Stability and Rheological Behavior of Mayonnaise-like Emulsion Co-Emulsified by Konjac Glucomannan and Whey Protein

The aim of this work was to study the physical stability and rheological properties of an oil-in-water emulsion stabilized by a konjac glucomannan–whey protein (KGM-WP) mixture at a konjac glucomannan concentration of 0.1–0.5% (w/w) and a whey protein concentration of 1.0–3.0% (w/w). The droplet size, microstructure, stackability, flow behavior, and viscoelastic properties were measured. The experimental results showed that with an increase in KGM and WP concentrations, the droplet size (D4,3) of the emulsion gradually decreased to 12.9 μm, and the macroscopic performance of the emulsion was a gel-like structure that can be inverted and resist flow and can also be extruded and stacked. The static shear viscosity and viscoelasticity generally increased with the increase of konjac glucomannan and whey protein concentration. Emulsions were pseudo-plastic fluids with shear thinning behavior (flow behavior index: 0.15 ≤ n ≤ 0.49) and exhibited viscoelastic behavior with a storage modulus (G′) greater than their loss modulus (G″), indicating that the samples all had gel-like behavior (0.10 < n′ < 0.22). Moreover, storage modulus and loss modulus of all samples increased with increasing KGM and WP concentrations. When the concentration of konjac glucomannan was 0.3% w/w, the emulsion had similar rheological behavior to commercial mayonnaise. These results suggested that the KGM-WP mixture can be used as an effective substitute for egg yolk to make a cholesterol-free mayonnaise-like emulsion. The knowledge obtained here had important implications for the application of protein–polysaccharide mixtures as emulsifiers/stabilizers to make mayonnaise-like emulsions in sauce and condiments.


Introduction
Mayonnaise is an oil-in-water emulsion with gel-like properties that can be manufactured by emulsifying egg yolk, oil, vinegar, and other spices [1]. Because of its excellent taste and flavor, mayonnaise has become an important food sauce. With the convenience food and prepared dish industries growing rapidly, the development and application of mayonnaise have great prospects. However, traditional mayonnaise is high in cholesterol (prepared using egg yolk) and fat (containing above 65% oil), which can increase the risk of various chronic diseases such as obesity, cardiovascular disease, and diabetes [2]. In addition, the use of egg yolk used as an emulsifier poses a risk of Salmonella contamination [3,4]. For these reasons, traditional mayonnaise has been unable to meet consumer demand for healthy food. Therefore, recent research has focused on optimization the of mayonnaise formulas to enhance its nutritional function and health safety.
The improvement of mayonnaise formula was mainly focused on two effective strategies. The first strategy involved partially or completely replacing the egg yolk with alternative emulsifiers to fabricate low-cholesterol or cholesterol-free mayonnaise, which addressed the issue of high cholesterol content in traditional mayonnaise. These alternative emulsifiers included small-molecule surfactants [5], animal/plant proteins [6,7],

Droplet Size Distribution and Mean Droplet Size Measurements
The droplet size distribution and mean droplet size measurements (D 4,3 ;D 3,2 ) of the samples were measured using Mastersizer 2000 (Malvern Instruments, Malvern, UK). The refractive indexes of flaxseed oil and water used in calculations were 1.474 and 1.33, respectively.

Microstructure Analysis
The microstructure of the samples was observed using CX40 metallographic microscope (Ningbo Shunning Instrument, Ningbo, China) equipped with a digital camera. A drop of the diluted emulsion sample was placed on a microscope slide and then covered with a coverslip. The micrographs of the samples were captured at 400× magnification.

Appearance Texture and Stackability
To observe the appearance texture and stackability of the emulsion, it was squeezed onto the slide using a dropper, and its appearance texture was recorded after 5 min. Additionally, the sample was drawn on the slide using a disposable plastic head dropper, and the letter "Z" was left for 5 min to observe its extrusion formation and stackability [5]. This method used to characterize the texture and extrusion characteristics of mayonnaiselike sauce was simple and intuitive, and it was also suitable for the actual process of extrusion from bottles when the system was applied to semi-solid condiments.

