The Stability of Four Kinds of Cellulose Pickering Emulsions and Optimization of the Properties of Mayonnaise by a Soybean Byproduct Pickering Emulsion

Abstract

Soybean residue, kudzu root residue, astragalus residue and pomegranate peel residue are byproducts of food processing with high yield. In the food processing industry in Northwest China, these waste residues contain a large amount of nutrients and have a large amount of emissions. In this study, cellulose was extracted from four kinds of waste residue and characterized to study its emulsification performance and application effect. The results are as follows: The extracted cellulose had typical cellulose crystal structure and good thermal stability. Among the four kinds of cellulose, the physical, chemical and functional properties of the soybean byproduct were significantly better than those of standard cellulose and other sources of cellulose. The Pickering emulsions fixed by four kinds of cellulose and soybean lecithin have similar properties. The emulsification performance of the immobilized soybean byproduct cellulose Pickering emulsion is the best. Soybean byproduct cellulose was used as an oil substitute for the development of new mayonnaise. The results showed that when 8% soybean byproduct cellulose Pickering emulsion was used to replace vegetable oil, the quality of reduced-fat mayonnaise was better. This soybean byproduct cellulose has potential development and application value in industrial food.

1. Introduction

Soybean, kudzu root, astragalus and pomegranate are widely used in the food processing industry in northwest China, and the amount of residual waste after processing is extremely large. These wastes are mainly composed of cellulose, hemicellulose, pectin, lignin and other components, which are rich in nutrition [1,2]. At present, most of the waste residue is used as agricultural fertilizer or even directly burned, which will not only cause environmental pollution but also waste a lot of material resources [3]. The rich content of cellulose in these waste residues makes it an ideal candidate resource for cellulose, which provides a good direction for its application [4].

Cellulose is non-toxic and harmless and has the advantages of sustainability and biocompatibility [5]. The use of cellulose to stabilize Pickering emulsions has gradually become a focus of attention [6]. An emulsion is a mixture in which homogeneous small droplets of one phase are uniformly dispersed in the other phase. In the past, traditional small molecules were often used as surfactants to stabilize Pickering emulsions. In recent years, Pickering emulsions stabilized by solid particles as surfactants have been widely used in food, medicine, cosmetics and other fields [7,8,9]. A difference in cellulose raw materials will cause differences in the properties of the cellulose, such as surface charge, crystal structure and so on [10]. Therefore, different celluloses have different effects on stabilizing Pickering emulsions, and related research needs to be further deepened.

High-fat eating habits can cause obesity and other diseases. Mayonnaise has a unique flavor and taste, and has been loved by consumers for a long time. The main components of mayonnaise generally include fat, egg yolk/whole egg, salt, lemon juice, etc., usually with a fat content of 70–80% [11]. Therefore, mayonnaise is a high-fat and high-cholesterol food [12]. At present, a large number of studies have been devoted to solving the problems of high-fat, high-cholesterol and high-calorie properties of mayonnaise, reducing its calorie content while ensuring its nutritional value and taste/flavor [13]. Cellulose Pickering emulsion can replace part of the oil in the mayonnaise by playing an emulsifying role, and the fat reduction may also have a positive impact on the properties of the mayonnaise.

In this experiment, cellulose was extracted from the processing residues of soybean, kudzu root, astragalus and pomegranate, and their microstructures and physicochemical properties were studied. The Pickering emulsions were prepared with four kinds of cellulose, and the effects of various factors on the stability of the emulsions were studied. Reduced-fat mayonnaise was prepared by replacing part of vegetable oil with a Pickering emulsion fixed by soybean byproduct cellulose, and some quality indexes of mayonnaise were studied. This experiment provides a new idea for the treatment of byproducts in food processing such as soybean byproducts and provides basic research for the subsequent application of cellulose Pickering emulsions in the food industry.

2. Materials and Methods

2.1. Materials

Soybean byproduct was provided by Shaanxi Qindouyuan Agricultural Science and Technology Co., Ltd. (Xianyang, China). Astragalus byproduct was purchased from Anhui Haozhou Baixinglin. Kudzu root byproduct was purchased from Guangzhou Guangxi. Pomegranate peel obtained from Xi’an, Shaanxi.

