# Co-stabilization mechanisms of solid particles and soluble compounds in hybrid Pickering emulsions stabilized by unrefined apple pomace powder

Charlotte Hollestelle, Camille Michon, Nathalie Fayolle, Delphine Huc-Mathis

<figure><img src="image_1.png" /></figure>

## To cite this version:

Charlotte Hollestelle, Camille Michon, Nathalie Fayolle, Delphine Huc-Mathis. Co-stabilization mechanisms of solid particles and soluble compounds in hybrid Pickering emulsions stabilized by unrefined apple pomace powder. Food Hydrocolloids, 2024, 146 (A), pp.109184. 10.1016/j.foodhyd.2023.109184. hal-04465833

HAL Id: hal-04465833

https://hal.science/hal-04465833v1

Submitted on 30 Jun 2025

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Distributed under a Creative Commons Attribution 4.0 International License
Co-stabilization mechanisms of solid particles and soluble
compounds in hybrid Pickering emulsions stabilized by
unrefined apple pomace powder

Charlotte Hollestelle<sup>a</sup>, Camille Michon<sup>a</sup>, Nathalie Fayolle<sup>b</sup>, Delphine Huc-Mathis<sup>a</sup>

Delphine HUC-MATHIS,

AgroParisTech, 22 Place de l'Agronomie, 91120 PALAISEAU, France

delphine.huc@agroparistech.fr

Abstract

Vegetal by-products powders, such as apple pomace powder, represent an efficient green alternative to conventional emulsifiers to stabilize oil-in-water Pickering emulsions. The natural material being used without purification, its multiple components shall impact differently the emulsion stability and interact with each other. These effects might be either synergic, competitive, or both depending on their ratio. Herein, the co-stabilizing effects of the apple pomace compounds on Pickering emulsions were explored from microscopic to macroscopic scales. The apple pomace powder was first fragmented into several fractions which were characterized (particle sizes, shape and wettability, oil-water surface activity). Emulsions were then prepared by varying the stabilizing material composition in using the different fractions obtained from the apple powder. Emulsions properties (droplet size, rheological properties) and stability were monitored over a month. The key role of solid particles in the surface activity of the apple pomace powder was showed by static interfacial tension measurements. All particle-stabilized emulsions were stable against coalescence during storage despite high droplet diameters (up to 80 µm) and low surface coverage. Soluble matter and particles show a synergic behaviour to stabilize oil-in-water emulsions. The soluble matter drives droplet size whereas insoluble cellulosic particles are responsible for emulsion stability by both *Pickering* adsorption to the oil droplet and oil droplet trapping in a three-dimensional network. This study provides new insights into the understanding of natural powders stabilizing mechanisms that may lead to a better prediction of the potential of vegetal by-products as stabilizing agents from their chemical composition.

<footnote><sup>a</sup>Université Paris-Saclay, INRAE, AgroParisTech, UMR SayFood, 91120, Palaiseau, France</footnote>

<footnote><sup>b</sup>JRS, Les Tanneries Royales, 20 quater rue Schnapper, 78100 Saint Germain en Laye, France</footnote>

<footnote>* corresponding author</footnote>
## Highlights

* Static interfacial tension measurements showed the mixture apple pomace micrometric particles + soluble matter had a surface activity higher than the soluble matter alone, highlighting the solid particles key role in the apple pomace powder surface activity.

* Interaction between solid particles and the soluble compounds of apple pomace powder on emulsion stabilization are either synergistic or competitive depending on their ratio

* Soluble compounds drive the emulsion droplet size

* Apple pomace particles protect the emulsions from coalescence even at very low surface coverage (below 5% of total interfacial areas being covered by solid particles)

## Keywords

Hybrid interface; Synergistic effect; By-product; Pickering; multi-scale; interfacial tension; rheology

## 1. Introduction

Emulsified systems are ubiquitous among processed food (mayonnaise, sauces, milk, ice-creams...). In response to the thriving consumers demand for healthier and more sustainable foodstuffs, increasingly referred to as 'clean label' products [1], the food industry has to change its well-established emulsifying strategies [2]. Indeed, the ingredientss commonly used to stabilize dispersed systems (surfactants, animals and dairy proteins) are being questioned and considered as synthetic or highly purified additives that can even cause some health issues [3]. Known as efficient alternatives to usual food emulsifiers for emulsion stabilization, solid particles could be a lever towards clean label emulsions. These particle-stabilized emulsions, also termed Pickering emulsions, have raised interest far beyond the food industry for decades. Indeed, they display very interesting features such as strong stability against coalescence, even at large droplet sizes, which makes them a powerful tool for many applications (catalysis, porous material conception, filtration, tissue engineering, drug delivery...) [4].

