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. 2026 Feb 4;34:103638. doi: 10.1016/j.fochx.2026.103638

Development of functional cheese sticks using konjac glucomannan: regulated pickering double emulsions under different heat treatments

Wei Xu a,⁎, Mengyao Kang b, Lanlan Zhang a, Yongpeng Yin b, Denglin Luo b, Xiaofan Zhang c, Lihua Zhang a, Lala Li a, Siqi Du a
PMCID: PMC12907721  PMID: 41704876

Abstract

In this paper, the structure, texture, thermal stability and controlled co-delivery characteristics of Pickering double emulsions (PDEs) regulated by konjac glucomannan (KGM) under different heat treatments were systematically studied. It showed heat treatments promoted the synergistic geation effect and transformed the system from the emulsion to the emulsion gel. The rheological results showed the increase temperature caused the loss factor first decreased and then increased and reached the minimum value at 70 °C. The texture test showed that after treatment at 70 °C, the hardness (0.79 N) and springiness (0.9 mm) of KGM-based PDEs were more similar to those of commercial cheese sticks. The gel matrix induced by heat treatments served as a physical barrier to improve the thermal stability of encapsulated EGCG and β-carotene in the KGM-based PDEs, and enhanced its gastrointestinal digestion stability by hindering lipid hydrolysis. These findings provide a theoretical foundation for developing functional plant-based gel foods.

Keywords: Pickering double emulsions, Konjac glucomannan, Epigallocatechin gallate, β-Carotene

Highlights

  • •

    KGM significantly improved the stability of PDEs under heat stress.

  • •

    KGM-based PDEs displayed solid-like behavior at 70 °C.

  • •

    The thermal stability of EGCG and β-carotene in KGM-based PDEs improved.

  • •

    The heated KGM-based PDEs have the potential to substitute cheese sticks.

1. Introduction

Pickering double emulsions (PDEs) are compartmentalized systems in which the dispersed droplets of simple emulsions contain smaller oil-in-water (O/W) or water-in-oil (W/O) droplets (Tenorio-Garcia et al., 2022). PDEs can be used as a promising approach to improve the nutritional profile or decreasing the calorific content. Recently, it has been used to design different types of meat, cheese, mayonnaise (Niroula et al., 2025). However, during the heating process, the thermodynamic instability of PDEs (such as flocculation, coalescence and stratification) will be increased. Besides, heat treatment accelerated lipid oxidation and altered protein structure, and finally ultimately affected the structural and functional properties (Chen et al., 2023; Li et al., 2020). As one kind of packaging carrier, the resistance of PDEs to heat treatment not only reflects their structural stability but also affects the activity of encapsulated substances (such as EGCG, β-carotene, and curcumin). The thermal degradation of these active substances mainly triggered by heat treatment (Ramzan et al., 2025). Therefore, search for feasible strategies to improve the thermal stability of PDEs is crucial for practical application in food field.

Typically, polysaccharides addition is an effective method to regulate the thermal stability of emulsions. Particularly KGM, containing numerous hydroxyl groups, exhibits strong hydrophilicity and influence the rheological properties of emulsions. Additionally, as a dietary fiber, KGM contributes to the regulation of body weight, blood glucose, and lipid metabolism (Ding et al., 2025). Research reports indicate that when KGM was added to whey protein-stabilized oil-in-water emulsions, curcumin degradation caused by heat treatment was reduced by approximately 20% (He et al., 2020). It is generally believed that KGM increase the viscosity of the emulsion system and reduce heat transfer efficiency. Additionally, the network structure formed by KGM between oil droplets effectively inhibits the flocculation caused by heat treatment, which is beneficial for maintaining the stability of the entire system. Taken together, KGM is a promising plant-based and low-calorie stabilizer for PEDs.

