. ²⁷

Table 3.

Critical micelle concentrations for different PGFEs

Sample	Critical micelle concentration (g kg−1)
PG‐2 MS	1.22
PG‐3 MS	1.87
PG‐6 MS	2.42
PG‐10 MS	3.68

Further analysis showed that, at similar concentrations, PG‐10 MS exhibited the highest interfacial tension‐reducing capacity, followed by PG‐6 MS, PG‐3 MS, and PG‐2 MS. This could be attributed to its long hydrophilic chain, which occupies a larger area at the system interface, thereby enabling stronger anchoring and a tighter arrangement of PGFE molecules. ³⁰ Such a compact packing can more effectively shield oil–water interactions, leading to a significant reduction in interfacial tension. ³¹ Therefore, it could be inferred that 5 g kg⁻¹ PG‐10 MS likely possessed the strongest dynamic adsorption capacity along with the most favorable emulsification performance at the lard–rapeseed oil interface.

Analysis of interfacial adsorption kinetics

Interfacial tension reflects the mechanical properties of an emulsifier at the oil–water interface, while adsorption kinetics describes the rate at which the emulsifier molecules migrate from the host phase to the interface. In this case, the parameter K _diff represents the diffusion of the emulsifier molecules from the solution to the interface, while K _p and K _r reflect their subsequent adsorption and molecular rearrangement at the interface, respectively. Table 4 summarizes the adsorption kinetic parameters of the different PGFEs at the oil–water interface. Overall, the results showed that, as the PGFE concentration increased, the K _diff value initially rose and then declined. At lower concentrations, the increase in K _diff could be attributed to a larger concentration gradient that enhanced the driving force for PGFE molecules to diffuse towards the interface. ³² However, at higher concentrations, steric hindrance and molecular crowding at the interface likely limited the insertion of additional molecules, thereby reducing K _diff. ³³ Subsequent analyses revealed that, among all PGFEs, 5 g kg⁻¹ PG‐10 MS had the highest adsorption kinetics, along with diffusion and rearrangement rates of 2.33 mN m⁻¹ s^−0.5 and 5.06 s⁻¹, respectively. These results, which were consistent with the observed reduction in interfacial tension (Fig. 1(D)), could be attributed to the higher solubility of PG‐10 MS in aqueous solution as this resulted in lower diffusion resistance and subsequently in quicker migration towards the interface. ³² Similar observations have been reported by Zhang et al., who found that the length of the hydrophobic chain of polyglycerol ester emulsifiers significantly influenced their interfacial adsorption behavior and, consequently, their emulsification performance. ³⁴

Table 4.

Adsorption kinetic parameters of PGFE at different concentrations at the oil–water interface

Type of emulsifier	Concentration (%, w/w)	K diff (mN m−1 s−0.5)	K p (× 103 s−1)	K r (× 103 s−1)
PG‐2 MS	0.1	0.25 ± 0.01i	0.98 ± 0.06c	2.87 ± 0.05e
	0.5	1.07 ± 0.00e	0.75 ± 0.04d	3.32 ± 0.06d
	1.0	0.85 ± 0.06f	0.64 ± 0.04e	3.25 ± 0.05d
PG‐3 MS	0.1	0.46 ± 0.04fg	1.47 ± 0.06a	2.42 ± 0.05g
	0.5	1.45 ± 0.04bc	0.29 ± 0.06f	4.50 ± 0.03b
	1.0	0.65 ± 0.03g	0.23 ± 0.04f	3.27 ± 0.07d
PG‐6 MS	0.1	0.55 ± 0.06h	1.41 ± 0.06a	2.13 ± 0.06h
	0.5	1.52 ± 0.03b	1.24 ± 0.05b	3.91 ± 0.05c
	1.0	1.30 ± 0.05d	1.05 ± 0.04c	2.42 ± 0.06g
PG‐10 MS	0.1	0.58 ± 0.05gh	1.27 ± 0.06b	2.70 ± 0.09f
	0.5	2.33 ± 0.04a	0.98 ± 0.06c	5.10 ± 0.06a
	1.0	1.38 ± 0.04cd	0.68 ± 0.06de	0.80 ± 0.05i

Note: Different superscript letters in the same column denote significant differences (p < 0.05). Same letters indicate no significant difference.

