Development and characterization of high-performance macadamia oil-based oleogel emulsions

Abstract

Macadamia oil is rich in monounsaturated fatty acids and bioactive compounds, making it highly valued in the food industry. However, its high fluidity and susceptibility to oxidation limit its broader applications. This study utilized macadamia oil as a base material to develop highly stable oleogels incorporating a β-sitosterol/γ-oryzanol (BS-GO) composite system. The gelation threshold was identified at 6 %, while oleogels prepared with 7 % -12 % BS-GO exhibited a dense three-dimensional network structure, transitioning the oil into a solid state. The oleogel at the optimized 12 % BS-GO concentration demonstrated a hardness of 6283.96 g, an oil-binding capacity of 99.51 %, and significantly enhanced antioxidant capacity, with DPPH scavenging activity reaching 1922.18 μmol TE/100 g. Emulsions derived from these oleogels revealed that emulsifier type significantly influenced stability. Emulsions stabilized with whey protein isolate achieved the smallest droplet size (256.86 nm) and the highest zeta potential (−32.93 mV). The 7 % BS-GO emulsion exhibited remarkable stability under thermal treatment (30–80 °C), freeze-thaw cycles, and varying ionic strengths (0–100 mmol/L). Furthermore, this emulsion effectively inhibited oxidation, with peroxide values ranging from 7.07 to 12.67 mmol/kg and TBARS values between 2.83 and 4.96 μmol/kg during accelerated storage. This research provides a theoretical foundation and practical guidance for the high-value utilization of macadamia oil and the advancement of oleogel-based emulsions in the food industry.

Highlights

• •Macadamia oil oleogels enhanced stability and antioxidative properties.
• •Optimized BS-GO concentration improves emulsion performance and stability.
• •WPI achieves superior emulsification and particle size distribution.
• •Oil gels exhibit excellent thermal, freeze-thaw, and ionic strength stability.

1. Introduction

Macadamia (Macadamia integrifolia) oil is a valuable woody plant oil, which unique nutritional value and high content of monounsaturated fatty acids (MUFA) make it an important player in the vegetable oil market (Shuai et al., 2021). In addition to that, it is also rich in minor compounds, such as phytosterol, tocopherols, polyphenols, and squalene, which confer antioxidant, immune-modulating, anti-inflammatory, and cardiovascular protective effects (Shuai, Dai, Chen, Liang, et al., 2022). However, despite the many health benefits of macadamia oil, factors such as its susceptibility to oxidation and instability have been a bottleneck in its application in food industry. These properties make macadamia oil susceptible to oxidative deterioration during storage and processing, making it difficult to meet the food industry's stringent requirements for stability and functionality (Shuai, Dai, Chen, Liu, et al., 2022). In order to give full play to the nutritional value and health functions of macadamia oil, it is necessary to develop more stable and functional oil products through technological innovation and process improvement. This study will address the food industry's requirements while offering consumers healthier and more delicious options, thereby driving the macadamia oil industry's advancement.

Oleogel technology represents an innovative lipid structuring strategy that incorporates gelling agents to form 3D networks, effectively converting liquid oils into solids. This approach enhances both oxidative stability and physical performance (Okuro et al., 2020). β-sitosterol and γ-oryzanol function as natural gelling agents with remarkable self-assembling properties and antioxidative capabilities. Through molecular hydrogen bonding and hydrophobic interactions, these compounds form stable fibrous or nanotubular structures, endowing oleogels with superior mechanical strength and oxidative stability (Wang & Liu, 2023). Despite these advantages, studies investigating β-sitosterol/γ-oryzanol systems in Macadamia oil remain limited (Shuai et al., 2024), particularly concerning their functionality in complex systems such as emulsions. On this basis, oleogel emulsion further expands the application field of oleogel. Formulating oleogels into stable emulsions by blending with an aqueous phase enhances oleogel solubility and dispersibility and confers new functional attributes like emulsification stability, moisturization, and nutrient delivery.

Developing oleogels into oil-in-water (O/W) emulsions can significantly upgrade oil functional properties and broaden their applications across diverse food systems. (Wang et al., 2024). However, current research on oleogel-based emulsifiers is still insufficient. Limited studies have systematically examined the role of oleogel-based emulsifiers or elucidated their optimization mechanisms (Chen et al., 2024; Perez-Santana et al., 2023), which undoubtedly limits the further application and development of oleogel emulsions in the food industry. The selection and optimization of emulsifiers, as the key components connecting the oil and water phases, directly affect the structural stability, mouthfeel and nutrient retention of emulsions (Tao et al., 2025). In addition, the stability of oleogel emulsions under different environmental conditions needs to be addressed. Environmental factors such as temperature, pH, and ionic strength may significantly affect the stability of emulsions. Evaluating the specific effects of these factors on the stability of oleogel emulsions and seeking corresponding stabilization strategies are crucial to ensure that they can maintain good performance in practical applications. The simultaneous preparation of oleogels and oleogel emulsions using natural materials is in line with the concept of green manufacturing and sustainable development. Therefore, through comprehensive evaluation and optimization, oleogel emulsions are expected to become a new generation of functional food ingredients that will bring innovation and change to the food industry.

This study aimed to address these gaps by developing high-performance oleogels from macadamia oil using a BS-GO composite system. The research focused on optimizing oleogel preparation conditions, evaluating their physicochemical and antioxidative properties, and investigating their synergistic effects with emulsifiers on emulsion stability. Additionally, the stability and oxidation inhibition mechanisms of oleogel emulsions under thermal, freeze-thaw, and ionic strength variations were systematically analyzed. This work provides a theoretical foundation and technical support for the high-value utilization of macadamia oil and the advancement of oleogel and emulsion technologies in the food industry.