Concurrently, existing research indicates that while heat treatment may induce thermodynamic instability in emulsions, it is widely employed for texture regulation and shelf life of emulsions (Tao et al., 2025; Wang et al., 2024). For instance, Choi et al. experimentally demonstrated that soy protein isolate emulsions with curdlan and transglutaminase can form elastic gels after heat treatment, which can be used as a potential fat substitute (Choi et al., 2024). Miao et al. showed that curcumin-loaded high internal phase emulsions with chitosan addition stabilized by beet pectin-tannic acid complexes exhibited higher viscoelasticity, delayed lipid hydrolysis as suffered 90 °C treatment for 30 min (Miao et al., 2021). Inspired by the aforementioned research, heat treatment offers new possibilities for regulating the texture of PEDs in presence of KGM. However, limited studies conducted systematic research on the structure and digestion behavior of KGM-based PDEs under different heat stress.

Therefore, in this paper, KGM-based co-loaded PDEs published in previous study were subjected to thermal treatments (Xu et al., 2025). The effects of different thermal treatments (50–90 °C) on water-holding capacity, microstructure, and rheological properties of PEDs with and without KGM loading were investigated. Additionally, the texture characteristics of the heated treated PDEs were determined and compared with those of commercially available cheese sticks to explore their potential application in the development of functional soft gel foods. Finally, the retention rate of the encapsulated active substances was evaluated, and in vitro gastrointestinal digestion test was conducted to assess the enhanced gastrointestinal tolerance of the PDEs after thermal treatment.

2. Materials and methods

2.1. Materials

KGM (Mw 1.4 × 106 Da) is supplied by Yizhi Konjac Industry Co. Ltd. (Yichang, China). Lysozyme (Ly, from chicken egg white purity >90%, Mw 1.43 × 104 Da) and xanthan gum (XG, 3.0 × 106–2.0 × 107 Da) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and Shanghai Yuanye Biotechnology Co., Ltd. (Shanghai, China). Camellia oil was obtained from local supermarket that produced by Henan Luda Tea Oil Co., Ltd. Nile red, nile blue A and other reagents were purchased from Sinopharm Chemical Reagent Co., Ltd.

2.2. Preparation of KGM-based PDEs

PDEs were prepared by a two-step emulsification method with previous method (Yin et al., 2025). Briefly, sucrose (2.0%, w/w) was dissolved in W1 phase as osmotic pressure regulator. Oil phase was prepared by mixing PGPR (5%, w/w) and camellia oil. W2 phase was prepared with (0% or 0.2%, w/w) KGM and XG/Ly NPs (2.0%, w/w) that prepared using our previous method (Xu et al., 2015; Xu, Yin, Cao, et al., 2024). The W1/O emulsions were prepared by homogenizing (18,000 rpm, 3 min) W1 phase and the oil phase (70%) for 3 min using an Ultra turrax T18 homogenizer (IKA, Germany). KGM-based PDEs was obtained by homogenizing (12,000 rpm, 2 min) 40% W1/O emulsion with 60% W2. During preparation, EGCG (8 mg/g) and β-carotene (4 mg/g) were added in W1 phase and oil phase, respectively. Finally, each sample was suffered to different heat treatments (50 °C for 30 min, 70 °C for 30 min and 90 °C for 30 min), and then cooled to room temperature.

2.3. Water holding capacity of KGM-based PDEs

The water holding capacity (WHC) of PDEs was determined using previous method with a slight modification (Qian et al., 2023). Briefly, KGM-based PDEs (1 g) were placed in a 1.5 mL plastic centrifuge tube and subjected to high-speed centrifugation (10,000 rpm, 20 min). And then the supernatant was removed, and the WHC of PDEs with different heat treatments was calculated using the following formula (1):

WHC%=W2−W0W1−W0×100 (1)

where W0 represents the mass of the plastic centrifuge tube (g), W1 represents the total mass of PDEs and the plastic centrifuge tube before centrifugation (g), W2 represents the total mass of PDEs and the plastic centrifuge tube after remove the supernatant (g).