Rheological Properties
A Kinexus Lab+ rheometer (Malvern Instruments, Malvern, UK) equipped with a cone plate (40 mm, and 4 • ) was employed to characterize the rheological properties of the emulsion. The test was conducted at 25 • C. After loading the sample, two minutes were required for the sample to return to its original state. After the sample was loaded onto the stage, the viscosity of emulsion was recorded at the range of 0.1~100 s −1 shear rate. To assess the non-linear relationship between shear stress and shear rate of emulsions and obtain the rheological parameters, such as consistency coefficient (K) and flow index (n), the Herschel-Bulkley model (Y = Y 0 + Kx n ) (Y: shear stress; x:shear rate) was used. Before the dynamic frequency sweeping experiments, the linear viscoelastic regions (LVR) were defined using the strain sweep test: angular frequency, 1.0 rad/s, amplitude strain 0.01-10%. According to the LVR, the subsequent frequency sweeping was performed at 0.5% stain, and the elastic modulus G and viscous modulus G were recorded at the range of 0.628~62.8 rad/s. The correlation between G and G was determined using the power law model (G = G 0 ·ω n and G = G 0 ·ω n ), where G and G were the storage modulus (Pa) and loss modulus (Pa), respectively; G 0 and G 0 were the storage modulus (Pa) and loss modulus (Pa) at angular frequency 1.0 rad/s, respectively; ω was the angular frequency (rad/s); n and n indicated the frequency dependence of the elastic modulus and loss modulus, respectively.

Statistical Analysis
All experiments were repeated three times. The results were expressed as means ± standard deviations (SD). Data and figures were processed using Excel and origin 2022. The differences between samples were calculated using LSD (SPSS 22.0). The letters a-d or A-D indicate significant differences between the samples (p < 0.05).