2. Materials and methods

2.1. Materials

Freshly harvested macadamia nuts (A4) were sourced from Guangxi province, China, in September 2023, and promptly transported to the laboratory upon harvest. The nuts were dried in an oven (DHG-9245 A, Yiheng, Shanghai, China) at 45 °C until their moisture content was reduced to below 2.0 %. The dried nuts were ground into uniform powder using a grinder (RHP-100, Ronghao, Zhejiang, China). The resulting powder was mixed with n-hexane at a 1:6 (g/mL) ratio and soaked in a water bath (Bangxi, Shanghai, China) maintained at 45 °C for 12 h. Following soaking, the mixture was filtered using a vacuum filtration device (2XZ-4, Lichen, Shanghai, China) to collect the supernatant. Residual organic solvents were eliminated with a rotary evaporator (YRE-2000E, Yuhua, Zhengzhou, China) set at 50 °C, and the oil was further dried in a vacuum oven (DZF-6050, Hengyi, Shanghai, China) at 45 °C for 24 h to ensure complete removal of solvents. The final product was pure macadamia oil. β-sitosterol (purity ≥95 %) and γ-oryzanol (purity ≥98 %) were obtained from Yuanye Biotechnology Co., Ltd. (Shanghai, China). Other reagents were sourced from Shanghai Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).

2.2. Preparation for Oleogels and O/W emulsions

Macadamia oil (20 g) was mixed with BS-GO at concentrations of 6 %, 7 %, 8 %, 9 %, 10 %, 11 %, and 12 % (β-sitosterol: γ-oryzanol = 2:3, w/w). The mixtures were transferred to 150 mL glass beakers and heated in a magnetic stirring water bath at 90 °C until the gelling agents dissolved completely. These mixtures were subsequently cooled at room temperature for 12 h. The cooled mixtures were refrigerated at 4 °C to form oleogels.

O/W oleogel emulsions were prepared using a modified protocol derived from previous studies (Gao et al., 2024). Each emulsifier (1 g; Whey Protein Isolate, WPI; Soy Protein Isolate, SPI; Gum Arabic, GA; Chitosan, CS; Lactose, LT; and Sodium Caseinate, SC) was dissolved in 200 mL ultrapure water and stirred at room temperature for 2 h. Insoluble substances were removed by centrifugation at 8000 rpm for 5 min, and the clarified aqueous phase was collected for subsequent use. The oil phase was prepared by adding BS-GO (0 %, 7 %, 8 %, 9 %, 10 %, 11 %, or 12 %) to macadamia oil, preheated at 70 °C for 10 min. One hundred milliliters of the aqueous phase were incrementally added to 1 % of the oil phase and stirred at 12,000 rpm for 3 min. Coarse emulsions were subjected to ultrasonic treatment at 45 W for 5 min (KQ-800KDE, Suzhou, China) and promptly cooled in an ice-water bath. Prepared emulsions were stored at 4 °C before testing.

2.3. Morphological observations

The macroscopic appearance of the oleogels was visually assessed after the gelling agents were completely dissolved and the samples were cooled at room temperature for 12 h. For microscopic morphology, melted oleogel samples were prepared on microscope slides and stored at 4 °C overnight to form gel structures. The gel microstructure was observed using a polarized light microscope (BX53M, Olympus Corporation, Japan) equipped with a 20× objective lens. Confocal laser scanning microscopy (TCS SP8, Leica, Germany) with a 63× oil immersion lens was employed to examine the microstructure of emulsions, with oil droplets stained using Nile red (50 μg/mL, excitation at 500 nm, emission at 570–640 nm).

2.4. Physicochemical properties

The acid value (AV) and peroxide value (POV) of the oleogels were quantified according to ISO 660, and ISO 3960 methods.

Oil binding capacity (OBC) was determined using a modified method based on Marsh et al. (2024). Five grams of oleogel samples were placed in 10 mL centrifuge tubes and allowed to stand at room temperature for 24 h. The samples were then centrifuged at 10,000 rpm for 15 min. Excess oil was removed by inverting the tubes onto absorbent paper, and the combined weight of the oleogel and centrifuge tube after centrifugation was recorded. OBC was calculated as follows:

$$\mathrm{OBC}=\frac{\mathrm{m}}{\mathrm{M}}\times 100.$$

where m is the weight of the oleogel and centrifuge tube after centrifugation, and M is the initial weight before centrifugation.

The DPPH radical scavenging activity was determined by weighing 0.15 g of the sample and diluting it with ethyl acetate. The diluted sample was vortexed and mixed with the DPPH solution. The mixture was incubated in the dark at room temperature for 2 h. The absorbance was measured at 517 nm. A Trolox standard curve was used to calculate antioxidant activity. The results were expressed as μmol TE/kg of oil.