Although knowledge on these systems has been built with inorganic particles (silica, alumina, titanium oxides, clay...) [5], their use in formulated products is limited by their poor biodegradability and biocompatibility [6]. To this end, literature focused on bio-based particles
such as starch [5], proteins [7] and fat crystals [6]. However, the particles preparation
procedure often requires chemicals and solvents, whether to modify particles wettability, shape
the material into a particle or in most cases to isolate a target fraction from the original matrix.
In addition to energy consumption, the separation of one component from its source leads to
the loss of healthful and bioactive ingredients (polyphenols, flavonoids...) [8]. Considering that
two main pillars towards a sustainable food production system are prioritizing functionality over
purity [9] and reducing the food losses [10], using the by-products derived from food
manufacturing as raw material for emulsion stabilization is of great interest. Indeed, vegetal
by-products may represent a new sustainable source of particles for Pickering emulsion
stabilization. Some efficient proofs of concept were established on particles from varied vegetal
sources [11]–[14].

Conversely to most particles that have been studied so far, unpurified natural ingredients are
complex multicomponent systems constituted of numerous compounds which can be both
insoluble or soluble (insoluble solid particles, surfactants, proteins, thickening polymers...).
The stabilizing mechanisms of these compounds used alone are already plural, and become
even more complex when mixed together. Indeed, surfactants can both reduce the interfacial
tension and modify particles wettability to increase their anchoring ability. Polymers can adsorb
at interfaces through amphiphilic regions and thicken the continuous phase. Even the particle-
induced stabilizing mechanisms appear many-faceted as they can involve both interfacial
colonization and droplets trapping into a three-dimensional network [15]. Therefore, the
soluble components effect on the stabilizing properties of insoluble particles might be either
synergetic or competitive [16] [17]. Identifying, decorrelating and prioritizing the impact of each
fraction in the stabilizing mechanisms of vegetal powders is hence a major challenge. The
combined use of particles with different species for emulsions co-stabilization start to be
studied: 1) particles and surfactants [18]–[21]; 2) particles and proteins [22]; 3) particles and
polysaccharides [23]; 4) particles, surfactant and polymer [24]. Yet, these researches mainly
focus on only two or three species at a time and studies centred on emulsifying mechanisms
on more complex mixes remain rare. However, knowledge on mechanisms is crucial to master
and thus democratize the use of unpurified natural material as emulsion stabilizers.

This work intends to provide in-depth insight into the multiple emulsion stabilizing mechanisms
of natural vegetal powders. The study was conducted on apple pomace powder as it
constitutes the major by-product of one of the most processed fruit [25]. It focusses on
identifying and decorrelating the role of their insoluble (solid particles) and soluble fractions on
the stabilizing properties of apple pomace powder as their different stabilization modes were
previously highlighted [26]. The study voluntarily focused on micron-sized particles because
the evolution of regulations governing the use of nanoparticles is still uncertain.
**2. Materials and methods**

All experiences were conducted at ambient temperature. All centrifugation steps were carried out 30 mins at 14,000 g at 20°C except stated otherwise in dedicated paragraph. The term "soluble" is referring to all the components that do not sediment during a 30mins centrifugation at 14,000 g of a 10% (w/w) dispersion of apple pomace powder in distilled water throughout the article.