2.2. Extraction of Cellulose from Soybean Byproduct and Other Waste Residue

Cellulose was extracted according to the method of Saleheen Bano et al. [14]. After passing through a 40-mesh sieve, the four byproducts were placed in distilled water and stirred at 80 °C for 90 min, filtered and washed. Sodium chlorite was added to the distilled water, and the pH was adjusted to 3.6–4.0 with hydrochloric acid. The filtered byproduct was stirred at 80 °C for 150 min, then filtered and washed until neutral. After adding 9.0 g NaOH, 0.30 g MgSO₄ and 0.30 g EDTA to 300 mL of 3% H₂O₂ solution, the filtered byproduct was stirred at 100 °C for 120 min. The filtered byproduct was tiled in a freeze-drying tray (thickness ≤ 1 cm) and freeze-dried using a vacuum freeze-drying machine (LGJ-25, Sihuan Keyi Technology Development (Beijing) Co., Ltd., Beijing, China). The freeze-drying parameters were set as follows: cold trap temperature −50 °C, vacuum degree ≤ 30 Pa, drying time 24 h. After drying, the soybean byproduct cellulose was obtained. The cellulose extraction from the byproduct of kudzu root, astragalus and pomegranate peel is also in accordance with the above method, only the raw materials need to be changed.

2.3. Experimental Study on the Microstructure of Cellulose

A Fourier transform infrared spectrometer (INVENIO S, Bruker Corporation, Ettlingen, Germany) was used to record the infrared spectrum (FTIR). Four dried cellulose powder samples were thoroughly mixed with KBr (1:100, w/w) and pressed into a transparent sheet. The IR spectra were recorded in spectrometer from 400 to 4000 cm⁻¹, with 16 scans and 4 cm⁻¹ resolution [15].

The X-ray diffraction (XRD) analyses of the films were conducted using a diffractometer (SU8020, Hitachi High-Tech Corporation, Tokyo, Japan). The diffractometer operated at 40 kV and 40 mA, scanning from 5° to 50° in 2θ with a step size of 0.02° and a scanning speed of 2°/min [16].

Thermogravimetric analysis (TGA) of the samples was carried out by a thermogravimetric analyzer (Q-600/Q1000, TA Instruments, New Castle, DE, USA). Sample amounts of 5–10 mg were placed in an aluminum crucible and heated from 30 °C to 600 °C at a rate of 15 °C/min in an N₂ environment with a flow rate of 50 mL/min [17].

The dried cellulose powder was pressed into flat sheets and the surface was cleaned with an argon ion beam. X-ray photoelectron spectroscopy (XPS) measurements were performed using an AXIS Ultra DLD spectrometer (Kratos Analytical Ltd., Manchester, UK) with Al Kα radiation. The chemical state information of each sample was obtained by detecting the photoelectron kinetic energy by energy analysis.

2.4. Determination of the Physicochemical Properties of Cellulose

2.4.1. Water-Holding Capacity (WHC)

According to the method of Lin Tong et al., 0.5 g dry powder sample (M₀) was placed in 15 mL distilled water, centrifuged after incubating in a water bath and the wet weight (M₁) of the centrifuge tube and the sample was recorded [18]. Dry weight (M₂) was recorded, and the WHC of the sample was calculated according to the Formula (1).

$$WHC\left({\displaystyle \frac{g}{g}}\right)={\displaystyle \frac{M_1-M_2}{M_0}}$$

(1)

2.4.2. Oil-Holding Capacity (OHC)

A total of 0.5 g of each dried powder sample was placed in a centrifuge tube (M), 15 mL of corn oil was added, shocked and mixed, then incubated at 37 °C in a water bath for 2 h. The samples were centrifuged for 20 min at 4000 r/min, the supernatant was discarded, the wet weight (M₁) of the centrifuge tube and the sample was recorded and the OHC of the sample was calculated according to the Formula (2).

$$OHC\left({\displaystyle \frac{g}{g}}\right)={\displaystyle \frac{M_1-M_0-M}{M_0}}$$

(2)

2.4.3. Swelling Capacity

A total of 0.5 g of each dried powder sample (M₀) was placed in 10 mL distilled water, and the initial volume (V₀) was recorded. The sample was allowed to fully swell, and the final volume (V₁) was recorded. The SC of the sample was calculated according to Formula (3).

$$SC\left({\displaystyle \frac{mL}{g}}\right)={\displaystyle \frac{V_0-V_1}{M_0}}$$

(3)

2.4.4. Angle of Repose

Each dried powder sample was placed in a drying funnel and dumped at a height of 3 cm from the desk top such that it naturally accumulated to form a cone. The height (H) and radius (R) of the cone were measured and recorded, and the angle of repose of the sample was calculated according to Formula (4).

$$\theta ={\mathit{tan}}^{-1}{\displaystyle \frac{H}{R}}$$

(4)

2.4.5. Bulk Density (BD)

The sample was added to a 2 mL dry centrifuge tube to the scale line, and the mass of the empty centrifuge tube (M₁) and the total weight of the centrifuge tube and the sample (M₂) were weighed and recorded. BD of the sample was calculated according to Formula (5).

$$BD\left({\displaystyle \frac{g}{mL}}\right)={\displaystyle \frac{M_2-M_1}{V}}$$

(5)

2.5. Preparation of the Pickering Emulsions

Four kinds of cellulose samples were prepared with ultrapure water into cellulose particle suspensions (concentration 2%), and the oil phase was added under the stirring of a magnetic stirrer (oil–water ratio 1:9). The mixed oil–water system was ground three times by a colloid mill to achieve the effect of ultrafine grinding of the material, and then homogenized by high-pressure homogenizer (70 MPa, 7 min) to obtain a Pickering emulsion.