The fatty acid composition was analyzed using a modified protocol based on Shuai, Dai, Chen, Liu, et al. (2022). A 0.1 g oil sample was mixed with 2 mL of 0.5 mol/L potassium hydroxide-methanol solution in a 10 mL centrifuge tube. The mixture was vortexed and heated in a water bath at 60 °C for 30 min. Afterward, 2 mL of boron trifluoride-methanol solution was added, and the reaction continued for 3 min at the same temperature. Saturated sodium chloride solution (2 mL) and chromatography-grade n-hexane (2 mL) were added to extract the fatty acid methyl esters (FAMEs). The mixture was ultrasonicated for 5 min and allowed to settle. The upper layer was extracted and filtered through a 0.22 μm organic membrane. The FAMEs were analyzed using a gas chromatograph (7890 A, Agilent, USA) equipped with an HP-88 capillary column (100 m × 0.25 mm × 0.20 μm, Agilent, USA). The injector temperature was 270 °C, and the detector temperature was 280 °C. Nitrogen was used as the carrier gas. The column temperature was programmed to increase from 100 °C to 230 °C in multiple steps. Fatty acids were identified and quantified by comparison to retention times of FAME standards.

Textural properties of the oleogels were measured using a texture analyzer (TA-XT plus, Surface Measurement Systems, UK) equipped with a TA/0.5 probe. Ten grams of melted oleogel were poured into 25 mL beakers and stored at 4 °C for 24 h. Pre-test and post-test speeds were maintained at 2 mm/s, while the test speed was set at 1 mm/s. The probe penetrated to a depth of 15 mm, with a 50 % compression ratio and a trigger force of 5 g.

2.5. Fourier transform infrared spectroscopy (FTIR)

FTIR analysis was conducted to characterize molecular interactions within the oleogels (Shuai et al., 2023). Potassium bromide was dried at 50 °C for 12 h and ground into powder under infrared light using a mortar. The powder was compressed into translucent thin sheets using a mold. A small amount of melted oleogel was applied to the potassium bromide sheet and dried under vacuum. Samples were analyzed using a Fourier transform infrared spectrometer (Nicolet iS10, Thermo Fisher, USA) in the range of 500–4000 cm⁻¹.

2.6. Rheological properties

Rheological properties of the oleogels were measured using a rheometer (DHR-2, Waters, USA) with a 40 mm parallel plate geometry and a gap setting of 1.0 mm. Rheological measurements were conducted at 25 °C. Apparent viscosity was measured after 1 min of equilibration, with a total measurement time of 2 min, as the shear rate increased logarithmically from 0.1 s⁻¹ to 100 s⁻¹. The linear viscoelastic region (LVR) was determined at 1.0 Hz, with the stress range set between 0.001 % and 10 %. Frequency sweep tests were performed at a constant strain of 0.1 % over a frequency range of 0.01 Hz to 100 Hz.

2.7. Antioxidation ability

2.7.1. Free radical content

The free radical content was measured using electron spin resonance (ESR, Magnettech ESR 5000, Bruker, Germany) by Jiang et al. (2020) with some modification. Samples were prepared by mixing the oleogel with DMPO free radical scavenger in a 5 mL centrifuge tube. The mixture was drawn into a capillary tube, sealed, and placed in the ESR instrument. The parameters were set as follows: a central magnetic field of 335 mT, a magnetic field width of 20 mT, a modulation amplitude of 0.22 mT, and a modulation frequency of 100 kHz.

2.7.2. Oxidation stability of emulsions

The oxidation stability of emulsions was evaluated by storing samples at 60 °C under accelerated conditions for up to seven days. Samples were collected on days 0, 1, 3, 5, and 7 to measure the content of primary and secondary oxidation products, including POV and thiobarbituric acid reactive substances (TBARS).

POV was determined using a modified method based on Zhang et al. (2024). One milliliter of emulsion was added to 1.5 mL of a mixed solution of isooctane and isopropanol (3:1, v/v) and centrifuged at 5500 rpm for 10 min. The upper phase was separated and diluted with a methanol-n-butanol solution (2:1, v/v) to a final volume of 5 mL. Potassium thiocyanate and ferrous sulfate solutions (15 μL) were added, and the mixture was incubated in the dark for 20 min. Absorbance was recorded at 510 nm using a UV–Vis spectrophotometer, and cumene hydroperoxide was used as the standard.

TBARS were quantified by mixing 2 mL emulsion with 4 mL trichloroacetic acid-thiobarbituric acid solution (1:1, v/v). The mixture was vortexed for 2 min and heated in a water bath at 90 °C for 15 min. After cooling, the samples were centrifuged at 4000 rpm for 10 min, and the supernatant was filtered. Absorbance was recorded at 532 nm using a UV–Vis spectrophotometer (UV755B, Keyou, Shanghai, China), and malondialdehyde was used as the standard.

2.8. Stability measurements

2.8.1. Thermal stability of Oleogels

The thermal stability of the oleogels was assessed using a modified method based on Gao and Wu (2019) with some modification. Approximately 3 g of oleogel samples were placed in 5 mL centrifuge tubes and heated in a water bath at 40 °C, 50 °C, or 60 °C for 30 min. After heating, the samples were cooled to room temperature and centrifuged at 10,000 rpm for 15 min. Excess oil was removed using absorbent paper, and the remaining weight of the oleogels and centrifuge tubes was recorded. Thermal stability was expressed as the percentage of oil retained within the oleogel matrix.

2.8.2. Freeze-thaw stability of emulsions

Freeze-thaw stability was evaluated by subjecting emulsions to three consecutive cycles of freezing at −20 °C for 24 h and thawing at room temperature for 30 min. After each cycle, emulsions were diluted 200-fold in ultrapure water and analyzed for changes in particle size, polydispersity index (PDI), and zeta potential using dynamic light scattering (DLS; Zetasizer Nano-ZS, Malvern Instruments, UK) following the methodology of Chang et al. (2025). Visual observation of creaming or phase separation was recorded to assess macroscopic stability.