2.4. Rheological behavior of PDEs after heat treatment

The rheological behavior of PDEs were measured by DHR-2 rheometer (USA) with a 40 mm diameter plate and 1.0 mm gap at 25 °C. Fresh KGM-based PDEs were conducted temperature sweep to monitor the viscoelastic characteristics during different heat treatment process (50 °C for 30 min, 70 °C for 30 min, and 90 °C for 30 min). The initial and final temperatures were 25 °C. The strain was 0.1%, the frequency was 1.0 Hz, and the rate was 1 °C/min. The apparent viscosity as a function of the shear rate was determined during the range of 0.1–100 s−1. Frequency sweep of PDEs after heat treatment was conducted with frequency ranging from 0.1 rad/s to 100 rad/s at 0.1% strain. The shear applied to the three-interval thixotropic (3-ITT) intervals were 0.1 s−1, 120 s; 10 s−1, 30 s and 0.1 s−1, 120 s, respectively. The data under different strains (1.00%, 5.04%, 10.12%, 51.63%, 103.50%, 506.73%) scanning were collected.

2.5. Microstructure observation of PDEs after heat treatment

The microstructure of PDEs after different heat treatment was viewed by a confocal laser microscope (SP8, Leica, Germany) and cryo-scanning electron microscope (Quanta 450, FEI, USA). For CLSM, PDEs were dyed with 40 μL Nile Red/Nile Blue A mixed solution (0.1%, w/v) for 0.5 h. And then it was observed under a 40× objective microscope with excitation light sources of 488 nm and 633 nm. For cryo-SEM observation, the PDEs were first frozen in liquid nitrogen for 30 s. Subsequently, they were observed after sublimation and plating gold under high-vacuum conditions. The accelerating voltage and temperature were 10 kV and − 140 °C, respectively.

2.6. Appearance and TPA characteristics of PDEs after heat treatment

KGM based PDEs suffered different heat treatments were poured into commercial cheese sticks molds. And then then refrigerated at 4 °C for 2 h before demolding. Their appearances were photographed and recorded. The TPA tests were conducted with P/0.5 cylindrical probe with 30% deformation. The test speed was 60 mm/min and the minimum gripping force was 0.1 N. Each sample was tested five times.

2.7. Stability of EGCG and β-carotene in PDEs after heat treatment

The thermal stability of PDEs was evaluated by measuring the retention rates of EGCG and β-carotene after heat treatment. Before the test, PDEs (1 g) was added to 5 mL of water, and the structure was thoroughly disrupted through high-speed shearing (10,000 rpm, 2 min). Then, the retention rates of EGCG and β-carotene in the sample were determined according to our previous method (Han et al., 2023).

2.8. In vitro digestion behavior and bioaccessibility of PDEs after heat treatment

The PDEs suffered 70 °C heat treatment for 30 min was used to investigate the in vitro digestion behavior and bioaccessibility. In vitro digestion, simulated gastric fluid (SGF) and simulated intestinal fluid (SIF) were prepared according to previous method (Thomassen et al., 2024). During gastric digestion, PDEs (9 mL), mixed with SGF (9 mL) were stirred at 37 °C for 2 h. And then, the pH value was adjusted to 7.0. Subsequently, SIF (18 mL) was added into the mixture and stirred at 37 °C for another 2 h. The release of Free fatty acids (FFAs) was determined by measuring the amount of 1 mol/L NaOH needed to keep the pH at 7.0 using eqs. (2).

FFAs%=VN×CN×ML2WL×100 (2)

where VN and CN were the consumption of NaOH (L) and the concentration of NaOH (1 mol/L). ML and WL indicated the total weight (g) and molecular weight (g/mol) of camellia oil, respectively.