**2.1. Particles processing and emulsion phases**

Apple pomace powder (VITACEL® CS 5), i.e. apple pomace from the fruit industry thermomechanically processed into very fine powder, was kindly provided by JRS (Rosenberg, Germany). The supplier compositional information is the following: dietary fiber content (according to AOAC-method) is of around 55% of which 45% of insoluble dietary fibers and 10% of soluble dietary fibers mainly composed of pectins (9%). The total amount of proteins in the powder was estimated to be around 7% by the Kjeldahl method. Former work showed that some of these proteins are soluble in our operating conditions [26]. From this native natural powder (NP), several fractions were prepared. The soluble fraction was obtained by isolating the supernatant collected after centrifugation as described below. Part of it was freeze-dried to get pure soluble matter. The insoluble fraction of the powder, i.e. the part that does not dissolve into the continuous phase during the emulsification, will be referred to as particles in the whole article. In order to obtain fractions with a wide range of water soluble components concentrations, the NP was washed several times following a protocol adapted from [27]. The NP was first suspended (10% w/w) in distilled water using a Polytron rotor-stator operating at 10,000 rpm for 3 min (PTG 30/2, Kinematica AG, Switzerland). The supernatant was then isolated after centrifugation. A part of the remaining pellet was freeze-dried and referred to as 1-time-washed powder (1WP). The other part was washed again by 6 successive resuspensions of the pellet in distilled water with 30 s of vortex. After each washing, water-soluble compounds were removed by isolating the supernatant retrieved after centrifugation. The remaining pellet after 2 and 6 washing steps were freeze-dried to give the powders washed respectively 3 and 7 times (3WP & 7WP).

The oil phase of the emulsions consisted of commercially available sunflower oil (purchased in a local supermarket). Distilled water was used for all samples. Potassium sorbate was added through the aqueous phase at 2.6 g.L⁻¹.

**2.2. Dispersion and emulsification processes**
Oil-in-water emulsions were prepared following a lab protocol adapted from [11]. The powders previously prepared (NP, 1WP, 3WP, 7WP) were first dispersed in the oil phase using a Polytron rotor-stator operating at 10,000 rpm for 2 minutes (PTG 30/2, Kinematica AG, Switzerland). The aqueous phase was then added to the dispersion and the emulsification was carried out using the same device at 10,000 rpm for 3 minutes. 150 g of the reference emulsion were prepared with the NP. The powder concentration was set at 5% wt/wt and both oil and water phase concentrations were set at 47.5% wt/wt. For other emulsions, oil and water masses were maintained equal to the reference emulsion. Powders amounts were adapted so that particles content remains equal for all emulsions except for the emulsion with soluble part only. Emulsions enriched in soluble compounds were obtained by dissolving specific amounts of freeze-dried soluble fraction by magnetic stirring in the aqueous phase. All emulsions were processed in a 250 mL tall glass beaker. Emulsions compositions are summarized in Table 3.

## 2.3. Powders characterization

### 2.3.1. Particles sizing

Particle size distribution in powder aqueous suspension was measured using a Mastersizer 2000 analyzer (Malvern Instruments, Worcestershire, UK). Measurements were performed using a liquid cell at room temperature (RI=1.5, RI dispersant 1.33, Ab 0.01). Powder aqueous suspensions were 10-fold diluted and suspended in the same conditions as applied for emulsions manufacture. Particle size distribution is expressed as volume of particles (%) = f(diameter). The diameter of the main population in volume (Dmax) was retrieved as main particle size indicator. Only one measurement was carried out for washed powders.

### 2.3.2. Morphology assessment with electronic microscopy

Powders morphology was observed with an environmental scanning electronic microscope (ESEM, Quanta 200, FEI Company, Oxford Instruments). Samples were fixed on an aluminium stub with a carbon double-side tape and were then coated with gold (Emitech K550). Images were obtained with the following conditions: secondary electron detector, low vacuum mode (53 Pa), an accelerating voltage of 12 kV, a spot size of 4 and a work distance of approximate 13 mm. 5 images were collected for each sample.

### 2.3.3. Dry matter content and particle content

The dry matter content (DM) of different samples was calculated according to Equation 1.

Equation 1

$$DM (\%) = (1 - \frac{m(in) - m(f)}{m(in)}) \times 100$$
m(in) being the mass of sample put into the oven and m(f) the mass of oven-dried sample (7 hours at 105 °C) weighted after being cooled down to ambient temperature in a desiccator for 30 min.