2.8.3. Ionic strength stability of emulsions

The stability of emulsions under different ionic strengths was tested by adding sodium chloride to achieve final concentrations of 0 mmol/L, 50 mmol/L, and 100 mmol/L. Samples were diluted 200-fold in ultrapure water and analyzed for particle size, PDI, and zeta potential using dynamic light scattering (DLS; Zetasizer Nano-ZS, Malvern Instruments, UK). Macroscopic stability was assessed by visually inspecting for signs of creaming or phase separation after 24 h of storage at 25 °C.

2.8.4. Thermal stability of emulsions

The thermal stability of emulsions was tested by heating samples in a water bath at 30 °C, 60 °C, or 80 °C for 30 min. Heated emulsions were cooled to room temperature, diluted 200-fold in ultrapure water, and analyzed for changes in particle size, PDI, and zeta potential using dynamic light scattering (DLS; Zetasizer Nano-ZS, Malvern Instruments, UK). Observations of creaming or phase separation were recorded for each condition.

2.9. Statistical analysis

All experiments were performed in triplicate, and the results are expressed as mean ± standard deviation. Data were analyzed using one-way analysis of variance (ANOVA) with SPSS 19.0 software. Significant differences (P < 0.05) were denoted by different superscript letters.

3. Results and discussion

3.1. Morphology of Oleogels

The appearance of macadamia oil containing varying BS-GO concentrations is presented in Fig. 1a. Oleogels did not form at a BS-GO concentration of 6 %. However, at concentrations of 7 % or higher, solid oleogels were observed, with increasing opacity as the BS-GO concentration increased. This change is attributed to the formation of smaller crystals at lower concentrations, allowing light transmission, and the generation of larger crystals at higher concentrations, resulting in a denser network structure that blocks light.

Fig. 1.

Macroscopic appearance (a) and microstructure (b) of macadamia oleogels prepared by BS-GO.

Polarized light microscopy (Fig. 1b) revealed differences in aggregate size and distribution with varying BS-GO concentrations. At lower BS-GO concentrations, small “spherulitic crystals” were observed. In contrast, higher concentrations produced larger crystal aggregates, forming a dense three-dimensional network dominated by “spherulitic crystals” (Sawalha et al., 2010). This compact and stable structure exhibited strong intermolecular forces within the crystal lattice, effectively restricting the diffusion of macadamia oil. As the BS-GO concentration increased from 7 % to 12 %, the network structure became denser and more uniform, further enhancing stability.

3.2. Physicochemical properties of Oleogel

AV and POV which are critical indicators of oil quality, are summarized in Table 1. The AV of the oleogels ranged from 1.09 to 1.44 mg/g, and the POV ranged from 6.16 to 9.14 mmol/kg, both of which were below the standard limits (AV < 3 mg/g; POV < 10 mmol/kg). However, both AV and POV values in the oleogels exceeded those of macadamia oil (AV: 0.32 mg/g; POV: 4.73 mmol/kg), likely due to the thermal oxidation and hydrolysis reactions occurring during the heating process. As the BS-GO concentration increased, AV and POV values initially rose, peaking at 9 %, before subsequently decreasing. At lower BS-GO concentrations, an incomplete network structure exposed more oil to oxidative reactions. Higher concentrations led to the formation of a stable 3D network, which acted as an oxygen barrier, reducing peroxide formation and slowing oxidation.

Table 1.

Effect of OZ-PS addition on physicochemical properties of oleogels.