2.6. Determination of Pickering Emulsion Stability

2.6.1. The Effects of Ionic Strength and pH

The balanced fresh soybean byproduct cellulose, kudzu root byproduct, astragalus cellulose and pomegranate peel byproduct cellulose, as well as a commercial emulsifier Pickering emulsion were taken, and different gradients of ion concentration (0, 50, 100, 150 and 200 mmol/L) and pH (4, 5, 6, 7, 8) were set. Ion concentration and pH were adjusted with NaCl (0, 2.92, 5.89, 8.77, 11.7 mg/mL) and hydrochloric acid, respectively. After 24 h at 4 °C, the stratification of the emulsion was observed, and the emulsification index, Zeta potential and particle size of the emulsion were measured [19]. Before the measurement, the emulsion was diluted 50 times to achieve dispersion. The absorption parameter was 0.001, the refractive index of the emulsion was 1.46, and the measurement temperature was 25 °C.

2.6.2. The Effects of Centrifugation, Storage and Temperature

The five kinds of Pickering emulsions were treated as follows: centrifuged at 4050 r/min for a certain time (0, 1, 2, 3 and 5 min); the emulsion was observed after storage at 4 °C for a period of time (3, 6, 12 and 24 h); continued heat treatment for 2 min after reaching the specified temperature (30, 40, 60, 70 and 80 °C). After completion, the emulsification index, Zeta potential and emulsion particle size were measured. The determination method is the same as 2.6.1.

2.6.3. The Effect of Solid Emulsifier and Oil Phase Ratio

In the preparation of the emulsions, the following treatments were carried out, and different amounts (0.2, 0.4, 0.6, 0.8, 1.0, 1.5 and 2.0%) of soybean byproduct cellulose, kudzu root byproduct, astragalus cellulose, pomegranate peel byproduct cellulose and the commercial emulsifier were added. Different amounts (5, 10, 15, 20, 25 and 30%) of soybean oil were added. After the emulsion was stable, the emulsification index, Zeta potential and emulsion particle size were determined. The determination method is the same as 2.6.1.

2.7. Application Effect of the Cellulose Pickering Emulsion

2.7.1. Production of Mayonnaise

The raw materials for the production of mayonnaise were 15 g of egg yolk, 250 g of rapeseed oil, 30 g of water, 1 g of edible salt, 1 g of lemon juice, 3 g of white vinegar, 6 g of white sugar and 3 g of corn starch. The above raw materials were mixed and stirred for 10 min to obtain mayonnaise. Reduced-fat mayonnaise was prepared by replacing rapeseed oil (2, 4, 6, 8 and 10%) with a Pickering emulsion fixed with soybean cellulose.

2.7.2. Determination of Fat, Moisture and Dietary Fiber Content in Mayonnaise

The fat content in mayonnaise was determined by a Soxhlet extraction method. The moisture content in mayonnaise was determined by a direct drying method. The impurities were removed by enzymatic hydrolysis, and the total dietary fiber content was determined by ethanol precipitation and liquid chromatography.

2.7.3. Determination of Some Indicators Affecting the Quality of Mayonnaise

Based on the method proposed by Yu et al., Zeta potential and a particle size analyzer (NanoBrook 90 PlusPALS, Brookhaven Instruments, Holtsville, NY, USA) were used to determine the potential and particle size of mayonnaise [20]. The mayonnaise was diluted 100 times with ultrapure water to achieve dispersion, gently stirred and stood for 5 min. The instrument absorption parameter was set to 0.001, the refractive index of the emulsion was 1.46 and the measurement temperature was 25 °C.

A total of 100 mL petroleum ether with a boiling range of 30–60 °C was added to a conical flask containing 50 g mayonnaise and shaken well for 12 h. The sample was prepared by filtration with a funnel containing anhydrous copper sulfate and a water bath at 60 °C.