Impact of KGM Concentration Droplet Size
Distribution and mean size of droplets were crucial parameters in assessing the physical stability of the emulsion system. The impact of KGM concentration (at WP concentration 2.0% w/w) on the distribution and mean droplets size of emulsions was characterized ( Figure 1). For samples with no KGM (0 KGM), maximum D 4,3 values of 27.7 µm and D 3,2 values of 23.1 µm were obtained, and they showed uniform unimodal distributions. However, emulsions with varying concentrations of KGM had smaller D 4,3 and D 3,2 values but a bimodal distribution (a main peak located in small droplet size region and a shoulder peak located in a large droplet size region) ( Figure 1A), indicating lower droplet uniformity.
Furthermore, with the increase in KGM concentration, D 4,3 and D 3,2 were significant decreased from 27.7 µm to 12.9 µm and from 23.1 µm to 6.6 µm, respectively. Although the minimum D 4,3 of 12.9 µm and D 3,2 of 6.6 µm were obtained at KGM 0.5% w/w, a wider shoulder peak located in the large droplet size region (>100 µm) was measured. This was because KGM was a non-adsorbed linear neutral polysaccharide, which increased the viscosity and steric hindrance effect of the system. The increase in viscosity had two effects on the droplet size of the emulsion: (1) Due to the high viscosity and large steric hindrance effect, the coalescence, aggregation, and agglomeration of oil droplets in the system were inhibited, leading to a reduction in droplet size and an improvement in the stability of the emulsion. (2) In the process of mayonnaise-like emulsion preparation, excessive viscosity was not conducive to the dispersion of the oil phase and the adsorption of emulsifiers at the oil-water interface, which increased droplet size, resulting in an uneven droplet size distribution. Therefore, the addition of KGM could reduce the droplet size of emulsion, but if the concentration is too high, the uniformity of droplet size will be reduced. In addition, the appearance of the shoulder peak located in the larger droplet size range might also be due to the uneven dispersion caused by the high viscosity of sample during droplet size test, as supported by the microstructure observations. distributions. However, emulsions with varying concentrations of KGM had smaller D4,3 and D3,2 values but a bimodal distribution (a main peak located in small droplet size region and a shoulder peak located in a large droplet size region) ( Figure 1A), indicating lower droplet uniformity. Furthermore, with the increase in KGM concentration, D4,3 and D3,2 were significant decreased from 27.7 µm to 12.9 µm and from 23.1 µm to 6.6 µm, respectively. Although the minimum D4,3 of 12.9 µm and D3,2 of 6.6 µm were obtained at KGM 0.5% w/w, a wider shoulder peak located in the large droplet size region (>100 µm) was measured. This was because KGM was a non-adsorbed linear neutral polysaccharide, which increased the viscosity and steric hindrance effect of the system. The increase in viscosity had two effects on the droplet size of the emulsion: (1) Due to the high viscosity and large steric hindrance effect, the coalescence, aggregation, and agglomeration of oil droplets in the system were inhibited, leading to a reduction in droplet size and an improvement in the stability of the emulsion. (2) In the process of mayonnaise-like emulsion preparation, excessive viscosity was not conducive to the dispersion of the oil phase and the adsorption of emulsifiers at the oil-water interface, which increased droplet size, resulting in an uneven droplet size distribution. Therefore, the addition of KGM could reduce the droplet size of emulsion, but if the concentration is too high, the uniformity of droplet size will be reduced. In addition, the appearance of the shoulder peak located in the larger droplet size range might also be due to the uneven dispersion caused by the high viscosity of sample during droplet size test, as supported by the microstructure observations. Visual Appearance, Texture, and Microstructure Stability of the mayonnaise-liked emulsion was observed visually by taking photos 0 days and 270 days after preparation. All the samples with different KGM concentrations were found to be stable ( Figure 2A), even after 9 months of storage. Although there were no visible differences in the appearance of the samples, their textures and microstructures were noticeably different. If emulsions had stacking properties after extrusion, they were considered to possess a gel-like behavior [5]. As shown in Figure 2C, at 0-0.5% of KGM fractions, all samples displayed uniform texture. Notably, the emulsion stabilized by WP alone (in the absence of KGM) was flowable, while those co-stabilized by WPI and KGM exhibited a distinct self-supporting texture. Furthermore, with the KGM concentration increasing from 0.1% to 0.5%, the stackability and extrusion formability of samples were Visual Appearance, Texture, and Microstructure Stability of the mayonnaise-liked emulsion was observed visually by taking photos 0 days and 270 days after preparation. All the samples with different KGM concentrations were found to be stable ( Figure 2A), even after 9 months of storage. Although there were no visible differences in the appearance of the samples, their textures and microstructures were noticeably different. If emulsions had stacking properties after extrusion, they were considered to possess a gel-like behavior [5]. As shown in Figure 2C, at 0-0.5% of KGM fractions, all samples displayed uniform texture. Notably, the emulsion stabilized by WP alone (in the absence of KGM) was flowable, while those co-stabilized by WPI and KGM exhibited a distinct self-supporting texture. Furthermore, with the KGM concentration increasing from 0.1% to 0.5%, the stackability and extrusion formability of samples were improved, indicating that the presence of KGM with increasing concentrations facilitated gel-like network formation [29].
The strengthening effect of the emulsions due to the presence of KGM could also be augmented by their microstructure ( Figure 2B). With increasing KGM concentration, the droplet size of samples gradually decreased. This result was in good accordance with Figure 1. Moreover, droplets became more tightly packed as the KGM concentrations increased. It had been reported that an increase in packing density could increase the interactions of droplets and the viscosity of emulsion, promoting the formation of network structure [30]. improved, indicating that the presence of KGM with increasing concentrations facilitated gel-like network formation [29]. The strengthening effect of the emulsions due to the presence of KGM could also be augmented by their microstructure ( Figure 2B). With increasing KGM concentration, the droplet size of samples gradually decreased. This result was in good accordance with Figure 1. Moreover, droplets became more tightly packed as the KGM concentrations increased. It had been reported that an increase in packing density could increase the interactions of droplets and the viscosity of emulsion, promoting the formation of network structure [30].