	BS-GO/0 %	BS-GO/7 %	BS-GO/8 %	BS-GO/9 %	BS-GO/10 %	BS-GO/11 %	BS-GO/12 %
AV (mg/g)	0.32 ± 0.02e	1.09 ± 0.02d	1.43 ± 0.02a	1.44 ± 0.11a	1.15 ± 0.01c	1.17 ± 0.04c	1.21 ± 0.04b
POV (mmol/kg)	4.73 ± 1.58d	6.70 ± 0.58cd	7.43 ± 0.14bcd	9.14 ± 0.49a	8.74 ± 0.12ab	7.94 ± 0.90abc	6.16 ± 0.07d
OBC (%)	/	98.21 ± 2.31b	98.37 ± 1.45b	98.81 ± 2.21ab	98.95 ± 2.49ab	99.56 ± 1.97a	99.71 ± 2.51a
DPPH (μmol TE/100 g)	24.33 ± 0.77f	1463 ± 53e	1644 ± 24d	1713 ± 22c	1806 ± 37b	1857 ± 25b	1922 ± 46a
Fatty acid composition (%)
C16:0	9.29 ± 0.05a	6.22 ± 0.00d	5.87 ± 0.00f	6.11 ± 0.00e	5.42 ± 0.01g	6.32 ± 0.01c	6.90 ± 0.00b
C16:1	12.95 ± 0.07d	13.66 ± 0.01b	13.17 ± 0.01c	13.11 ± 0.06d	12.02 ± 0.02e	13.81 ± 0.12a	9.70 ± 0.03f
C18:0	4.93 ± 0.01c	4.85 ± 0.00d	4.94 ± 0.00c	4.92 ± 0.00c	5.13 ± 0.00b	4.85 ± 0.01d	5.18 ± 0.00a
C18:1	63.08 ± 0.10g	65.11 ± 0.02c	64.14 ± 0.03e	64.64 ± 0.05d	65.21 ± 0.01b	63.62 ± 0.03f	67.23 ± 0.04a
C18:2	1.27 ± 0.00c	2.43 ± 0.00a	2.41 ± 0.00a	2.35 ± 0.00b	2.42 ± 0.00a	2.40 ± 0.00a	2.42 ± 0.00a
C20:1	3.99 ± 0.01b	3.30 ± 0.00f	3.74 ± 0.01c	3.49 ± 0.00e	3.98 ± 0.00b	3.63 ± 0.01d	4.58 ± 0.00a
C18:3	3.80 ± 0.05f	4.43 ± 0.00e	5.72 ± 0.01c	5.38 ± 0.01d	5.82 ± 0.00b	5.37 ± 0.00d	6.19 ± 0.00a
SFA	14.90 ± 0.05a	11.07 ± 0.01d	10.81 ± 0.01f	11.03 ± 0.00e	10.55 ± 0.02g	11.17 ± 0.02c	12.08 ± 0.01b
MUFA	80.02 ± 0.06e	82.07 ± 0.03a	81.05 ± 0.12d	81.24 ± 0.07c	81.21 ± 0.03c	81.06 ± 0.11d	81.51 ± 0.08d
PUFA	5.07 ± 0.25g	6.86 ± 0.02e	8.14 ± 0.00b	7.73 ± 0.00d	8.24 ± 0.01a	7.77 ± 0.02c	6.41 ± 0.01f
Textural properties (g)
Hardness	/	1449 ± 122f	2697 ± 105e	3699 ± 260d	4659 ± 186c	5280 ± 91b	6284 ± 141a
Viscoelasticity	/	0.55 ± 0.02e	0.84 ± 0.04c	0.74 ± 0.02d	0.93 ± 0.00b	0.96 ± 0.01ab	0.99 ± 0.00a
Cohesiveness	/	0.13 ± 0.02a	0.12 ± 0.01a	0.15 ± 0.03a	0.13 ± 0.02a	0.14 ± 0.01a	0.14 ± 0.02a
Thermal stability (%)
40 °C	/	98.05 ± 1.32d	98.77 ± 1.13c	98.88 ± 0.96b	98.92 ± 1.04b	98.95 ± 2.04b	99.51 ± 1.01a
50 °C	/	89.47 ± 1.03f	94.96 ± 0.95e	95.10 ± 2.01d	97.28 ± 1.87c	97.46 ± 2.03b	97.65 ± 1.03a
60 °C	/	61.62 ± 1.01f	74.64 ± 1.43e	90.41 ± 2.01d	93.35 ± 1.43c	93.49 ± 1.72b	93.86 ± 1.02a

The OBC values of the oleogels prepared with varying BS-GO concentrations are presented in Table 1. OBC reflects the ability of the oleogel's three-dimensional network to retain liquid oil (Okuro et al., 2018). The OBC of oleogels with 7 % BS-GO concentration was the lowest (98.21 %), likely due to the incomplete formation of the gel network at lower concentrations. This incomplete structure exhibited weak intermolecular forces, reducing the oleogel's capacity to trap oil effectively. As the BS-GO concentration increased, the OBC values progressively improved, reaching 99.51 % at 12 % BS-GO. The denser and more stable network structure at higher concentrations effectively restricted oil mobility, thereby minimizing oil loss. These findings underscore the critical role of BS-GO in enhancing the OBC of oleogels, which is crucial for applications requiring oil retention under various conditions.

The antioxidant capacity of the oleogels, measured by DPPH radical scavenging activity, also exhibited a positive correlation with BS-GO concentration. As shown in Table 1, DPPH scavenging activity increased from 1462.86 to 1922.18 μmol TE/100 g as BS-GO concentrations rose, which 50 times higher than the macadamia oil (24.33 TE/100 g). This enhancement is attributed to the potent antioxidant properties of β-sitosterol and γ-oryzanol, which effectively neutralize free radicals (Liu et al., 2021). Furthermore, the gel network provided a protective microenvironment that shielded the antioxidant components within the oleogels from oxidative degradation, thereby preserving their activity. These results highlight the dual functionality of BS-GO in oleogel systems: improving oil retention and enhancing antioxidative stability, both of which are critical for prolonging shelf life and maintaining product quality in food applications.

3.3. Fatty acid composition of Oleogel

The fatty acid composition of macadamia oil-based oleogels is summarized in Table 1. The oleogels primarily consisted of unsaturated fatty acids (UFAs), with monounsaturated fatty acids (MUFA) being the dominant type, accounting for 81.05 % -82.07 % of the total fatty acids. Oleic acid (C18:1) was the most abundant fatty acid, ranging from 63.62 % to 67.23 %, followed by palmitoleic acid (C16:1), which ranged from 9.70 % to 13.81 %. Polyunsaturated fatty acids (PUFA), including linoleic acid (C18:2) and linolenic acid (C18:3), ranged from 6.41 % to 8.24 %. Saturated fatty acids (SFA) accounted for 10.55 % -12.08 % of the total fatty acids. Palmitic acid (C16:0) was the primary SFA, with concentrations decreasing slightly in oleogels compared to raw macadamia oil. This reduction could be attributed to the structural arrangement of the BS-GO network, which might preferentially encapsulate unsaturated fatty acids due to their higher fluidity. The MUFA content slightly increased in oleogels, with the highest MUFA levels observed at 7 % BS-GO (82.07 %). Conversely, PUFA content peaked at 10 % BS-GO (8.24 %) but decreased at higher BS-GO concentrations, indicating that the gel network structure may influence the distribution and retention of fatty acids. Notably, the oleogelation process did not significantly alter the fatty acid composition of the oil, preserving its nutritional value while enhancing stability and functionality. This result shows that oleogel can become an ideal substitute for solid-fat in the food industry, which can not only eliminate the harm of SFA, but also meet the growing demand of consumers for healthier substitutes.