After vitro digestion, the mixtures were centrifuged (8000 rpm, 15 min) at 4 °C. The obtained micelle solutions were used to measure the bioavailability of β-carotene and EGCG. The contents of EGCG and β-carotene were determined according to previous method (Liu et al., 2025; Tang et al., 2024). The bioaccessibility of them was calculated using eqs. (3).

Bioaccessibility%=C1C0×100 (3)

where C1 and C0 represent the content in the micelles, and the initial content of EGCG or β-carotene in the PDEs respectively.

2.9. Statistical analysis

Data were expressed as mean ± standard deviation (SD, error bar) from triplicate experiments. One-way ANOVA and Duncan's multiple comparison tests were performed using SPSS 25 software (SPSS Inc., Chicago, Illinois, USA), with a significance level of P < 0.05.

3. Results and discussion

3.1. WHC of KGM-based PDEs after heat treatment

WHC represents a crucial index to evaluate the water retention ability of emulsion gel system. In general, WHC can be regulated by many factors, such as protein, polysaccharide concentration and environmental conditions (Han et al., 2024). Emulsion gels with WHC are essential for the development of gel-based foods. Compared with PDEs, the WHC of KGM-based PDEs was significantly increased (P < 0.05). In the non-heat treated group, the WHC of PDEs increased from 53.5% to 91.1% (Fig. 1). This phenomenon can be attributed to the fact that KGM, abundant in hydrophilic hydroxyl and carboxyl groups, is capable of binding to water molecules in the water phase via hydrogen bonding. Consequently, this interaction enhances the WHC of the whole system (Tu et al., 2025). After heat treatment at 50 °C–90 °C, the WHC of PDEs remained at a low level with a slight decrease. However, the WHC of KGM-based PDEs was further increased, reaching up to 100% which exhibited a significant difference compared to that of PDEs (P < 0.05). This may be due to the fact that heat treatment promotes the interaction between KGM and XG/Ly NPs in oil droplets. This interaction leads to the formation of a dense cross - linked network, which can effectively counteract the instability induced by high centrifugal force (Liu, He, et al., 2024). Similarly, previous reports confirmed that pasteurization (65 °C for 30 min) can significantly improve the WHC of KGM-based Pickering emulsion, which is consistent with the results (Xu, Yin, Yue, et al., 2024).

Fig. 1.

Fig. 1

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Water holding capacity of PDEs after different heat treatments.

3.2. CLSM observations of PDEs after heat treatment

From the results of CLSM images (Fig. 2), all PDEs still have the typical structure of double emulsion after heat treatment, but there are differences in the size of oil droplets. According to Stokes' law, the smaller the particle size the higher the stability of the emulsion (Zhang, Zhang, et al., 2024). For PDEs, the flocculation of oil droplets increased with the increase of temperature, and led to the increase of oil droplet size. This was mainly due to the fact that temperature promoted the denaturalization and aggregation of XG/Ly NPs, resulting in reduced adsorption at the oil-water interface. Besides, heating accelerated the collision of oil droplets, and resulted in an increase in the size of oil droplets. The phenomenon was more obvious when suffered 90 °C treatment. However, the flocculation is obviously repressed in the presence of KGM, which enhanced the steric hindrance between oil droplets by increasing the viscosity of W2 phase and inhibited the movement of oil droplets. Additionally, there is a synergistic stabilization effect between XG/Ly NPs and KGM. The effect is a dense three-dimensional network caused by heat-induced gelation (Liu, Wang, et al., 2024).

Fig. 2.

Fig. 2

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CLSM images of PDEs after different heat treatments, scale bar 25 μm.