Impact of WP Concentration
Droplet Size Figure 3 shows droplet size distribution of emulsion with different WP concentration at KGM 0.3% w/w. In the absence of WP (0 WP), emulsion had a main peak located in the large droplet size range with a shoulder peak located in the small droplet size range. A maximum D4,3 123.1 µm and a maximum D3,2 93.8 µm were recorded, suggesting that the droplet size of the system was large and uneven. KGM, a hydrophilic polysaccharide, could not stabilize the oil-water interface alone. However, in the presence of WP, the main peaks of emulsions were all located in the smaller droplet size range, and the average droplet size of samples significantly decreased to less than 20 µm (D4,3) and 10.0 µm (D3,2).
However, the average droplet size of the samples with different WP concentrations showed no significant difference. When the emulsifier amount was sufficient to cover the oil droplets, the concentration of emulsifier in aqueous phase did not significantly affect droplet size [31]. Therefore, in this work, 1.0% WPI was found to be sufficient to cover the oil-water interface formed by a 70% oil phase.  Figure 3 shows droplet size distribution of emulsion with different WP concentration at KGM 0.3% w/w. In the absence of WP (0 WP), emulsion had a main peak located in the large droplet size range with a shoulder peak located in the small droplet size range. A maximum D 4,3 123.1 µm and a maximum D 3,2 93.8 µm were recorded, suggesting that the droplet size of the system was large and uneven. KGM, a hydrophilic polysaccharide, could not stabilize the oil-water interface alone. However, in the presence of WP, the main peaks of emulsions were all located in the smaller droplet size range, and the average droplet size of samples significantly decreased to less than 20 µm (D 4,3 ) and 10.0 µm (D 3,2 ). The impact of WP concentration on the visual appearance, texture, and microstructure of the emulsion was observed (Figure 4). It was found that using KGM alone at c = 0.3% (in absence of WP) resulted in an unstable emulsion system, with free oil still noticeable and an oily yellow visual appearance ( Figure 4A). This was consistent with  However, the average droplet size of the samples with different WP concentrations showed no significant difference. When the emulsifier amount was sufficient to cover the oil droplets, the concentration of emulsifier in aqueous phase did not significantly affect droplet size [31]. Therefore, in this work, 1.0% WPI was found to be sufficient to cover the oil-water interface formed by a 70% oil phase.

Impact of WP Concentration Droplet Size
The impact of WP concentration on the visual appearance, texture, and microstructure of the emulsion was observed (Figure 4). It was found that using KGM alone at c = 0.3% (in absence of WP) resulted in an unstable emulsion system, with free oil still noticeable and an oily yellow visual appearance ( Figure 4A). This was consistent with the understanding that KGM was a water-soluble hydrophilic colloid with weak surface activity. Microstructure and stackability results further showed that in the absence of WP, the system was unstable, with large particle size. However, samples co-stabilized by WP and KGM at different WP concentrations displayed a uniform milky white color and showed excellent stacking and extrusion properties ( Figure 4B,C), indicating their effectiveness in forming a gel-like emulsion. The impact of WP concentration on the visual appearance, texture, and microstructure of the emulsion was observed (Figure 4). It was found that using KGM alone at c = 0.3% (in absence of WP) resulted in an unstable emulsion system, with free oil still noticeable and an oily yellow visual appearance ( Figure 4A). This was consistent with the understanding that KGM was a water-soluble hydrophilic colloid with weak surface activity. Microstructure and stackability results further showed that in the absence of WP, the system was unstable, with large particle size. However, samples co-stabilized by WP and KGM at different WP concentrations displayed a uniform milky white color and showed excellent stacking and extrusion properties ( Figure 4B,C), indicating their effectiveness in forming a gel-like emulsion.  Visual Appearance, Texture, and Microstructure In fact, as previously mentioned in Section 3.1.1, the formation of small oil droplets in the continuous phase with high viscosity was a crucial factor in the preparation of a stable emulsion to prevent coalescence. The microstructure results indicated that emulsions stabilized by KGM alone contained large droplets, whereas emulsions co-stabilized by KGM and WP had noticeably smaller droplets. This was observed regardless of the WP concentration and was in good accordance with Figure 3.

Rheological Behavior of Mayonnaise-like Emulsion
In this study, steady flow and dynamic viscoelasticity were used to test the rheology of the emulsion. The Herschel-Bulkley model (Y = Y 0 + Kx n ) (Y: shear stress; x: shear rate) was widely used to assess the non-linear relationship between shear stress and shear rate of emulsions and to determine rheological parameters, such as consistency coefficient (K) and flow index (n). The rheological parameters of samples were depicted in Tables 2 and 3. The determination coefficient (R 2 ) was higher than 0.9, indicating that the Herschel-Bulkley model showed a good fit to the experimental data.