The particle content (%) was calculated as 1 minus the powder soluble fraction (%). The latter was measured by the following protocol: the powders (10% wt/wt) were suspended in distilled water using conditions similar to the ones encountered during the emulsification process (3 min of Polytron rotor-stator at 10,000 rpm). The suspension was centrifuged and the retrieved mass of supernatant (m(s)) was weighted. Then the supernatant dry matter content (SuDM) was calculated from Equation 1. The soluble fraction was given by the following formula (Equation 2):

Equation 2

$$ \text{Soluble fraction (\%)} = \frac{m(s) \times \text{SuDM} \times 100}{m(p, in)} $$

m(p,in) being the mass of powder present in the centrifuged suspension.

### 2.3.4. Water binding capacity (WBC)

The water binding capacity was calculated based on the centrifugation described above, according to the following formula (Equation 3):

Equation 3

$$ \text{WBC (\%)} = \frac{100 \times (m(p, in) - m(S))}{m(p, in)} $$

m(p,in) being the mass of powder present in the centrifuged suspension and m(S) the mass of supernatant retrieved after centrifugation.

### 2.3.5. Surface activity

Interfacial tension (σ, mN.m⁻¹) measurements were performed using a Tracker tensiometer (Teclis, Tassin, France) at 25°C with a droplet volume of 25 µL in a rising mode (sunflower oil injected from the Teflon syringe into a quartz glass cuvette filled with the different samples solutions under weak magnetic agitation) for approximately 6 h. For some measurements, a purification of the sunflower oil was required. The oil was treated by mixing it with some Florisil® (Merck) (10% wt/wt) for 30 min by magnetic stirring. The adsorbent was then isolated from the oil by centrifugation. Measurements were performed 1) on the purified sunflower oil as reference; 2) on a NP dispersion (0.01% w/w) in distilled water after suspension with a Polytron rotor-stator operating at 10,000 rpm for 3 min (PTG 30/2, Kinematica AG, Switzerland); 3) on the supernatant retrieved from the centrifugation of the 0.01% suspension.

## 2.4. Emulsions characterization
### 2.4.1. Droplet size distribution

Droplet size distribution was measured using a Mastersizer 2000 analyzer (Malvern Instruments, Worcestershire, UK) on emulsions 10-fold diluted with distilled water. Measurements were made using a liquid cell at room temperature (RI=1.47, RI dispersant 1.33, Ab 0.01). Three repetitions were performed for each sample at day 0, 7, 14, 21 and 28.

### 2.4.2. Rheological properties

Rheological properties of the emulsions were measured with a MCR 301 rheometer (Anton Paar, Graz, Austria) using a striated plate-plate geometry (5 cm diameter, 1 mm gap, using a solvent trap to avoid water loss from the sample). The temperature was set at 25 °C and controlled with a Peltier plate. For the particular case where the emulsion drained over time, the rheological measurements were performed on the upper emulsified phase (thus concentrated in dispersed phase).

The flow properties were measured at 25°C in the shear rate range from 10 s⁻¹ to 100 s⁻¹. Five different shear rates were applied (10, 25, 50, 75, 100 s⁻¹) during 1 min each. Apparent viscosity was taken at 50 s⁻¹, which corresponds to the approximate shear rate in mouth [28].

The storage G' (Pa) and loss G" (Pa) moduli were measured through a frequency sweep carried out from 100 to 0.1 Hz, with a 0.1% maximum amplitude strain, taken from the viscoelastic linear domain of all samples.

### 2.4.3. Microstructure imaging

Emulsions were observed under a light microscope (MZ6, Leica, Germany) at 50-fold magnification. Confocal scanning electron microscopy (CLSM) was also performed on emulsions 10-fold diluted with distilled water using a reversed CLSM (CLSM, TCS SP8 AOBS, Leica, Germany). The samples were put on a cover-slide already bearing a drop of Red Congo probe (Sigma-Aldrich, USA) with λ<sub>excitation</sub> = 497 nm for cellulosic particles (pink), and a drop of DyLight (Thermo Fisher Scientific, USA) with λ<sub>excitation</sub> = 488 nm for proteins (green). x40 (water) and x100 (oil) objectives were used. For both methods, at least 3 pictures were taken for each sample. The images displayed in the present article were selected by the authors as representative of the obtained data.