3.3. Rheological behaviors of KGM-based PDEs after heat treatment

Temperature sweep of PDEs under different heat treatments is shown in Fig. 3. In the heating stage, G“ and G' decreased rapidly due to the destroyed internal structure resulting from the depolymerization of polymer molecular chains in the whole system. Besides, the reduction of modulus value is positively correlated with the temperature. When there is no KGM, the curves of G “and G' cross as the temperature increases. While for KGM-based PDEs, the G' higher than G“, displaying elastic characteristics. The values of G" and G' of increase slowly in the constant temperature stage. And then the value of G“ and G' increase sharply in the subsequent cooling stage. This phenomenon is attributed to the enhancement of hydrogen bond interaction induced by cooling. It promotes the reconstruction of a stable gel network between XG/Ly NPs and KGM. Generally, tanδ <1 indicates that the material mainly exhibits elastic characteristics. The smaller the tanδ value, the stronger the elasticity. It found that KGM addition significantly increases the G' and G" of PDEs. The obvious reduction of tanδ also reflects the enhancement of viscoelasticity. However, the viscoelasticity of KGM-based PDEs is different under different heat treatments. As the temperature rises from 50 °C to 90 °C, the tanδ decreases from 0.095 to 0.058, and then increases to 0.062. The KGM-based PDEs has the smallest tanδ at 70 °C heat treatment. It means that the gel network of the PDEs system is more stable and elastic under the heat treatment. Similar studies have confirmed that Pickering emulsions co-stabilized by melolin/sodium caseinate nanoparticles and KGM showed higher viscoelasticity under heat treatment at 70 °C in the temperature range of 40 °C and 100 °C (Xu et al., 2023).

Fig. 3.

Fig. 3

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Temperature sweep curves (A: 50 °C for 30 min, B: 70 °C for 30 min, C: 90 °C for 30 min) as well as modulus values and tanδ (D: 0% KGM, E: 0.2% KGM) at the end of the temperature sweep for PDEs after different heat treatments.

Fig. 4 shows the viscosity of PDEs with shear rate after different heat treatments. With the increase of shear rate, the internal structure in PDEs is destroyed, which resulted in the sharp reduction in the shear viscosity. While the heat treatment temperature is favorable to increase the viscosity of PDEs. With the help of KGM, PDEs exhibits higher viscosity (Fan et al., 2023). Besides, the viscosity increases first and then decreases at high shear rate. The highest viscosity appears at 70 °C because KGM forms a more stable network structure at the temperature where PDEs becomes emulsion gel with stronger solid-like behavior and shows strong shear resistance. For all PDEs, both G' and G" increased as the frequency gradually increased (Fig. 5). PDEs with 2% XG/Ly NPs displayed viscous characteristics at low frequencies, while elastic feature is dominant at higher frequencies. The heat treatment temperature reduced the frequency dependence and improved the viscoelasticity. In the presence of KGM, the G' and G" values significantly increased in the whole frequency range. When the heat treatment temperature was 70 °C, the high viscosity of PDEs gave it a more stable internal network structure, making G' value higher than that of other heat treatment groups. However, the G' of 90 °C heat treatment group was lower than that of the 70 °C heat treatment group. The reason may result from the reduced interface stability and structural strength of the network due to excessive cross-linking of KGM and wettability changes of XG/Ly NPs caused by high temperature.

Fig. 4.

Fig. 4

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Steady-state shear curves of PDEs after different heat treatment (A: 0% KGM, B: 0.2%).

Fig. 5.

Fig. 5

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Frequency sweep of PDEs after heat treatment (A: 0% KGM, B: 0.2% KGM).