3.4. Textural properties of Oleogel

The textural properties of the oleogels, including hardness, elasticity, and cohesiveness, are presented in Table 1. These parameters are crucial for assessing spreadability, adhesiveness, oil migration, and storage stability (Thomas et al., 2023). As the BS-GO concentration increased, the hardness of the oleogels rose from 1449.41 g to 6283.96 g, while elasticity increased from 0.55 to 0.99. This enhancement can be attributed to the formation of a denser and more interconnected network structure at higher BS-GO concentrations, which strengthened intermolecular interactions, and restricted oil molecule movement (Martins et al., 2019). Cohesiveness, ranging from 0.12 to 0.15, exhibited minimal variation across different concentrations, likely due to saturation of the intermolecular forces governing gel cohesiveness.

3.5. Thermal stability of Oleogel

The thermal stability of the oleogels, evaluated through their oil retention capacity after heating, is summarized in Table 1. As the heating temperature increased from 40 °C to 60 °C, all oleogels exhibited a reduction in oil retention. This decrease can be attributed to the disruption of non-covalent interactions, including hydrogen bonds and van der Waals forces, which weakened the three-dimensional network structure. At a BS-GO concentration of 7 %, the oleogel showed the lowest thermal stability, with a 36.43 % reduction in oil retention at 60 °C. In contrast, oleogels prepared with higher BS-GO concentrations demonstrated enhanced thermal stability, with the 12 % BS-GO oleogel retaining 93.86 % of its oil at 60 °C. These findings indicate that higher BS-GO concentrations form stronger networks capable of withstanding temperature-induced stresses.

3.6. Rheological properties of Oleogel

The rheological properties of oleogels, including apparent viscosity and viscoelastic behavior, were evaluated to understand their structural dynamics (Fig. 2). The apparent viscosity of the oleogels, as presented in Fig. 2a, was analyzed to evaluate flow behavior under shear stress. All oleogel samples exhibited shear-thinning behavior, characterized by a reduction in viscosity as the shear rate increased. At low shear rates, the molecular network within the oleogels resisted deformation, resulting in higher viscosity. As the shear rate increased, external forces disrupted the molecular entanglements, reducing viscosity (Nutter et al., 2022). Higher BS-GO concentrations corresponded to increased apparent viscosity at the same shear rate, reflecting the formation of a denser and more robust network structure. This denser network provided greater resistance to deformation, requiring more energy to disrupt.

Fig. 2.

Effect of different concentrations of BS-GO macadamia oleogels on apparent viscosity (a), strain scans (b), frequency scanning (c), Fourier transform infrared spectra (d), fitted mapping (e), free radical content (f).

The strain scan results (Fig. 2b) revealed that the storage modulus (G′), which represents the elastic properties of the oleogels, increased with BS-GO concentration. Higher G′ values indicate enhanced structural strength and energy storage capacity under deformation. The frequency sweep results (Fig. 2c) demonstrated that G′ consistently exceeded the loss modulus (G″) across all frequencies tested, confirming the dominance of solid-like behavior in the oleogels. Both G′ and G″ exhibited frequency dependence, with no crossover points observed, indicating that the oleogels retained their gel-like structure even under dynamic oscillatory conditions. These findings highlight the role of BS-GO concentration in modulating the rheological behavior of oleogels, thereby influencing their application potential in various food systems.

3.7. FTIR

FTIR is capable of detecting changes in molecular vibration modes induced by intermolecular forces during the formation of oleogels. This capability provides insights into the oleogels' network structure. The phase transition of oleogels, from liquid to solid, results in alterations in molecular arrangement. Consequently, structural changes in oleogels can be inferred from shifts in displacement, intensity, or peak shape of the absorption peaks in the FTIR spectrum. Fig. 2d illustrates the infrared light changes in oleogels with varying contents of BS-GO. The result shows that the infrared absorption of macadamia oil and oleogels primarily exhibits absorption peaks at 3009, 2925, 2854, 1746, 1464, 1377, 1164, 1039, and 720 cm⁻¹. Specifically, the absorption peak at 3009 cm⁻¹ is likely due to the stretching vibration of C=C-H. The absorption peaks at 2925 and 2854 cm⁻¹ correspond to the stretching vibrations of CH₃ and CH₂, respectively. The peak at 1746 cm⁻¹ is associated with the stretching vibration of the carbonyl group C O in esters. The peaks at 1464 and 1377 cm⁻¹ are related to the C—H bending vibrations of the CH₂ group. The peaks at 1164 and 1093 cm⁻¹ are attributed to the stretching vibrations at C—O in the C-O-H and C-O-C groups, respectively. Finally, the peak at 720 cm⁻¹ is due to the bending vibration of (CH₂)_n in the alkyl chain. Upon the addition of BS-GO, no new absorption peaks appeared or disappeared compared to macadamia oil, indicating the absence of covalent interactions during oleogel formation. However, the absorption peak in the range of 3200–3600 cm⁻¹ continues to increase, suggesting that the interaction force may be related to hydrogen bonding (Alongi et al., 2022). Additionally, the absorption peaks at 2925 cm⁻¹ and 2854 cm⁻¹ in the oil gel tend to shift to the right, which may be attributed to van der Waals forces. The oil gel system formed by BS-GO is a result of the synergistic effects of intermolecular hydrogen bonding and van der Waals forces.