### 2.4.4. Anchoring rate & coverage rate

The anchoring rate (AR) is defined as the percentage of particles attached to the droplets, relative to the total particles content (Equation 4):
Equation 4

$$AR (\%) = (m_p^i - m_p^f) / m_p^i$$

with $m_p^f$ being the mass of non-adsorbed particles and $m_p^i$ the mass of particles introduced in emulsion. The protocol used to quantify $m_p^f$ is adapted from [29]. The emulsions 10-fold diluted with distilled water were stored 1 day to allow natural creaming. The cream was carefully removed with a Pasteur pipette, 10-fold diluted and stored for 1 day. Both subnatant phases were gathered and centrifugated 5 min with increasing speed (20 g, 100 g, 1000 g, 7000 g and 14,000 g) to remove the residual oil droplets while preventing particle desorption. The thin cream layer was removed from the tube and replaced by distilled water after each centrifugation step. After the last centrifugation, a fully transparent upper phase was observed, reflecting the absence of droplets. This latter phase was removed to delete remaining soluble compounds. The remaining solid particles were freeze-dried and weighted to obtain the mass of free particles.

The precise coverage rate (C) was calculated as the average oil droplets area ($A_d$) to adsorbed particles area ($A_p^{ads}$) ratio.

To calculate the average $A_d$, droplets were hypothesized to be spheres having the diameter of the main population in volume ($D_{max}$) obtained during the droplet size distribution evaluation which gives Equation 5:

Equation 5

$$A_d = n_d * A_{1d} = \frac{V_t}{V_d} \times 4\pi\left(\frac{d_{max}}{2}\right)^2 = \frac{V_t * 4\pi\left(\frac{d_{max}}{2}\right)^2}{\frac{4}{3} * \pi * \left(\frac{d_{max}}{2}\right)^3} = \frac{V_t * 6}{d_{max}}$$

Where $n_d$ is the total number of droplets, $A_{1d}$ the area of surface of 1 droplet, $V_t$ the total volume of oil dispersed in emulsion, $V_d$ the volume of 1 droplet.

To calculate the total area of particles introduced in emulsion ($A_p$) particles were assimilated to spheres, that can cover an area of the corresponding disk. Particle size distribution measured is expressed as volume of particles (%) = $f(diameter)$. As the particles size distribution in not monomodal with one specific diameter, we calculate the total particle area as the sum of the areas corresponding to each particle size (Equation 6):

Equation 6

$$A_p = \sum_i A_p^i(d_i) = \sum_i \frac{V_{p,t}^i}{V_p^i} \times \pi\left(\frac{d_i}{2}\right)^2 = \sum_i \frac{\frac{m_p}{\rho_p} \times \%_{di}}{\frac{4}{3} * \pi * \left(\frac{d_i}{2}\right)^3} \times \pi\left(\frac{d_i}{2}\right)^2 = \sum_i \frac{m_p \times 3 \times \%_{di}}{2 \times \rho_p \times d_i}$$
Where $V_{p,t}^i$ is the total volume of particle of diameter $d_i$, $V_p^i$ the volume of 1 particle of diameter $d_i$, $m_p$ the total mass of particles introduced, and $\rho_p$ the particles density.

Thus, the coverage rate, C, is calculated using the obtained Equation 7 :

Equation 7

$$C (\%) = \frac{A_p^{ads}}{A_d} \times 100 = \frac{AR \times A_p}{A_d} = AR \times \frac{d_{max} \times m_p \times \sum_i \frac{\%di}{d_i}}{4 \times V_t \times \rho_p}$$

## 2.5. Statistical analysis

All measurements were made in triplicates unless indicated in the dedicated paragraph. The significance of the differences was checked through ANOVA test using XLStat (Addinsoft, USA) software with a 5% trust interval.

# 3. Results & discussion

## 3.1. Characterization of the powders

As particles sizes, shape and wettability are key parameters of Pickering emulsion stability, the obtained powder fractions were first characterized.