Flow Behavior Impact of KGM Concentration
The effect of different concentrations of KGM on the changes in apparent viscosity and shear stress of emulsions containing 2.0% w/w WP was shown in Figure 5. Visual Appearance, Texture, and Microstructure In fact, as previously mentioned in Section 3.1.1, the formation of small oil droplets in the continuous phase with high viscosity was a crucial factor in the preparation of a stable emulsion to prevent coalescence. The microstructure results indicated that emulsions stabilized by KGM alone contained large droplets, whereas emulsions costabilized by KGM and WP had noticeably smaller droplets. This was observed regardless of the WP concentration and was in good accordance with Figure 3.

Rheological Behavior of Mayonnaise-like Emulsion
In this study, steady flow and dynamic viscoelasticity were used to test the rheology of the emulsion. The Herschel-Bulkley model (Y = Y0 + Kx n ) (Y: shear stress; x: shear rate) was widely used to assess the non-linear relationship between shear stress and shear rate of emulsions and to determine rheological parameters, such as consistency coefficient (K) and flow index (n). The rheological parameters of samples were depicted in Tables 2 and  3. The determination coefficient (R 2 ) was higher than 0.9, indicating that the Herschel-Bulkley model showed a good fit to the experimental data.

Impact of KGM Concentration
The effect of different concentrations of KGM on the changes in apparent viscosity and shear stress of emulsions containing 2.0% w/w WP was shown in Figure 5. The results revealed that apparent viscosity of all samples decreased with an increase in shear rate, indicating strong shear thinning behavior ( Figure 5) and pseudoplastic fluids (n < 1) ( Table 2), which is in keeping with previous studies on the rheology of mayonnaiselike sample [2,13]. The microstructure of systems was destroyed due to shear action, with the degree of damage increasing as the shear rate increased. At shear rate of 100 s −1 , the shear viscosities of samples were all reduced to lower than 10 Pa·s, indicating that the microstructure of the network was seriously damaged and that the new microstructure of the network cannot be formed in a short time [26]. Additionally, the viscosity and shear stress of emulsions increased with increasing KGM concentration in the range of 0.1-100 s −1 shear rate. The results revealed that apparent viscosity of all samples decreased with an increase in shear rate, indicating strong shear thinning behavior ( Figure 5) and pseudoplastic fluids (n < 1) ( Table 2), which is in keeping with previous studies on the rheology of mayonnaiselike sample [2,13]. The microstructure of systems was destroyed due to shear action, with the degree of damage increasing as the shear rate increased. At shear rate of 100 s −1 , the shear viscosities of samples were all reduced to lower than 10 Pa·s, indicating that the microstructure of the network was seriously damaged and that the new microstructure of the network cannot be formed in a short time [26]. Additionally, the viscosity and shear stress of emulsions increased with increasing KGM concentration in the range of 0.1-100 s −1 shear rate.
In addition, as the KGM concentration increased, the consistency coefficient (K) increased, while flow index (n) decreased. This could be attributed to several factors, including (1) KGM increasing the viscosity of the continuous phase; (2) as the oil droplets size decreased, the mean distances between them were smaller and the interactions between them were stronger, leading to a viscosity increase [32]; (3) the hydrogen bond interaction between KGM and WP might be another reason for the increased viscosity. The rheological properties of the emulsion at KGM 0.3% w/w were similar to the commercial mayonnaise.
Impact of WP Concentration The apparent viscosity and shear stress of emulsions (KGM c = 0.3% w/w) with varying WP concentrations were plotted in Figure 6. The parameters from the Herschel-Bulkley fit were listed in Table 3. In addition, as the KGM concentration increased, the consistency coefficient (K) increased, while flow index (n) decreased. This could be attributed to several factors, including (1) KGM increasing the viscosity of the continuous phase; (2) as the oil droplets size decreased, the mean distances between them were smaller and the interactions between them were stronger, leading to a viscosity increase [32]; (3) the hydrogen bond interaction between KGM and WP might be another reason for the increased viscosity. The rheological properties of the emulsion at KGM 0.3% w/w were similar to the commercial mayonnaise.