It was also observed that the K _p values of all samples were greater than their corresponding K _r, hence indicating that PGFE molecular rearrangement was the rate‐determining step and played a dominant role in driving the formation of interfacial layers. ³² In addition, increasing PGFE concentrations led to higher K _p values, possibly due to rapid PGFE adsorption at the interface and the formation of a compact, ordered structure through hydrogen bonding, thus facilitating faster molecular rearrangement. ³⁵ Collectively, these findings indicated that the diffusion and rearrangement of PGFE molecules reached maximum efficiency near the CMC, resulting in a rapid and substantial reduction in interfacial tension.

Evaluation of interfacial dilatational properties

Dilatational rheology is a key parameter for evaluating the film‐forming performance of emulsifiers as it provides insight into the viscoelasticity and deformation resistance of the interfacial layer. ³⁶ Figure 2 shows the time‐dependent changes in the dilatational modulus (E), elastic modulus (E _d) and viscous modulus (E _v) for the different PGFEs. Overall, for all samples, the moduli gradually increased over time due to the continuous adsorption of PGFE molecules at the interface. In particular, E _d was much higher than E _v and closer to E, hence suggesting that PGFE adsorption formed an elastic network film with strong deformation resistance. ³⁷ At low concentrations, partial coverage of PGFE molecules at the interface limited the interfacial film's rigidity which then increased at higher concentrations. ³⁸ However, excessive emulsifier concentrations promoted overcrowding, and the resulting repulsion between molecules made the interfacial layer more susceptible to rupture or rearrangement under deformation, thus reducing the dilatational modulus. ³⁹ Similar findings were previously reported for Tween 80, where the E value decreased with increasing concentration. ⁴⁰

Figure 2.

Expansion rheological plots of PGFE at different concentrations: (A) composite modulus E; (B) elastic modulus E _d; (C) viscous modulus E _v.

Although the different samples exhibited the same trend, at similar concentrations, PG‐3 MS consistently showed the highest E values, followed by PG‐2 MS, PG‐6 MS, and PG‐10 MS. This superior interfacial stability of PG‐3 MS could be attributed to its lower hydrophilicity, which enhanced anchoring with the oil phase via hydrophobic and van der Waals interactions, thereby facilitating optimal orientation of its hydrophilic moiety towards the aqueous phase. ⁴¹ This arrangement, in turn, enabled efficient intermolecular interactions to yield a stronger and more elastic interfacial film. ⁴²

Evaluation of functional properties of oleogels

Hardness analysis

The hardness of an oleogel is a key indicator of its structural integrity and mechanical stability, both of which are crucial in baking applications where moderate firmness ensures ideal spreadability and workability. ⁴³ As shown in Fig. 3, the addition of PGFEs significantly reduced the hardness of the oleogels in a concentration‐dependent manner. In particular, incorporating 10 g kg⁻¹ of PG‐2 MS yielded an oleogel with the lowest hardness. This finding, which was consistent with the observed decrease in water–oil interfacial tension (Fig. 1), suggested that PGFEs likely promoted a uniform distribution of lard droplets in RLCO, thereby modifying the gel network structure. ⁴⁴

Figure 3.

Comparison of hardness of RLCO with different contents of PGFE.

Analysis of rheological properties

The viscoelastic modulus serves as a key rheological parameter for evaluating oleogel hardness, and it provides insights into the structural stability of the gel network as well as the latter's ability to withstand external stresses, both of which are essential indicators for baking and related processing. ⁴⁴ As shown in Fig. 4, none of the prepared oleogels exhibited a gel–sol transition across the frequency range tested, thus indicating strong mechanical stability and resistance to deformation. ⁴⁵ Furthermore, the viscoelasticity of the oleogels reflected a concentration‐dependent effect of the PGFEs. Notably, compared with the RLCO control, the addition of 1 and 5 g kg⁻¹ PGFE increased the viscoelastic modulus (G′), with the 5 g kg⁻¹ formulation exhibiting the highest G′. These findings aligned with those of Wang et al., who found that a 30 g kg⁻¹ hemp seed protein concentration could minimize emulsion particle size and improve the viscoelasticity of emulsion gels. ⁴⁶ Conversely, increasing the PGFE concentration to 10 g kg⁻¹ reduced the storage modulus. This was likely attributable to disruption of the structural rearrangement at the lard–rapeseed oil interface by excessive PGFE, thereby compromising the integrity of the interfacial membrane and weakening the overall oleogel network. ⁴⁷

Figure 4.

Effects of different ratios of emulsifier and gelling agent on the composite modulus of oleogel during frequency scanning (G′, modulus of elasticity; G″, modulus of viscosity).