<table><caption>Table 1 - Physical properties of native apple powder (NP), 1-time-washed powder (1WP), 3-times-washed powder (3WP) and 7-times-washed powder (7WP). All the values are Mean ± Standard Deviation (n = 3).</caption><thead><tr><th>Powder</th><th>DM (%)</th><th>PC (%)</th><th>WBC (g/g)</th><th>D<sub>max</sub> (μm)</th></tr></thead><tbody><tr><td>NP</td><td>97.4±0.0003</td><td>83.92±0.03</td><td>3.00±0.01</td><td>6.6±0.5</td></tr><tr><td>1WP</td><td>98.9±0.003</td><td>93.28±0.06</td><td>3.64±0.01</td><td>7.8±0.5</td></tr><tr><td>3WP</td><td>98.1</td><td>97.03±1.04</td><td>4.31±0.01</td><td>/</td></tr><tr><td>7WP</td><td>97.2±0.0007</td><td>97.92±0.07</td><td>4.23±0.02</td><td>7.1±0.0</td></tr></tbody></table>

### 3.1.1. Composition and water binding capacity (WBC)

Table 1 summarizes the powder dry matter content (DM), the particle content (PC), the $D_{max}$ and the water biding capacity (WBC) of the different powders. Logically the particle content significantly increased through the successive washing cycles. Soluble compounds were thus efficiently removed by the washing method. In the meantime, the water binding capacity of the powder increased while the dry matter content was globally constant (1 to 3 % w/w). The significant water binding capacity increase between the native powder (NP) and the freeze-dried powders (1WP, 3WP and 7WP) can be explained by the microstructure change induced
by freeze-drying. Indeed, higher porosity has been previously reported for freeze dried fruits and vegetable powders, which improves their rehydration capacity [30], [31]. The water binding capacity increase between 1WP, 3WP and 7WP can be related to sugars or proteins removal while washing as both compounds can impair hydration capacity [32], [33].

### 3.1.2. Morphology

The surface morphology of dry powders was assessed by environmental scanning electronic microscopy to further support the hypothesis of the freeze-drying impact on their morphology and functionality (Figure 1b-d). The surface morphology was highly dependent on the freeze-drying pre-treatment. The microstructure was highly porous when freeze-dried. This result is consistent with the enhanced water binding capacity observed for 1WP, 3WP and 7 WP powders and described in the literature [8, 9]. Moreover, freeze-dried samples exhibited larger aggregates than NP.

### 3.1.3. Particles size distribution

<figure><img src="image_3.png" /></figure>
Figure 1 - Droplet size distribution of NP, 7WP and 7WP-US (a), Scanning electron micrographs taken by Dr. Giana Almeida with a x2000 magnification of the native apple powder (b), powder washed 1 time (c) and powder washed 7 times (d)

To assess the processing effect on particles size, the particles size distribution of NP and of the most processed powder (7WP) are compared after dispersion by rotor/stator turbulent mixing. Results are presented Figure 1a. The diameters of the main population in volume $D_{max}$ are summarized in Table 1. $D_{max}$ are comprised between around 6-7µm for both powders, which is similar to A. Schröder et al. particles (from spirulina cake or matcha tea powders) used to stabilize emulsion [14]. However, whereas the NP particles only display one particle size class, 7WP particles have three size classes: 1) the main population in volume with a diameter of 7 µm; 2) a second population attributed to particles aggregates with a diameter of 100 µm; 3) a few larger particle aggregates around 1 mm (Figure 1a). The presence of particle aggregates was corroborated by CLSM images (white arrows on Figure 2f).

<figure><img src="image_4.png" /><figcaption>Figure 2 - Emulsions images at day 0 (D0) and day 28 (D28), Emulsion Light microscopy images at D0 for (a) E-S (b) E-NP (c) E-7WP magnification x50, CLSM image of (d) E-S (e) E-NP (f) E-7WP magnification x100, Oil droplet size distributions of (h) E-S (i) E-NP</figcaption></figure>

To verify that the clusters were aggregates and not a single entity resulting from particles coagulation, the 7WP was treated by ultrasounds for 30s. The ultrasounds treated 7WP and
NP superimposed curved proved it was indeed particle aggregation. Particle size distribution of 3WP and 1WP were similar to the one of 7WP (results not shown). Powder processing had no impact on the main particle size population although it increased its water binding capacity. However, some particle aggregates were formed during powder processing and can remain in final emulsions as they are not all disaggregated during the rotor-stator suspension.

### 3.1.4. Interfacial activity

The comparison between the surface-active properties of the NP and its soluble fraction is summarized in Table 2.