The phenolphthalein indicator was added to 5 g of the mixed preparation sample and 50 mL of an ethanol–ether mixture, and the acid value was calculated by 0.05 mol/L sodium hydroxide standard titration. A total of 5 g of the above sample was added to 30 mL of a chloroform–glacial acetic acid mixture with a ratio of 2:3, and the mixture was fully shaken and dissolved. A total of 1.0 mL of potassium iodide was added and the sample was placed in the dark for 3 min, then 50 mL of pure water was added and the sample was shaken. The solution was titrated with 0.02 mol/L sodium thiosulfate solution. When the color became light yellow, the starch indicator was added, and the solution was titrated until the solution became blue and then the color disappeared. The results were calculated.

The swelling rate of the sample was determined by referring to the method of Ma et al. [21]: 5 g samples were weighed and placed in a calibration tube, 50 mL distilled water was added and soaked for 12 h at room temperature. After soaking, the scale before and after swelling was read and recorded, and the swelling rate was calculated.

Centrifugal oil precipitation rate was determined. The mayonnaise was placed at room temperature for 12 h, and part of it was placed in a centrifuge tube and weighed. The mayonnaise was centrifuged for 3 min at a temperature of 25 °C and a speed of 4020 r/min. The upper layer of oil was removed and the quality of the oil was calculated.

2.8. Data Analysis

All experimental data are expressed as mean ± standard deviation, and the number of experimental repetitions in each group was 3. One-way analysis of variance (ANOVA) was performed using SPSS Statistics 25 software for significance analysis, at a 95% confidence level (differences were considered significant at p < 0.05).

3. Results and Discussions

3.1. Microstructure of Cellulose

From the Fourier transform infrared spectrum of Figure 1a, the celluloses extracted from the four substances of soybean byproduct, kudzu root, astragalus and pomegranate peel had a wide absorption peak near 3400 cm⁻¹, which were the stretching vibration peaks of the -OH group on the sugar ring of cellulose, indicating that the cellulose had a certain hydrophilic tendency [22]. The absorption peak near 2900 cm⁻¹ is attributed to the stretching vibration of the -CH/-CH₂ group on the cellulose sugar ring. The absorption band between 900–1100 cm⁻¹ is related to the C-H stretching vibration of CH-OH and CH-OH on the cellulose sugar ring. Compared with kudzu root, astragalus and pomegranate peel, the absorption peak of cellulose in soybean byproduct near 1700 cm⁻¹ is sharper, and its carbonyl content is higher.

The crystallinity and crystalline structure of cellulose and cellulose nanofibers of the four substances were determined by X-ray spectrometer. The results are shown in Figure 1b. Soybean cellulose has two peaks between 2θ = 14–17°, and peaks at 2θ = 23.5° and 36°, corresponding to (110), (200) and (040) crystal planes, respectively. It is a typical type I cellulose. The cellulose spectra of kudzu root, astragalus and pomegranate peel were similar; there were peaks at 2θ = 14.5°, 22.5° and 33°, and there were no typical characteristic peaks of type II cellulose at 12° and 20°. The extraction method was not harsh enough to convert the cellulose between types, so these three kinds of cellulose should also belong to type I cellulose. The cause of its broad peak at 14° was analyzed. Firstly, the crystallinity was calculated to be relatively small, which proved that there were amorphous compounds in its composition. Then, combined with the FTIR spectrum and the selected cellulose extraction method, it was possible that it was caused by impurities such as pectin, lignin and hemicellulose [23].

The TGA analysis spectrum (Figure 1c) shows the TG curves of the cellulose of these four substances. In the N₂ environment, the decomposition of the substance can be divided into three stages. In the first stage, the mass of cellulose and cellulose nanofibers decreased rapidly at 50–75 °C, which was attributed to the rapid evaporation of water in the sample. The mass loss of the four substances in the first stage is similar, indicating that the water content of the two substances is similar. The second stage is 75–400 °C. More extensive dehydration and pyrolysis reactions reduce the sample quality the most at this stage, where most of the cellulose, hemicellulose, and lignin are degraded [24]. The quality of cellulose decreased by about 50% at this stage.

As shown in Figure 1d, the peak shape of the O element in the four celluloses is basically the same, and the highest peak appears in the range of 530–535 eV. The peak at 531–532 eV is a medium binding energy peak, and the peak at this position corresponds to hydroxyl or organic hydrogen. The peak at 533–534 eV is a high binding energy peak, and the peak at this position corresponds to adsorbed water or carboxyl group. The peaks of the soybean byproduct and pomegranate peel cellulose were higher than those of astragalus and kudzu root cellulose. In the detection of carbon atoms, the highest peak appears near 286 eV, which proves the presence of hydroxyl groups in the sample.