The viscoelastic behavior of PDEs after heat treatment was consistent with the results of frequency sweep (Fig. 6). KGM significantly improves the viscoelasticity of PDEs and prolongs the length of the linear viscoelastic region. In addition, the small loss factor (tanδ < 0.2) in the linear viscoelastic region of the KGM-based PDEs after heat treatment further proved its stronger solid-like behavior. In the nonlinear visco-elastic range, the G' of KGM-based PDEs decreased, and the G“ first increased and then decreased. While both G' and G" of PDEs decreased gradually without KGM. The phenomenon reflected the collapse of the internal structure of PDEs. Therefore, the relative modulus (G/G0) was calculated by normalized treatment of G' and G" to further analyze the impact of heat treatment on the nonlinear rheological behavior of PDEs (Fig. 7). The relative modulus values of PDEs decreased with the increase of the oscillatory strain under heat treatments. The displayed typical strain thinning behavior resulted from the excessive strain destroying the intermolecular or intramolecular interaction. In contrast, for heat treatment KGM-based PDEs, G"/G0 first increased and then decreased, showing weak strain overshoot behavior. In addition, the strain corresponding to the maximum value of G"/G0 reached at 70 °C (Fig. 7D). This result indicated after heat treatment and oscillatory strain, the network of KGM based PDEs reconstructed through hydrogen bond, and the reconstruction speed exceeded the destruction speed, finally forming a stronger elastic structure (Zhang et al., 2022). The weak strain overshoot behavior was also observed in the high-interior phase Pickering emulsion stabilized by chitosan nanoparticles. In the study, the value first improved and then weakened with the increase of heat treatment (25 °C–80 °C), and reached the maximum value at 65 °C (Yang et al., 2024).

Fig. 6.

Fig. 6

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Oscillatory strain sweep curves (A: 0% KGM, B: 0.2% KGM) and tanδ variation curves (C: 0% KGM, D: 0.2% KGM) of PDEs after different heat treatments.

Fig. 7.

Fig. 7

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Relative modulus change curves of PDEs after different heat treatments (A, C, E: 0% KGM, B, D, F: 0.2% KGM).

Fig. 8 showed the elastic Lissajous curve of PDEs after heat treatment. At a small strain (1%), the Lissajous curves of all PDEs displayed as narrow regular ellipses, mainly exhibiting linear viscoelastic responses of elastic-like solid behavior. The results were consistent with Fig. 6. While at high oscillatory strain (506.3%), the Lissajous curve transformed from ellipse to a non-ellipse. This nonlinear response also indicated that the network structure of PDEs was destroyed under large deformation. The increased energy dissipation resulted in the evolution process from elasticity to viscosity (Zhao, Chu, et al., 2023). It found that the elliptic area of Lissajous curve displayed a decreasing trend with the increase of temperature under 1% and 10.18% strain. Especially for KGM-based PDEs shows a smaller elliptic area and stronger elastic-like behavior. The lower strain sensitivity at 70 °C further indicated that moderate heat treatment promoted the formation of a more stable gel network and the anti-deformation ability of KGM-based PDEs (Zhao, Li, et al., 2023).

Fig. 8.

Fig. 8

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Lissajous curves of PDEs after heat treatment (A: 0% KGM, B: 0.2% KGM).

3.4. Texture characteristics of KGM-based PDEs after heat treatment

Fig. 9 displayed the appearance of cheese sticks and PDEs after heat treatments. Due to the lack of KGM, PDEs has no fixed shape after heat treatment and has a certain fluidity, making it impossible to simulate cheese sticks. In contrast, after adding 0.2% KGM, heat treatment caused gelation of KGM-based PDEs, which enabled it to exhibit good self-supporting ability after molding and demolding with a mold. It had an appearance similar to commercially available cheese sticks. Meanwhile, β-carotene, as an edible natural pigment, endows KGM-based PDEs with a unique orange-yellow color, which is conducive to attracting consumers by enhancing the acceptance of sensory color. It showed that the KGM-based PDEs after heat treatments have the potential to substitute commercial cheese sticks.

Fig. 9.

Fig. 9

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Appearance of cheese stick and PDEs after heat treatments.