<table><caption>Table 2 - Surface tension (σ) measurements at the initial and final (6h) times obtained for commercial and purified sunflower oil, with distilled water, 0.01% (wt/wt) suspension of apple powder (NP), supernatant of the 0.01% (wt/wt) suspension. All the values are Average ± Standard Deviation (n = 3).</caption><thead><tr><th></th><th>Oil type</th><th>Distilled water</th><th>NP (0.01%)</th><th>Soluble fraction of NP at 0.01%</th></tr></thead><tbody><tr><td>σ Initial (mN.m<sup>-1</sup>)</td><td rowspan="2">Commercial</td><td>25.1±0.1</td><td>24.8±0.4</td><td>25.1±0.16</td></tr><tr><td>σ Final (mN.m<sup>-1</sup>)</td><td>20.2±1.6</td><td>19.3±1.0</td><td>20.2±1.3</td></tr><tr><td>σ Initial (mN.m<sup>-1</sup>)</td><td rowspan="2">Purified</td><td>27.3±0.2</td><td>27.3±0.6</td><td>27.8±0.7</td></tr><tr><td>σ Final (mN.m<sup>-1</sup>)</td><td>25.6±1.3</td><td>22.3±0.3</td><td>24.2±0.4</td></tr></tbody></table>

The curves (results not shown) displayed a typical shape as can be seen in Z. Huang et al. work [34]. The first measurements were performed on the commercial sunflower oil without further purification. The initial commercial oil-pure water interfacial tension (σ) was 25.1 mN.m⁻¹ which is slightly higher than in literature ([34], [35]). However, it is closer to the σ of a deacidified sunflower oil [35]. σ gradually decreased over time because of surface-active impurities going to the interface. As this decrease was important, no significant additional σ decrease was observed with the powder or with the supernatant. The oil was thus purified. Then, a significant effect of the NP was observed as it decreased the final σ from 3.3 mN.m⁻¹ compared to pure distilled water (p=0.014). The decrease is lower than the 7 mN.m⁻¹ observed by A. Tamayo-Tenorio et al. for cellulosic particles of approximately 0.4-10 µm at 0.001% [37]. However, it is consistent with the 3 mN.m⁻¹ observed by Z. Huang et al. for a 1% dispersion of carboxylated nanodiamonds [34]. Thus, although the purification did not remove all the surface-active impurities (σ still decreased from 27.3 mN.m⁻¹ to 25.6 mN.m⁻¹), it still enabled to highlight the powder surface activity. The native powder efficiently decreased σ while soluble compounds alone did not. Solid particles can thus be considered as surface active in the presence of soluble species. This surface activity might come from proteinic residues on cellulosic particles [36] or capillary interactions between particles due to their irregular shape previously highlighted by morphology assessment or the soft character induced by their high WBC which both favour local interface distortions [37].
## 3.2. Characterization of apple powders stabilized emulsions

### 3.2.1. Influence of the different powder fractions on emulsion characteristics

The emulsifying properties of the different fractions of micronized apple powder were assessed on oil-in-water emulsions. Table 3 summarizes emulsions composition and characteristics (main droplet size at day 0 (D0) and after 28 days of aging (D28), viscosity).

#### 3.2.1.1. Anchoring and coverage rate

Anchoring rate of E-NP, E-1WP, E-7WP and E-NP+S5% were respectively of 46±3%, 18±3%, 12±6% and 22±2%. The anchoring rate obtained for NP is consistent with literature on cocoa particles emulsified with a similar protocol [29]. As only 46% of particles are adsorbed onto the interfaces, many particles remain free in the continuous phase. Since no sedimentation was observed over time, it can be concluded that free particles form a network reinforced by embedded oil droplets. This is supported by previous results showing that apple pomace powder suspension alone displayed solid visco-elastic properties with $G' > G''$ and were stable against sedimentation for 3 days [11]. Washing the apple powder significantly decreased the anchoring ratio. Two hypotheses might explain this: 1) the porosity increase induced by freeze-drying decreased particles ability to anchor at oil-water interfaces as their hydrophobicity is lowered; 2) washing steps removed some soluble species such as proteins and surfactants linked to particles surface through hydrophobic or electrostatic interactions, easing their diffusion and adsorption to the interface [17], [38]. Indeed, surface activity of particles can be modified in the presence of surfactants [22], their removal by washing decreased particles attachment ability. However, when the concentration in soluble compounds was almost tripled, the anchoring rate was halved. This might be due to the competition between soluble compounds and particles at the interface. Indeed, while particles-small surfactants stabilizing effects are synergistic at low surfactant concentration, their effects become antagonistic when surfactant concentration increases as particles are removed from the interfaces [19], [39]. The surfactant-to-particle ratio is thus known to be a key parameter for Pickering emulsion stability [17].