3.2. Physicochemical Properties of Cellulose

The water-holding capacity of cellulose is its ability to absorb water, which is of great significance for its function in different environments. Figure 2a shows the water-holding capacity of cellulose in soybean byproduct, astragalus byproduct, kudzu root byproduct and pomegranate peel. The water-holding capacity of cellulose in soybean byproduct, astragalus byproduct, kudzu root byproduct and pomegranate peel was 5.67 g/g, 5.33 g/g, 4.51 g/g and 3.71 g/g, respectively. Among the four kinds of cellulose extracted, the water-holding capacity of soybean byproduct, astragalus byproduct and kudzu root byproduct was significantly higher than that of standard cellulose.

Figure 2b is the oil-holding capacity of cellulose in the four substances. The oil-holding capacity of cellulose in astragalus byproduct, kudzu root byproduct and pomegranate peel was 3.97 g/g, 3.87 g/g and 3.89 g/g, respectively. The oil-holding capacity of soybean byproduct cellulose was 4.22 g/g, which was significantly higher than that of the other three kinds of cellulose. The oil-holding capacity of standard cellulose was 4.11 g/g, which was slightly lower than that of soybean byproduct cellulose. The oil-holding capacity of cellulose is different due to the lipophilic groups of cellulose and protein, and it is also related to its surface characteristics, overall charge density and thickness [25]. The soybean byproduct has a good ability to maintain oil, which is conducive to the adsorption of oil to reduce the interfacial tension and can play a role in stabilizing the emulsion.

As shown in Figure 2c, the swelling properties of cellulose in the four substances were 0.92 mL/g, 0.85 mL/g, 0.86 mL/g, and 0.79 mL/g, respectively. The swelling property of soybean byproduct cellulose was slightly larger than that of the other three celluloses. The swelling capacity of the standard product was 1.02 mL/g, which was slightly larger than that of soybean byproduct cellulose, but there was no significant difference between the two. The swelling of cellulose is related to many factors. In the process of cellulose extraction, temperature, alkali concentration and heating time will affect the swelling of the final cellulose. The difference between the cellulose extracted from the byproduct and the standard cellulose is small, which also proves the correctness of the cellulose extraction method.

The angle of repose results of cellulose in the four substances are shown in Figure 2d, which are 57.21°, 54.38°, 56.81°, and 51.26°, respectively, and the angle of repose of the standard cellulose is 47.91°. The angle of repose of the four kinds of cellulose is significantly higher than that of the standard cellulose. Among them, the angle of repose of the soybean byproduct cellulose is the largest, which may be related to the friction force of the loose and porous structural material.

Figure 2e shows the volume density of the different celluloses. The volume density of standard cellulose is 0.13 g/mL, and the volume densities of soybean byproduct, astragalus byproduct, kudzu root byproduct and pomegranate peel cellulose are 0.19 g/mL, 0.15 g/mL, 0.16 g/mL and 0.18 g/mL, respectively. Among them, the bulk density of cellulose in soybean byproduct, kudzu root byproduct and pomegranate peel were significantly higher than that of standard cellulose, while the bulk density of cellulose in soybean byproduct was significantly higher than that of astragalus byproduct and kudzu root byproduct. In the process of cellulose extraction, both alkali treatment and freeze-drying will make the surface of the cellulose rough, a pore structure of different sizes visible to the naked eye will appear and the surface area will increase. In the process of decolorization, hydrogen peroxide will produce oxygen overflow, which will make the structure of the cellulose looser and the volume density increase accordingly.

3.3. Pickering Emulsion Stability

3.3.1. The Effects of Ionic Strength and pH on Stability

It can be clearly observed from Figure 3a that with the gradual increase of ionic strength, the emulsification index of Pickering emulsions stabilized by soybean byproduct cellulose, kudzu root byproduct cellulose, astragalus cellulose and pomegranate peel byproduct cellulose showed a decreasing trend. Specifically, when the ionic strength increases to 50, the emulsification index decreases, but the decrease is relatively small. At this stage, the electrostatic repulsion between particles is weakened to a limited extent, and the properties of the interfacial film remain relatively stable. When the ionic strength was further reduced to 150, the decline rate of the emulsification index was significantly accelerated, and the stability of the emulsion was significantly damaged. Finally, when the ionic strength reaches 200, the decline rate of the emulsification index slows down again, which may be related to the fact that the ionic strength in the emulsion has approached saturation. The stability of each Pickering emulsion is closely related to the ionic strength in the emulsion system. The change of ionic strength will affect the degree of flocculation between droplets in the emulsion system and the properties of the interfacial film, which in turn affects the stability of the emulsion [20]. When the electrolyte concentration is high, the electrostatic repulsion between the emulsion particles may be weakened, resulting in flocculation to form larger particles, which is not conducive to the adsorption of particles at the interface, destroys the original tightness of the interface film, accelerates the coalescence process of the emulsion and has a negative impact on the stability of the emulsion [26].