Texture characteristics of KGM-based PDEs after heat treatments were further investigated compared with commercial cheese sticks (Fig. 10). Hardness reflects the force required to cause the deformation of the emulsion gel. Springiness reflects the ability of an emulsion gel to return to its original shape after being compressed. For commercial cheese stick, the hardness and springiness were 1.28 N and 1.55 mm, respectively, which were higher than that of KGM based PDEs after heat treatments. It is worth noting that with the increase of heat treatment temperature, the hardness and springiness of KGM based PDEs first increase and then decrease. They reached the highest values when treated at 70 °C for 30 min, at which they were closest to the hardness and springiness of commercial cheese sticks. It demonstrated that moderate heat treatment endowed XG had a synergistic gelation effect with KGM, which improved the stability of the gel network in PDEs (Min et al., 2023). Similar reports showed that gellan enhanced the crosslinking density of the PDEs network, and exhibit similar hardness and elasticity with commercial cheese sticks (Xiong et al., 2022). However, when the heat treatment temperature reached 90 °C, the network structure stability of PDEs were weaken, displaying a decrease in hardness and springiness. After heat treatment, the gumminess of PDEs shared a similar trend to that of hardness and springiness. This is consistent with the fact that the high viscosity of KGM under heat treatment reduces droplet size and enhances the stability of PDEs (Fig. 2), as well as the results of the solid rheological properties of KGM after heating (Fig. 3 and Fig. 6). The gumminess value with 70 °C heat treatment group is closer to that of commercial cheese sticks. It is worth noting that regardless of the heat treatment temperature, the cohesiveness of PDEs is similar to that of commercial cheese sticks. Texture analysis indicated that PDEs with heat treatment at 70 °C had similar texture properties to commercial cheese sticks and can be used as a potential functional emulsion gel food.

Fig. 10.

Fig. 10

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Textural properties of heat treated PDEs and commercial cheese sticks. Different letters denote significant differences in textural characterizations (P < 0.05).

3.5. Stability of EGCG and β-carotene co-load in PDEs after heat treatment

Fig. 11 showed the retention rates of the two functional components after different heat treatments. With the increase of heat treatment temperature, the retention rates of EGCG and β-carotene showed a downward trend. When the heat treatment temperature increased from 50 °C to 90 °C, the retention rates of free EGCG and β-carotene decreased significantly from 79.2% and 90.3% to 69.7% and 74.7%, respectively (p < 0.05). When co-loaded in PDEs system, the dense interfacial film provided by XG/Ly NPs offered additional protection for EGCG and β-carotene, which significantly increased the retention rates of both functional components (p < 0.05). The retention rates of EGCG and β-carotene were further increased in presence of KGM. It because that heat treatment promoted the gelation of the system. The gel network provided by KGM reduced the heat transfer efficiency, thereby improving the thermal stability of the two heat-sensitive components (Bu et al., 2023). Specifically, the retention rate of EGCG decreased from 96.2% to 89.0%, β-carotene decreased from 98.7% to 96.7% when the temperature increased from 50 °C to 70 °C. The retention rate of the two functional components further decreased significantly as temperature increased to 90 °C. Additionally, they were all higher than those in the group without KGM (p < 0.05).

Fig. 11.

Fig. 11

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The retention of EGCG (A) and β-carotene (B) in PDEs after different heat treatments. Different letters denote significant differences (P < 0.05).

3.6. In vitro digestion and bioaccessibility of PDEs after heat treatment

Fig. 12 showed the CLSM images after gastric digestion and intestinal digestion. It can be seen that each sample displayed the typical structure of a double emulsion in the extremely acidic environment of gastric digestion regardless of whether KGM is present or not. It noted that PDEs without KGM obvious flocculation and aggregation of oil droplets in the gastric digestion stage, and the size of the oil droplets was larger as compared with that of KGM based PDEs. This indicates that KGM addition was beneficial to improving the resistance to extreme acids (Zhang, Zhang, et al., 2024). In the intestine digestion stage, the pH of digestive fluid changed from acidic to neutral, and the added lipase and bile salts started to replace the XG/Ly NPs adsorbed at the oil-water interface, causing lipid hydrolysis and releasing FFA (Lin et al., 2025). The CLSM images showed that after intestinal digestion, the oil droplets of heat-treated PDEs without KGM were hydrolyzed and did not have double emulsion structure. Therefore, the encapsulated EGCG and β-carotene are released in the stage. However, the lipolysis process was inhibited after the KGM addition. Some intact oil droplets with double emulsion structure were still observed, which provided a structural basis for the reduction of FFA release.