Impact of WP Concentration
The apparent viscosity and shear stress of emulsions (KGM c = 0.3% w/w) with varying WP concentrations were plotted in Figure 6. The parameters from the Herschel-Bulkley fit were listed in Table 3. The samples, including commercial mayonnaise, exhibited strong shear thinning behavior ( Figure 6) and a pseudo-plastic behavior since the values of flow behavior index (n) were less than 1 (n < 1) (Table 3). In the range of 0.1-100 s −1 shear rate, when WP was absent (0 WP), the viscosity and shear stress were both minimum. These results were consistent with the findings in appearance and droplet size, indicating KGM could not stabilize the emulsion alone. Nevertheless, when WP was present, there was a sight dependence of viscosity and shear stress on WP concentration. With an increase in WP concentration from 0 to 3.0% w/w, Herschel-Bulkley fitting parameters of samples were generally increased (K, 5.29-95.87 Pa s n ) and decreased (n, 0.31-0.15), respectively. The The samples, including commercial mayonnaise, exhibited strong shear thinning behavior ( Figure 6) and a pseudo-plastic behavior since the values of flow behavior index (n) were less than 1 (n < 1) (Table 3). In the range of 0.1-100 s −1 shear rate, when WP was absent (0 WP), the viscosity and shear stress were both minimum. These results were consistent with the findings in appearance and droplet size, indicating KGM could not stabilize the emulsion alone. Nevertheless, when WP was present, there was a sight dependence of viscosity and shear stress on WP concentration. With an increase in WP concentration from 0 to 3.0% w/w, Herschel-Bulkley fitting parameters of samples were generally increased (K, 5.29-95.87 Pa s n ) and decreased (n, 0.31-0.15), respectively. The increase in K value and decrease in n value with increasing WP concentrations suggested that WP molecules bridged droplet to droplet, enhancing the strength of the interactions between droplets [6,33]. Although the increase was at minimum 2.0% w/w, the emulsion at this WP concentration showed a similar trend to commercial mayonnaise. Figure 7 demonstrated the change in storage (G ) and loss (G ) modulus values with frequency for emulsions containing different KGM and WP concentrations. In general, when the G value is expected to be independent of frequency and G > G , the emulsion is defined as a gel-like behavior [32]. As can be seen in Figure 7, in the range of 0.628-62.8 rad/s frequency, the storage modulus (G ) was consistently higher than the loss modulus (G ), which indicated that the emulsion samples presented a gel-like behavior. Moreover, G rose with the in KGM concentration ( Figure 7A) and WP concentration ( Figure 7B). This result confirmed the predominant role of KGM and WP concentration in the structure formation of gel-like emulsion, as we discussed earlier. It should be noted that for the sample without WP (0 WP), a stable emulsified system could not be formed, and its property of gel-like behavior was mainly due to KGM. A slight frequency dependence of G was observed, which is typical of protein-stabilized emulsions. This could be attributed to the development of elastic networks resulting from extensive bridging and flocculation processes [6,34]. Figure 7 demonstrated the change in storage (G′) and loss (G″) modulus values with frequency for emulsions containing different KGM and WP concentrations. In general, when the G′ value is expected to be independent of frequency and G′ > G″, the emulsion is defined as a gel-like behavior [32]. As can be seen in Figure 7, in the range of 0.628-62.8 rad/s frequency, the storage modulus (G′) was consistently higher than the loss modulus (G″), which indicated that the emulsion samples presented a gel-like behavior. Moreover, G′ rose with the in KGM concentration ( Figure 7A) and WP concentration ( Figure 7B). This result confirmed the predominant role of KGM and WP concentration in the structure formation of gel-like emulsion, as we discussed earlier. It should be noted that for the sample without WP (0 WP), a stable emulsified system could not be formed, and its property of gel-like behavior was mainly due to KGM. A slight frequency dependence of G′ was observed, which is typical of protein-stabilized emulsions. This could be attributed to the development of elastic networks resulting from extensive bridging and flocculation processes [6,34]. The viscoelasticity of the mayonnaise-like emulsion was due to a network format, which was related to egg yolk proteins located between the interfaces of adjacent oil droplets [1] and also might be due to the hydrophobic interaction between lipids in egg yolk and oil droplets. In this work, the viscoelastic property (where G′ > G″) of the The viscoelasticity of the mayonnaise-like emulsion was due to a network format, which was related to egg yolk proteins located between the interfaces of adjacent oil droplets [1] and also might be due to the hydrophobic interaction between lipids in egg yolk and oil droplets. In this work, the viscoelastic property (where G > G ) of the emulsions co-stabilized by KGM and WP was similar to that of commercial mayonnaise, suggesting that a KGM and WP mixture could effectively help compensate for the absence of egg yolk and stabilize the oil droplet interface. Moreover, elastic interfacial networks among oil droplets were found to be enhanced as the concentration of KGM and WP increased, as evidenced by the increase in G and G of all samples with increasing concentrations of KGM and WP.