As shown in Figure 3b, the stability of the emulsion is closely related to the pH of the emulsion system. When the pH is 6 to 7, the emulsification index does not change significantly; the electrostatic interaction between droplets does not change significantly under this acid–base condition. The droplets are uniform and no coalescence occurs. When pH is 4 to 5, the emulsification index of the emulsion is small. The dispersion state of the emulsion is not ideal under the weak acid condition, and the droplets have aggregation behavior, which may be related to the acidic conditions. The electrostatic repulsion between the emulsion droplets is changed, and the larger electrostatic repulsion will effectively prevent the coalescence of the droplets, so that the emulsion maintains good stability. When pH is 8, the stability of the emulsion remains in a good state. In this state, the carboxyl group in the cellulose is transformed into a carboxyl group under alkaline conditions, and the amphiphilicity is increased. The pH of the emulsion system will affect the surface charge density of the solid particles in the Pickering emulsion, change the surface wettability and interfacial tension of the particles and affect the stability of the Pickering emulsion by adjusting the adsorption performance of the solid particles at the interface [27]. The pH also changes the charge properties and charge amount on the surface of the particles, the electrostatic repulsion or gravitation decreases, resulting in the easy aggregation and stratification of the emulsion droplets, and the emulsification index increases.

3.3.2. The Effects of Centrifugation, Storage and Temperature on Stability

Centrifugation will cause the emulsion to emulsify, stratify, precipitate and suspend [28]. Figure 4a shows the effect of centrifugation on the emulsification index, Zeta potential and particle size of the emulsion. The demulsification and phase separation of the emulsion will occur during the centrifugation of the emulsion. The essence is that the oil phase with lower density moves to the top of the emulsion, while the water phase with higher density moves to the bottom of the emulsion. During the centrifugation process, the unabsorbed cellulose particles will gather at the bottom of the sample tube. It can be seen from the emulsification index and Zeta potential that the stability of the four Pickering emulsions stabilized by cellulose decreased with the increase of centrifugation time. The soybean byproduct cellulose emulsion in the centrifugal treatment showed high EI retention rate and small particle size growth, which may be due to the high oil-holding capacity of its cellulose, ensuring the strong interaction between the particles at the oil–water interface and the oil droplets, and its good swelling provides effective steric hindrance.

As shown in Figure 4b, the emulsification index of various emulsions stored at 4 °C for 24 h is above 95%, the oil is not precipitated, and the emulsion state is good. The absolute value of the Zeta potential of the emulsion is always around 40. The particle size of the emulsion is small, indicating that the emulsion does not aggregate.

Figure 4c shows that the emulsion maintains a high emulsification index and the Zeta potential does not change significantly after heat treatment in the range of 30–80 °C. This result shows that the emulsion exhibits good thermal stability. This indicates that cellulose forms a more stable structure in the emulsion, and the interface layer and structure of the emulsion are not significantly affected by the increase of temperature. Combined with the TGA spectrum, several celluloses have a higher degradation temperature and better thermal stability, which may be one of the reasons why heat treatment has little effect on them. In addition, cellulose may also form a more stable structure in the emulsion, and the interface layer and structure of the emulsion are not significantly affected with the increase of temperature [29]. The emulsification index and Zeta potential of the emulsion decreased slightly, which may be related to the destruction of the hydrogen bond interaction between cellulose molecules by high temperature, and the interaction between cellulose molecules was weakened, resulting in the decrease of emulsification ability.

3.3.3. The Effect of Solid Emulsifier Concentration and Oil Phase Ratio on Stability

When the concentration of solid emulsifier in the emulsion was set to 0.2% and 0.4%, the emulsification index of the emulsion stabilized by the four kinds of cellulose was small, and the Zeta potential value was small (Figure 5a). This is because the concentration of solid emulsifier is low, the cellulose is not enough to balance the oil–water interface. With the increase of cellulose concentration, the emulsion emulsification index and Zeta potential value increased, and the emulsion stability increased. The emulsions with the concentration of 1.5% and 2.0% have not been stratified after long-term storage at room temperature, indicating that the concentration is an ideal concentration to maintain the stability of the emulsion, and the emulsion within the concentration range of the emulsifier has better anti-aggregation stability. The cellulose stabilized the emulsion at a certain concentration in the emulsion and played a role in stabilizing the oil–water interface. It can be applied to the preparation and storage of Pickering emulsions.