Moreover, at equivalent concentrations, different PGFEs enhanced the viscoelastic properties to differing extents: PG‐10 MS yielded the highest G′, followed by PG‐6 MS, PG‐3 MS, and PG‐2 MS. This trend reflects the stronger and more compact interfacial ‘anchoring’ conferred by PG‐10 MS, which was more effective in reinforcing the architecture of the oleogel network.

Temperature sweep measurements further elucidated the thermal stability of the oleogel modulus. As shown in Fig. 5, G′ of RLCO decreased gradually at lower temperatures, followed by a sharp drop upon further heating, indicating a distinct transition in phase from gel to liquid. The unmodified RLCO exhibited the highest transition temperature, suggesting stabilization of its network by more persistent (as opposed to transient) molecular interactions, resulting in superior thermal stability. The transition temperatures were observed to decrease in the order of 10 > 5 > 1 g kg⁻¹ PGFE, confirming that PGFE incorporation systematically modulates the thermal behavior of the oleogel. ⁴⁸ At equivalent concentrations, PG‐10 MS again led to the highest G′ value, followed sequentially by PG‐6 MS, PG‐3 MS, and PG‐2 MS, implying that its multi‐site polymerization enhances the mechanical strength of RLCO, likely through reinforcement of the crystalline microstructure. ⁴⁹

Figure 5.

Effects of different ratios of emulsifier and gel on the composite modulus of oleogel in temperature scanning (G′, modulus of elasticity; G″, modulus of viscosity).

OBC analysis

While high viscoelasticity reflects an oleogel's resistance to strain, the OBC directly indicates its ability to bind liquid oil and maintain structural stability during baking. ⁵⁰ Figure 6 shows the effects of PGFE concentration on the OBC of the oleogels. Overall, the incorporation of PGFE increased the OBC, possibly due to a more uniform dispersion of lard droplets at high temperatures, resulting in a more stable oleogel structure. ⁵¹ Specifically, the highest OBC was observed at 5 g kg⁻¹ PGFE, hence indicating optimal oleogel stability at this concentration. However, further increases in the amount of PGFE reduced the OBC. This could be attributed to the excessive emulsifier molecules potentially dispersed within the continuous phase that disrupted the mechanical strength of the oleogel and reduced its oil‐binding performance. ⁵¹ Consistent with these results, Blount et al. also reported that the excessive addition of emulsifiers resulted in larger gaps between oleogel crystals and a weaker three‐dimensional network that decreased the OBC. ⁵² Collectively, these observed OBC trends aligned well with the results for interfacial tension shown in Fig. 1, hence indicating that the interfacial adsorption dynamics of PGFE plays a key role in determining the final OBC of the oleogel.

Figure 6.

Oil holding capacity of RLCO with different contents of PGFE.

Evaluation of oleogel structure

DSC analysis

Although OBC provides a macroscopic measure of an oleogel's stability, thermal analysis via DSC is essential for understanding temperature sensitivity and structure–function relationships. ⁴² Figure 7(A) shows the enthalpy changes of different samples during cooling. Overall, the samples exhibited a major exothermic peak at around 5 °C and a smaller shoulder peak at around 20 °C, hence indicating that crystallization occurred twice. ⁵³ Further analysis showed that the exothermic peak near 5 °C was steeper for oleogels containing 5 g kg⁻¹ PGFE, with the most significant effect observed for the 5 g kg⁻¹ PG‐10 MS sample. These results suggested a more uniform and stable crystal morphology within the oleogel network. ⁵⁴

Figure 7.

DSC traces (A), FTIR spectra (B, C), and XRD patterns (D) for the crystallization process of RLCO with different types and dosages of PGFE.

The above findings only provided an overview of the crystallization process but they lacked quantitative precision. Therefore, key DSC parameters related to crystallization were also calculated and are summarized in Table 5. Overall, the crystallization enthalpy of all PGFE‐containing samples was consistently lower than that of the control, hence confirming that the addition of PGFE altered oleogel crystallization. ⁵⁵ Interestingly, the enthalpy value also decreased gradually with increasing PGFE concentration. This could be attributed to the adsorption of PGFE molecules at the lard droplet interface, which influenced the reorganization of the RBW network, thereby modifying crystallization energetics. ⁴² Similar findings have been reported by Chen et al., who observed that soybean protein–ester emulsifiers could alter the crystal structure of oleogel due to differences in molecular crosslinking interactions. ⁵⁶

Table 5.