Viscoelastic Properties
In order to further ascertain the time-stability of gel and gel-like network, the power law model was used to fit G and G with ω to obtain the related rheological parameters [35][36][37][38]. The results indicated that the emulsion samples had gel-like characteristics due to their G and G being able to be fitted to the angular frequency (ω) using the power law model (R 2 > 0.9) ( Table 4). Except for 0 KGM and 0 WP, both G 0 and G 0 parameters (G 0 and G 0 are the storage and loss moduli at 1 rad/s, respectively) were noticeably greater in emulsion samples vs. the commercial mayonnaise, indicating greater gel strength [36]. The n value of all emulsion samples was lower than 1, also indicating a gel-like property [37]. Additionally, although there was no significant difference, n value generally decreased from 0.14 to 0.10 and from 0.22 to 0.10 as the concentrations of KGM and WP increased, respectively, indicating that the emulsion sample gradually developed a semi-solid form [38]. These results were consistent with physical stability and static viscosity results. In addition, the n value of all emulsion samples was less than the n value, meaning the rate of decrease in G was lower than in G and that the gel-like network was slightly reinforced with decreasing angular frequency [35]. Taken together, emulsions stabilized with a KGM-WP mixture had similar rheological behaviors to commercial mayonnaise, indicating that mayonnaise-like emulsion with high viscoelasticity, consistency, and texture could be fabricated by replacing egg yolk with a KGM-WP mixture.

Conclusions
This study proposed a strategy to prepare mayonnaise-like oil-in-water emulsions that can potentially replace traditional mayonnaise. The strategy involved the usage of mixtures solution of whey protein (WP) and konjac glucomannan (KGM) as the continuous phase and flaxseed oil as dispersed phase to fabricate emulsion. The impact of different concentrations of WP and KGM on droplet size, texture, microstructure and rheological properties of emulsion were characterized. The results showed that increasing the the concentrations of KGM (0.1-0.5% w/w) and WP (1.0-3.0% w/w) progressively decreased the oil droplet size. The static flow behavior revealed that emulsions showed a phenomenon of pseudo-plastic fluid characteristics. The dynamic rheological properties indicated that G was always higher than G in the range of 0.628-62.8 rad/s angular frequency. Moreover, apparent viscosity, G , and G showed increasing trends with increases in KGM and WP concentrations, thereby leading to an enhancement in the network structure. The network of emulsion co-stabilized by KGM and WP might be maintained through three aspects: (1) an increase in continuous phase viscosity, primarily from the contribution of KGM; (2) the excellent emulsifying activity of WP molecules; (3) the noncovalent interactions between KGM and WP. Furthermore, emulsions stabilized by a KGM-WP mixture exhibited shear thinning behavior and viscoelastic properties, which was similar to the commercial mayonnaise. These results suggested that the KGM-WP mixtures could effectively replace egg yolk to produce cholesterol-free mayonnaise-like emulsions. These results had significant implications for the development of protein-polysaccharide mixtures as promising egg yolk replacers in modification of traditional mayonnaise. However, thixotropy behavior and the oxidation stability of mayonnaise-like high-viscosity emulsions rich in unsaturated fatty acids (flaxseed oil), which are very important in practical applications, need to be further studied for future applications.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.