As shown in Figure 5b, when the oil phase concentration is less than 15%, the emulsion state stabilized by the four celluloses is good, the emulsification index is large and the Zeta potential value is high. The emulsification index and Zeta potential value gradually decreased with an increase in the oil phase ratio. When the oil phase ratio was greater than 15%, the emulsion displayed a creaming phenomenon, which was related to the increase in the oil phase mass fraction and the decrease in the particle adsorption rate at the emulsion interface. The particle size can usually effectively account for the stable state of the emulsion. In general, the smaller the average particle size of the emulsion, the better the stability of the emulsion and the higher the emulsifying ability of the emulsifier. With the increase of the proportion of oil phase in the emulsion, the change trend of the particle size of the emulsion decreases first and then increases. When the proportion of oil phase in the emulsion is 15%, the particle size of these emulsions is the smallest. This is because the cellulose extracted from the byproducts and the oil phase are emulsified to form a protective layer on the surface of the oil droplets. This irreversible protective layer can prevent the aggregation of oil droplets. When the oil phase ratio exceeds 15%, the particle size of the emulsion increases with the increase of the oil phase volume fraction. This is because the oil phase ratio increases and the emulsifier content remains unchanged, forcing the emulsion droplets to reduce the surface area to maintain the stability of the oil phase system. The oil droplets are close to or even polymerized, resulting in an increase in the particle size of the emulsion [30]. It is worth noting that the soybean byproduct cellulose emulsion always exhibits the highest EI value, which is consistent with its outstanding oil-holding capacity and good swelling property.

3.4. The Application of a Pickering Emulsion Immobilized by Soybean Byproduct Cellulose in Mayonnaise

3.4.1. Contents of Fat, Moisture and Dietary Fiber in Mayonnaise

As shown in

Table 1. Moisture, fat and dietary fiber content of mayonnaise.

Specimen	Moisture	Fat	Dietary Fiber
1	8.32 b	85.62 ± 0.76 c	/
2	5.12 a	88.34 ± 0.66 d	/
3	9.31 c	81.78 ± 0.15 b	/
Pickering-2%	8.78 bc	83.21 ± 0.65 bc	1.02 ± 0.05 a
Pickering-4%	11.16 d	81.44 ± 0.78 b	1.15 ± 0.03 b
Pickering-6%	14.14 e	79.61 ± 0.31 a	1.08 ± 0.03 ab
Pickering-8%	18.84 f	76.21 ± 0.36 a	1.11 ± 0.01 ab
Pickering-10%	20.45 g	74.19 ± 0.57 a	1.18 ± 0.03 b

Notes: Values are given as mean ± standard deviation (SD) of three independent tests. Different letters in the same column indicate significantly different results (p < 0.05).

and Figure 6a, with the increase of the proportion of Pickering emulsion, the fat content decreases, and the water and dietary fiber content increases, which is consistent with the results of the FTIR spectra. The results show that it contains -OH groups and has a certain hydrophilic tendency. The fat content of the mayonnaise with the emulsion was significantly different from that of mayonnaise without the emulsion, which indicated that the fat content could be reduced by replacing soybean oil with soybean cellulose Pickering emulsion, and the dietary fiber content increased due to the use of the soybean byproduct cellulose Pickering emulsion as emulsifier.

3.4.2. Mayonnaise Zeta Potential and Particle Size

As shown in Figure 6b,c, the Zeta potential of mayonnaise reached the maximum when the proportion of soybean byproduct cellulose Pickering emulsion was 8% and was significantly higher than that of mayonnaise with a ratio of 6% or 10%. The particle size of mayonnaise tends to increase gradually, which may be due to the coalescence of oil droplets in these dense emulsions with high oil content, which leads to the increase of droplet size, and the coalescence of oil droplets occurs after the oil droplets contact for a long time [31]. However, there was no significant difference in the particle size of mayonnaise between different proportions of soybean byproduct cellulose Pickering emulsion.

3.4.3. The Acid Value and Peroxide Value of Mayonnaise

The acid value is affected by many factors such as temperature, light, and oxygen. It is an indicator of oil rancidity. The higher the acid value, the worse the quality of the oil. It can be seen from Figure 6d that the acid value of the commercial mayonnaise and self-made mayonnaise is in the normal range (≤3 mg/g), which indicates that the mayonnaise has not turned rancid. When the soybean cellulose Pickering emulsion is greater than 6%, the acid value decreases, which is related to the increase of the emulsion ratio, the decrease of the oil content and the lower free fatty acid content. If mayonnaise needs long-term storage, soybean cellulose Pickering emulsion greater than 6% is more appropriate.