Thermal property parameters of the main endothermic and exothermic peaks of samples and control group during melting and crystallization

Sample	Crystallization process
	T on (°C)	T p (°C)	T off (°C)	ΔH c (J g−1)
PG‐2 MS (0.1%)	6.93	4.27	0.57	−9.45
PG‐3 MS (0.1%)	7.32	4.69	0.71	−8.85
PG‐6 MS (0.1%)	7.28	4.74	0.59	−9.52
PG‐10 MS (0.1%)	7.34	4.71	0.75	−9.04
PG‐2 MS (0.5%)	6.84	4.26	0.34	−9.04
PG‐3 MS (0.5%)	6.39	3.99	0.46	−8.32
PG‐6 MS (0.5%)	6.69	4.31	0.63	−8.80
PG‐10 MS (0.5%)	7.18	4.72	1.34	−8.62
PG‐2 MS (1%)	6.59	4.31	0.66	−8.03
PG‐3 MS (1%)	6.62	4.49	1.27	−8.20
PG‐6 MS (1%)	6.44	4.12	0.59	−7.67
PG‐10 MS (1%)	6.51	4.21	0.72	−8.06
RLCO	7.87	5.12	0.85	−9.83

T _on, starting temperature; T _off, end temperature; T _p, peak temperature; ΔH _m, melting enthalpy change; ΔH _c, crystallization enthalpy change.

FTIR analysis

FTIR spectroscopy was used to elucidate the molecular interactions underlying the thermodynamic properties of the oleogels, with Fig. 7(B),(C) showing the FTIR spectra of the oleogel samples and their individual components. ⁵⁶ Gradual spectral changes were observed with increasing PGFE concentrations. Specifically, the oleogel peak at 2924.7 cm⁻¹ became sharper and shifted to a lower frequency (red shift), with the most significant change noted for 5 g kg⁻¹ PG‐10 MS. Furthermore, the peak in the 2925 cm⁻¹ region corresponded to the tensile vibrations of CH₃ and CH₂, and the observed changes in the peak suggest the presence of increased van der Waals interactions, thereby maintaining structural stability. ⁵⁷ Similar to the trend observed at 2924.7 cm⁻¹, the intensity of carbonyl (C=O) stretching vibration peaks at 1700 cm⁻¹ increased significantly with the addition of PGFE. This enhanced peak intensity suggested that more C=O groups may have adopted an ordered conformation, typically associated with a tight crystalline packing driven by intermolecular forces such as hydrogen bonding and van der Waals interactions. ⁵¹ However, it is important to point out that changes in FTIR peak position and intensity alone do not provide a direct determination of the presence of hydrogen bonds or ordered conformations, which may also be affected by other factors.

Interestingly, the absorption peak at around 3480 cm⁻¹ was strongest for PG‐10 MS, followed by PG‐6 MS, PG‐3 MS, and PG‐2 MS. Based on this result, a possible mechanism can be proposed: the strength of hydrogen bonds within PGFE determines its adsorption efficiency at the lard–rapeseed oil interface. This reduces interfacial tension, producing smaller and more uniformly dispersed oil droplets. Such improved dispersion eventually promotes stronger van der Waals interactions, resulting in a denser and more uniform network structure. ⁵⁸ However, confirmation of this mechanism requires additional verification using other molecular spectroscopic techniques, such as Raman spectroscopy or nuclear magnetic resonance.

XRD analysis

XRD analysis provides insight into how the molecular interactions identified from FTIR spectra translate into the crystalline structure of oleogels. ⁵⁹ As shown in Fig. 7(D), all oleogel samples displayed characteristic diffraction peaks at d = 4.12 Å and d = 3.72 Å in the wide‐angle region, hence indicating a β′‐crystal structure. This finding suggested a common molecular packing pattern within the oleogel network, with differences mainly occurring in the relative abundance of the β′ form. ⁶⁰ Furthermore, with increasing PGFE concentration, the diffraction peak intensity initially increased and then decreased. Since peak intensity and sharpness directly reflect a crystal's integrity and content, the observed nonlinear trend demonstrated PGFE's ability to influence the crystallization process of RLCO. ² Therefore, this non‐monotonic change clearly demonstrates the ability of PGFE to regulate the crystallization process of RLCO. These structural changes were highly consistent with the reduction in interfacial tension (Fig. 1), thus establishing a clear relationship between PGFE interfacial activity and RLCO crystallization behavior.