Fig. 12.

Fig. 12

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Microscopic pictures of PDEs with or without KGM at the end of gastric and intestinal digestion after heat treatment at 70 °C for 30 min, scale bar 25 μm.

It is worth noting that the heat-treated PDEs with KGM showed a lower cumulative FFA release level (16.3%) compared to that without KGM (Fig. 13). The results indicated that heat treatment enhanced the strength of the cross-linked network formed by KGM and promoted gelation, which enabled the oil droplets to be effectively encapsulated (Lv et al., 2022). Under the protection, the adsorption of lipase and bile salts at the oil-water interface was further hindered, thereby delaying the hydrolysis of oils. The inhibited lipolysis means that there were fewer mixed micelles available for dissolving EGCG and β–carotene, which made the PDEs showed lower bioaccessibility (EGCG: 0.78%, β-carotene: 0.67%) (Chen et al., 2024), corroborated by the conclusions of research conducted by Lu et al. (Lu et al., 2018). The heat treatment promoted the gelation of KGM-based PDEs and effectively protected EGCG and β -carotene, endowing it with good gastrointestinal tolerance. Therefore, the PDEs may release EGCG and β-carotene to exert physiological functions after entering the large intestine stage, in which KGM could be degraded by polysaccharide hydrolases and the double emulsion structure also be destroyed (Wei et al., 2021).

Fig. 13.

Fig. 13

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FFA release (A) and EGCG/β-carotene bioaccessibility (B) in PDEs after heat treatment at 70 °C for 30 min.

4. Conclusion

Effects of heat treatments on the WHC, microstructure, rheological properties and textural properties of the KGM-based PDEs co-loaded system were explored. KGM-based PDEs showed a smaller oil droplet size after heat treatment than that without KGM. Meanwhile the water-holding capacity was significantly improved. Especially after heat treatment at 70 °C for 30 min, KGM-based PDEs had a more stable gel network, showing higher viscoelasticity. LAOS results indicated the treatment enhanced the elastic response and improved the anti-deformation ability of the PDEs structure. With the increase of heat temperature, the hardness, springiness and adhesive property of KGM-based PDEs first increased and then decreased. Under 70 °C treatment, the texture characteristics of KGM-based PDEs were close to those of commercial cheese sticks, and have the potential to construct functional emulsion gel-based foods. The degradation of EGCG and β-carotene caused by heat treatment was inhibited after gelation of KGM-based PDEs. After heat treatment, the gelation hindered the hydrolysis of lipids by bile salts and lipase, resulting in a decrease of FFA release and low bioaccessibility. Hopefully provide valuable theoretical guidance for the application of KGM-based PDEs in plant-based soft gel foods.

CRediT authorship contribution statement

Wei Xu: Supervision, Project administration, Methodology. Mengyao Kang: Methodology, Data curation. Lanlan Zhang: Data curation. Yongpeng Yin: Methodology, Data curation. Denglin Luo: Writing – review & editing. Xiaofan Zhang: Validation. Lihua Zhang: Supervision, Methodology. Lala Li: Validation. Siqi Du: Validation.

Declaration of competing interest

No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication.

Acknowledgment

This work was supported by Natural Science Foundation of Henan Province (Grant No. 252300420218), Program for Science & Technology Innovation Talents in Universities of Henan Province (Grant No. 24HASTIT060), Key Scientific Research Project of Higher Education Institutions in Henan province (24A416004), Key Research and Development Project of Henan Province (Grant No. 231111112400), National Natural Science Foundation of China (Grant No. U2004160), and Nanhu Scholars Program for Young Scholars of XYNU.

Data availability

The data that has been used is confidential.

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