The peroxide value reflects the content of hydroperoxide in oil, which is of great significance for mayonnaise with an oil content above 75%. The poorer the oil quality, the higher the peroxide value and the faster the oil oxidation rate. It can be seen from Figure 6e that once the proportion of soybean byproduct cellulose Pickering emulsion exceeds 4%, there is no significant difference in the peroxide value of various mayonnaises and no significant difference between them and the commercial mayonnaises. On the one hand, the emulsion replaces the oil, which reduces the proportion of oil in the system and reduces the concentration of the oxidized substrate. On the other hand, cellulose may form a three-dimensional network structure with the protein to encapsulate the oil droplets while exerting the total use of emulsification, thereby reducing the oxidation rate [32]. When the proportion of Pickering emulsion was 10%, the peroxide value was higher than that of the mayonnaise with 8% emulsion. This may be due to the excessive concentration of the emulsion, the instability of the system, the precipitation of oil and the oxidation.

3.4.4. Swelling Rate of the Mayonnaise

As shown in Figure 6f, with the increase of the proportion of emulsion, the swelling rate of the mayonnaise increases, but when the proportion reaches 10%, the swelling rate of the mayonnaise decreases. Cellulose has a strong ability to absorb water, which will expand rapidly after absorbing water, and can absorb water with its own weight of 50–100. Therefore, the substitution of soybean byproduct cellulose Pickering emulsion significantly increases the swelling rate of mayonnaise. When the proportion of soybean cellulose Pickering emulsion reached 10%, the swelling rate of the mayonnaise decreased after the moisture and lubricating oil in the mayonnaise were adsorbed by the cellulose. From the perspective of application, mayonnaise needs appropriate viscosity to ensure spreadability. When the amount of cellulose added exceeds 8%, it may cause an imbalance in the water–oil ratio; when the oil content is insufficient and the water content is too large, the excessive fluidity will damage the texture and edible feeling, which is also mentioned in other studies. Therefore, the proportion of soybean cellulose Pickering emulsion should be controlled below 8% [33]. The mayonnaise should be thick and suitable for easy application. When the oil content is insufficient and the water content is too large, the fluidity is too strong, which will make the application inconvenient and the eating experience is poor. Therefore, the proportion of soybean cellulose Pickering emulsion should be controlled below 8%.

3.4.5. Centrifugal Oiling Rate of Mayonnaise

Mayonnaise will undergo oil precipitation during storage. This phenomenon is related to the stability of the mayonnaise system and environmental conditions. Centrifugal milk absorption rate can predict the stability of soy sauce foods over a short time. The lower the centrifugal milk absorption rate, the more stable the mayonnaise system, and the environment in which it is located is suitable for its preservation. Too high indicates that the stability of the mayonnaise system is weak or its storage environment will break the stability of the system. To a certain extent, this may be related to the excellent stability of the soybean byproduct cellulose Pickering emulsion itself against centrifugal force. Its high oil-holding capacity and the formation of a stable interface layer may continue to play a key role in preventing oil droplets from coalescing and floating in the complex multiphase system of mayonnaise. Figure 6g shows that with the increase of the proportion of soybean byproduct cellulose Pickering emulsion, the centrifugal milk absorption rate of mayonnaise gradually decreases. When the concentration of soybean byproduct cellulose Pickering emulsion increases to 8%, the centrifugal milk absorption rate is not different from that of the two commercial mayonnaises, which indicates that a certain amount of emulsion instead of oil can increase the oil-holding capacity of mayonnaise. Various proportions of mayonnaise did not show serious oil precipitation, and its internal system stability was good. The emulsification of mayonnaise mainly depends on the natural lecithin contained in eggs. The addition of soybean byproduct cellulose Pickering emulsion reduces the oil in mayonnaise and introduces the emulsifier of cellulose, which significantly improves the oil precipitation phenomenon of mayonnaise and enhances its stability. This may be related to the fact that polysaccharides can fix the grease inside the mayonnaise by a gel network, limiting its flow [34].

4. Conclusions

In this study, cellulose with good crystal structure was successfully extracted from food industry processing residues such as soybean, kudzu root, astragalus and pomegranate peel. More importantly, it was confirmed that cellulose derived from these waste resources can be used as an effective Pickering emulsion stabilizer. Among them, soybean cellulose is particularly prominent in stabilizing emulsions with different oil–water ratios. Based on this core finding, this study developed a new type of low-fat mayonnaise by using the Pickering emulsion stabilized by soybean cellulose as a fat substitute. Through systematic analysis, it was determined that replacing 8% of the vegetable oil with the soybean byproduct cellulose Pickering emulsion was the optimal solution. The prepared mayonnaise not only significantly reduced the fat content, but also its key indicators (such as swelling rate, centrifugal oiling rate), and also met the quality requirements. In summary, this study not only provides a new way for the high-value utilization of food industry wastes, but also shows an effective strategy for the preparation of healthy low-fat foods using cellulose Pickering emulsions, which provides an important theoretical and practical basis for the development of sustainable and healthy new food ingredients and products.