The relative crystallinity of the different samples was further analyzed (Table 6). The results indicated that, at the same concentration, the PG‐10 MS formulation showed the highest relative crystallinity, followed by PG‐6 MS, PG‐3 MS, and PG‐2 MS. The oleogel containing 5 g kg⁻¹ PG‐10 MS exhibited the strongest relative crystallinity (31.73 ± 0.51^a%), suggesting that it possessed the highest content of thermodynamically stable β′ crystals. This uniform crystallization behavior could be attributed to the rapid adsorption capacity of 5 g kg⁻¹ PG‐10 MS at the lard–rapeseed oil interface, resulting in smaller and more uniformly dispersed oil droplets. This, in turn, promoted epitaxial growth of RLCO crystals to form a highly ordered crystal structure with fewer structural defects.

Table 6.

Comparison of relative crystallinity of RLCO with different PGFEs

Sample	Relative crystallinity (%)
PG‐2 MS (1.0 g kg−1)	22.89 ± 0.32g
PG‐3 MS (1.0 g kg−1)	23.75 ± 0.15f
PG‐6 MS (1.0 g kg−1)	25.14 ± 0.16e
PG‐10 MS (1.0 g kg−1)	25.97 ± 0.34d
PG‐2 MS (5.0 g kg−1)	27.18 ± 0.47c
PG‐3 MS (5.0 g kg−1)	27.77 ± 0.33c
PG‐6 MS (5.0 g kg−1)	29.22 ± 0.23b
PG‐10 MS (5.0 g kg−1)	31.73 ± 0.51a
PG‐2 MS (10.0 g kg−1)	21.58 ± 0.23h
PG‐3 MS (10.0 g kg−1)	19.15 ± 0.13j
PG‐6 MS (10.0 g kg−1)	17.58 ± 0.07k
PG‐10 MS (10.0 g kg−1)	17.36 ± 0.67k
RLCO	20.43 ± 0.32i

Note: Different superscript letters in the same column denote significant differences (p < 0.05). Same letters indicate no significant difference.

Microscopic analysis

The microstructure of the oleogel crystal can be evaluated visually using PLM. As shown in Fig. 8, the crystal morphology of RLCO in the control group was predominantly in the form of irregular sheets, accompanied by a small number of rod‐like crystals. It has been previously reported that β‐crystal forms are typically seen as large flakes or spherical crystals under PLM, while the β′‐crystal form appears as fine needle‐like or rod‐like crystals.61, 62 Therefore, the control group was dominated by flake‐like crystals with fewer rod‐like crystals, indicating a higher proportion of the β‐crystal form, accompanied by a low proportion of the β′‐crystal form. After the addition of PGFE, the proportion of flake‐like crystals decreased while that of the needle‐/rod‐like crystals increased significantly. In particular, a finely intertwined needle‐like network structure was found in the 5 g kg⁻¹ PG‐10 MS sample, which was highly consistent with the increase in the β′‐crystal form content inferred from the XRD results (Table 6). Further analysis of samples with different degrees of PGFE polymerization at the same concentration indicated that PG‐10 MS had needle‐like crystals interwoven into a mesh structure, with similar structures seen in PG‐6 MS, PG‐3 MS, and PG‐2 MS. This change indicates that PGFE inhibits the transition from the β′‐ to the β‐crystal form through interfacial adsorption and promotes the formation of β′ crystals.

Figure 8.

PLM microscopic images of RLCO with different concentrations and types of PGFE.

Bread characterization

Bread volume analysis

Figure 9(A) shows the specific volume of different bread samples. Overall, bread prepared using commercial butter exhibited the lowest specific volume, potentially due to its inadequate network structure that could not effectively capture and retain the carbon dioxide produced during fermentation, thereby limiting dough expansion. ⁶³ In contrast, all PGFE‐modified RLCO samples displayed significantly higher specific volumes, with those containing 5 g kg⁻¹ PGFE showing the greatest improvement. This enhancement was likely due to interfacial tension at the lard–rapeseed oil interface, which enabled the RLCO crystal network to more effectively stabilize the air chamber and promote dough expansion, resulting in improved bread volume.

Figure 9.

Specific volume (A), moisture content (B), hardness (C), and springiness (D) of bread with different oleogels.