3.8. Free radical content of oleogel

The free radical content of the oleogels, analyzed using electron spin resonance (ESR), is illustrated in Fig. 2e and f. The primary reactive oxygen species detected were superoxide anion radicals (•O₂⁻) and singlet oxygen (¹O₂). These reactive oxygen species accelerate oil oxidation, resulting in rancidity and off-flavors, ultimately degrading oil quality and reducing its nutritional value. Incorporation of BS-GO effectively inhibited the formation of •O₂⁻ and ¹O₂, with the inhibition effect positively correlated with BS-GO concentration. At higher concentrations, the antioxidant activity of BS-GO reached saturation, with additional increases yielding negligible effects. These findings underscore the critical role of BS-GO in mitigating oxidative degradation, thereby preserving oleogel stability and quality during storage.

3.9. Influence of emulsifier types

The influence of emulsifier types on the stability of macadamia oil gel emulsions is depicted in Fig. 3. Emulsifiers possess both hydrophilic and lipophilic groups, allowing them to enhance interfacial properties and inhibit phase separation in emulsions (Xu et al., 2018). The stability of the emulsions followed the order: WPI > SPI > GA > LT > CS > SC. Among these, WPI exhibited the highest stability due to its dual hydrophilic and lipophilic characteristics, enabling efficient encapsulation and stabilization of fat at the oil-water interface (Marcelo & Rizvi, 2008). SPI also demonstrated amphiphilic properties but with a slightly lower emulsification capacity compared to WPI. The remaining emulsifiers, such as GA and SC, exhibited weaker stabilizing effects due to either lower amphiphilic efficiency or poorer solubility, resulting in suboptimal performance (Herrero et al., 2018).

Fig. 3.

Effect of emulsifier type on particle size, PDI and zeta potential of macadamia oleogels emulsion.

Key parameters for evaluating emulsification performance include droplet size and zeta potential (Fig. 4). Smaller droplets typically indicate greater stability, as their larger surface area enables emulsifiers to create a robust interfacial layer, thereby reducing droplet aggregation. WPI-based emulsions achieved the smallest droplet size (256.86 nm) and the lowest polydispersity index (PDI, 0.32) in Fig. 4a, reflecting a more uniform distribution of oil droplets. Additionally, the absolute zeta potential value of WPI-based emulsions (−32.93 mV) in Fig. 4b was the highest among all samples, indicating stronger electrostatic repulsion between droplets and enhanced stability. These findings confirm the superior performance of WPI as an emulsifier and its suitability for stabilizing macadamia oil-based emulsions.

Fig. 4.

Effect of BS-GO addition on emulsion particle size, PDI and potential.

3.10. Emulsion microstructure

The effect of BS-GO concentration on the microstructure and droplet size distribution of emulsions is illustrated in Fig. 5. Red-colored droplets indicate the distribution and size of the oil droplets. Confocal laser scanning microscopy revealed that at the critical gelation concentration of 7 % BS-GO, oil droplets were densely packed and exhibited uniform sizes, consistent with particle size measurements. As the BS-GO concentration increased beyond 7 %, the droplet sizes and distribution became more heterogeneous. This phenomenon is likely attributable to the three-dimensional network structure formed by BS-GO, which promotes droplet aggregation and results in larger droplets in localized regions (Wei et al., 2019). These findings highlight the importance of optimizing BS-GO concentration to balance network formation and droplet stability.

Fig. 5.

Laser confocal maps and droplet size distribution of emulsions with different BS-GO additions.

3.11. Emulsion stability

3.11.1. Thermal stability

The thermal stability of emulsions is critical for maintaining quality during storage and processing. The effect of heating temperatures (30–80 °C) and BS-GO concentrations on emulsion stability is shown in Fig. 6. At lower temperatures (30–60 °C), emulsions containing 7 % BS-GO exhibited minimal changes in particle size and zeta potential, with particle size increasing from 218.03 nm to 245.00 nm and zeta potential decreasing slightly from −36.20 mV to −35.70 mV. This indicates excellent thermal stability. In contrast, emulsions with higher BS-GO concentrations showed larger particle size increases and more pronounced zeta potential reductions at higher temperatures. For example, the particle size of emulsions containing 12 % BS-GO increased significantly from 320.27 nm to 561.73 nm, accompanied by a decrease in zeta potential from −31.13 mV to −28.21 mV. This instability is likely due to excessive gel network rigidity at high BS-GO concentrations, which may reduce emulsion flexibility under thermal stress. The results suggest that the optimal BS-GO concentration for thermal stability is 7 %, as it provides a balanced network structure capable of withstanding temperature fluctuations without compromising emulsion integrity.

Fig. 6.

Effect of different heating temperatures of emulsions on the macroscopic morphology (a), particle size (b), and potential (c).

3.11.2. Freeze-thaw stability

The freeze-thaw stability of emulsions is essential for applications involving cold storage or transportation. Fig. 7 illustrates the effects of freeze-thaw cycles on macadamia oil gel emulsions. After three cycles, emulsions with 7 % BS-GO exhibited the smallest particle size increase, from 265.87 nm to 302.12 nm, indicating superior resistance to ice crystal formation and droplet aggregation. This stability can be attributed to the gel network's ability to regulate water distribution, reducing ice crystal damage during freezing and thawing processes. In contrast, emulsions with higher BS-GO concentrations (e.g., 12 %) experienced more significant particle size increases, exceeding 400 nm, and exhibited slight oil-water separation. These changes are likely due to excessive crosslinking in the gel network, which disrupts water arrangements and facilitates the formation of larger ice crystals. Additionally, zeta potential reductions were observed across all samples, ranging from −1.69 mV to −4.01 mV, suggesting partial emulsifier redistribution during freeze-thaw cycles. Among all samples, emulsions with 7 % BS-GO demonstrated the highest freeze-thaw stability, maintaining structural integrity and minimal aggregation.