Bread moisture content

As shown in Fig. 9(B), bread prepared with commercial shortening exhibited a relatively low moisture content, while those containing PGFE‐modified oleogels had a moisture content of at least 50 g kg⁻¹ higher than the control. In particular, the PG‐10 MS formulation increased the moisture content by 7.07%. This improvement suggested that the interfacial activity of PGFE could promote the formation of a denser three‐dimensional RLCO network structure that acted as a physical barrier against water migration during processing. This, in turn, helped to maintain a more uniform moisture distribution that enhanced the stability of the product's texture. ⁶⁴

Bread texture properties

TPA showed that the addition of PGFE‐modified oleogels significantly improved bread quality (Fig. 9(C),(D)). In addition, all PGFE‐containing samples exhibited significantly reduced hardness, with the greatest softening observed in bread prepared using 5 g kg⁻¹ PGFE. These results could be attributed to the formation of a dense and stable crystal network within the RLCO, with such a structure effectively supporting the dough matrix during baking and reducing structural collapse, thereby resulting in softer bread. ⁶⁵

Measurements of springiness (Fig. 9(D)) further confirmed the above findings. Notably, elasticity initially increased with PGFE addition before declining, hence reflecting the trend observed in the case of hardness. These findings were likely due to the well‐structured RLCO network that enhanced gluten expansion during proofing and baking, resulting in improved bread elasticity. ⁶⁵

Schematic mechanism

Based on the above analysis, we propose that PGFE regulates RLCO through the sequence ‘interface adsorption → crystal network formation → macroscopic performance → application’ (Fig. 10). Specifically, PGFE is adsorbed at the lard–rapeseed oil interface largely via hydrogen bonding, thereby reducing the interfacial tension and facilitating the uniform dispersion of lard droplets. This improved interfacial stability promotes the formation of a denser and more ordered β′‐crystalline network in RBW while effectively suppressing the β′‐to‐β polymorphic transition (Fig. 8; Table 6), ultimately enhancing the mechanical properties of RLCO and the quality of the baked bread. Among all tested samples, PG‐10 MS demonstrated the strongest interfacial adsorption capacity, the highest β′‐crystal content, and the greatest overall crystallinity, resulting in the most pronounced regulatory effect on RLCO functionality. Notably, the presence of hydrogen bonding was inferred primarily from indirect FTIR spectroscopic evidence, and therefore warrants further experimental verification using complementary techniques, such as Raman spectroscopy.

Figure 10.

Schematic representation of the formation mechanism of oleogel stabilized by RLCO.

CONCLUSION

This study showed that the concentration and type of PGFE small‐molecule emulsifiers significantly influenced the behavior of the internal interface, as well as the network structure and mechanical strength of RLCO, thereby altering bread quality. Specifically, PGFE enhances interfacial adsorption through its polyglycerol moieties and modulates the dispersion of lard droplets, thereby promoting the formation of a denser and more ordered β′‐crystalline network in RBW while suppressing its transition to the thermodynamically stable β form, ultimately increasing the elastic modulus of the system. Among all formulations, 5 g kg⁻¹ PG‐10 MS exhibited the most significant enhancement of the RLCO network, increasing the bread moisture content by 7.07% and yielding superior bread quality. This was followed by PG‐6 MS, PG‐3 MS, and PG‐2 MS in descending order of effectiveness. However, excessive PGFE addition can compromise the integrity of the RLCO network, functional characteristics, and final product quality.

Overall, these findings establish a predictive framework linking the interfacial properties of PGFE to the crystallization behavior and bread quality of RLCO. The results indicate the potential of interfacial engineering in the design of custom oleogel systems as fat substitutes in baked goods, thereby moving beyond traditional applications where emulsifiers are used only as structural fillers. However, this study has several limitations. First, FTIR spectroscopy provides only indirect evidence of molecular interactions, and the presence of hydrogen bonding and ordered carbonyl conformations requires verification by more direct techniques, such as Raman spectroscopy and NMR. Second, further investigation is needed to explore the effects of sensory evaluation, shelf‐life analysis, and long‐term storage of bread models on RLCO applications to fully elucidate and optimize emulsifier interfacial properties in industrial grease systems.

AUTHOR CONTRIBUTIONS

Renyu Fan and Zhuangpeng Wang: Conceptualization, formal analysis, resources, writing – original draft. Guihua Ye and Wuhui Dong: Software, validation, resources. Sulin Ouyang: Resources, software, project administration, validation. Kun Wang: Data curation, investigation, supervision. Leiyan Wu: Writing – review and editing, funding acquisition.

CONFLICT OF INTEREST

The authors declare no conflict of interest.

Supporting information

Table S1. Basic properties of oleogel components.

Table S2. Hydrophilic–lipophilic balance (HLB) value of the emulsifier.

Table S3. Rice bran wax (RBW) internal ingredient content.

Data S1.

DATA AVAILABILITY STATEMENT

The data that support the findings of this study are available in the supporting information of this article (Data S1).