Fig. 7.

Effect of 3 freeze-thaw repeated cycles of emulsions on the macroscopic morphology (a), particle size (b), and potential (c).

3.11.3. Ionic strength stability

The ionic strength stability of emulsions was evaluated under varying NaCl concentrations (0–100 mmol/L) to simulate food processing conditions. Fig. 8a shows that all emulsions maintained their structural integrity without phase separation, indicating excellent macroscopic stability. As shown in Fig. 8b, the particle size of emulsions increased slightly with higher NaCl concentrations. However, emulsions containing 7 % BS-GO exhibited the smallest particle size (232.37 nm) and the least increase under ionic strength variations, indicating superior resistance to electrostatic shielding effects. Absolute zeta potential values decreased slightly across all samples, from −28.31 mV to −33.62 mV, as salt ions neutralized surface charges on the droplets. Nonetheless, the emulsion with 7 % BS-GO showed the highest zeta potential stability, demonstrating that it effectively resists ionic destabilization. These findings highlight that emulsions with 7 % BS-GO exhibit optimal stability across thermal, freeze-thaw, and ionic strength conditions, making them suitable for diverse food processing and storage applications.

Fig. 8.

Effect of different salt ion concentrations of emulsions on the macroscopic morphology (a), particle size (b), and potential (c).

3.12. Oxidation stability of emulsions

The oxidation stability of emulsions was evaluated by monitoring POV and TBARS over seven days of storage at 60 °C (Fig. 9). Emulsions containing BS-GO exhibited significantly lower POV (7.07–12.67 mmol/kg) and TBARS (2.83–4.96 μmol/kg) compared to the control, which reached 18.26 mmol/kg and 5.76 μmol/kg, respectively. The enhanced oxidative stability of BS-GO emulsions is attributed to the physical barrier created by the three-dimensional network, which restricts oxygen diffusion to oil droplets. Furthermore, BS-GO's inherent antioxidative properties further mitigated oxidation rates, enhancing the emulsions' resistance to oxidative degradation.

Fig. 9.

Variation of POV and TBARS content for 7 d of storage at 60 °C temperature.

3.13. Rheological properties of emulsion

The rheological properties of emulsions provide valuable insights into their flow and deformation behavior under various conditions, such as storage, transportation, and application. Apparent viscosity represents the internal frictional resistance of a system under shear conditions, serving as an indicator of flowability. Fig. 10a shows that apparent viscosity decreased sharply as the shear rate increased from 0.1 to 10 s⁻¹, indicating shear-thinning behavior. At lower shear rates, tightly associated droplets and polymer chains within the emulsion formed robust networks, resulting in higher viscosity. As the shear rate increased, the network was disrupted, aligning molecules with the shear field and reducing internal resistance, thereby lowering viscosity.

Fig. 10.

Plot of apparent viscosity of emulsion (a) and variation of dynamic scanning frequency (b).

Fig. 10b and c depict the storage modulus (G′) and loss modulus (G″), which measure the elastic and viscous behaviors of the emulsions, respectively. Storage modulus (G′) reflects structural strength and energy storage capacity, characterizing the solid-like nature of the system. Loss modulus (G″) quantifies internal friction and fluidity, characterizing the liquid-like behavior of a system. Across all samples, G′ values exceeded G″ values, confirming the dominance of elastic behavior. Both G′ and G″ increased with frequency, exhibiting parallel trends that indicate strong frequency dependence without crossover points. This finding suggests that the emulsions maintained a gel-like structure throughout the frequency range tested. Emulsions with higher BS-GO concentrations exhibited greater G′ values, reflecting enhanced structural rigidity and stability due to the denser molecular networks formed at higher concentrations. These results highlight the critical role of BS-GO in modulating the rheological properties of emulsions to achieve desired textural and stability characteristics.

4. Conclusions

This study demonstrates the potential of β-sitosterol/γ-oryzanol composite systems (BS-GO) to convert macadamia oil into high-performance oleogels and emulsions, addressing its limitations of high fluidity and oxidative instability. Oleogels prepared with 7 % -12 % BS-GO formed dense three-dimensional networks, exhibiting superior oil-binding capacity, antioxidative properties, and textural stability. Emulsions derived from these oleogels showed excellent stability, particularly at the optimal 7 % BS-GO concentration. WPI emerged as the most effective emulsifier, ensuring smaller droplet sizes, higher zeta potential, and enhanced emulsion stability under diverse conditions, including thermal treatment, freeze-thaw cycles, and ionic strength variations. Furthermore, the emulsions demonstrated improved resistance to oxidation, as evidenced by lower peroxide and TBARS values during accelerated storage. These findings establish a scientific basis for the value-added utilization of macadamia oil in food products. Future research could explore the application of oleogel-based emulsions in functional foods and nutraceuticals, expanding the industrial scope and potential of macadamia oil.

CRediT authorship contribution statement

Pan Gao: Writing – original draft. Ying Liu: Formal analysis. Shu Wang: Methodology. Jiaojiao Yin: Data curation. Wu Zhong: Data curation. Xinghe Zhang: Resources. Xingguo Wang: Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

All data generated or analyzed during this study are included in this published article.