The coverage rates (surface actually covered by particles-to-total interfacial area ratio) of E-NP, E-1WP and E-NP+S5% were respectively of 2.7%, 1.8% and 1.5% which is globally low compared to the densely packed layer of solid particles described as being necessary to provide an efficient protection from coalescence [40]. Other authors already obtained stable silica particles-stabilized emulsions with very low droplet surface coverage: down to respectively 8% and 5% for F. Gautier et al. [41] and E. Vignati et al. [42]. However, these systems are only stabilized by particles. The even lower surface coverage obtained here are
attributed to the interface hybrid colonization by both particles and other non-cellulosic surface actives components of the vegetal powder [26]. Moreover, it is worth specifying that calculations were made by approximating particles as spherical whereas the scanning electron micrograph show it is not accurate (Figure 1). This approximation might impact the obtained coverage rates.

### 3.2.1.2. Droplet size distribution

The emulsifying properties of the different powders were compared by measuring the initial oil droplet size. Results are summarized in Table 3.

<table><caption>Table 3. Emulsions composition and characterization parameters</caption><thead><tr><th>Emulsion description [code]</th><th>Powder type</th><th>Powder fraction (wt%)</th><th>Oil fraction (wt%)</th><th>Water fraction (wt%)</th><th>Soluble concentration in aqueous phase (%)</th><th>D<sub>max</sub> (µm) at D0</th><th>D<sub>max</sub> (µm) at D28</th><th>Viscosity at 10s<sup>-1</sup> (mPa*s)</th><th>Ostwald</th></tr></thead><tbody><tr><td>Reference with NP [E-NP]</td><td>NP</td><td>4.9</td><td>47.5</td><td>47.6</td><td>1.7</td><td>24.7±2.8</td><td>28.3±0.0</td><td>1639±81</td><td>0.34±0.01</td></tr><tr><td>With 1WP [E-1WP]</td><td>1WP</td><td>4.2</td><td>47.9</td><td>47.9</td><td>0.6</td><td>47.3±7.9</td><td>49.1±0.0</td><td>716±7</td><td>0.36±0.03</td></tr><tr><td>With 3WP [E-3W]</td><td>3WP</td><td>4.2</td><td>47.9</td><td>47.9</td><td>0.3</td><td>42.8±0.0</td><td>/</td><td>664±20</td><td>0.29±0.01</td></tr><tr><td>With 7WP [E-7W]</td><td>7WP</td><td>4.1</td><td>47.9</td><td>48.0</td><td>0.2</td><td>81.6±6.4</td><td>98.0±0.0</td><td>451±15</td><td>0.40±0.01</td></tr><tr><td>Reconstituted [E-r]</td><td>1WP</td><td>4.2</td><td>47.9</td><td>47.9</td><td>1.7</td><td>27.0±2.1</td><td>/</td><td>1510±28</td><td>0.32±0.01</td></tr><tr><td>Enriched in soluble [E-NP+S3%]</td><td>NP</td><td>4.9</td><td>47.5</td><td>47.6</td><td>3.25</td><td>21.4±0.0</td><td>/</td><td>3300±170</td><td>0.34±0.01</td></tr><tr><td>Emulsion enriched in soluble [E-NP+S5%]</td><td>NP</td><td>4.9</td><td>47.5</td><td>47.6</td><td>4.8</td><td>25.8±2.1</td><td>34.0±3</td><td>4281±295</td><td>0.39±0.01</td></tr><tr><td>Soluble fraction only [E-S]</td><td>/</td><td>0</td><td>50.0</td><td>50.0</td><td>1.7</td><td>21.6±3.4</td><td>49.1±0.0</td><td>165±3</td><td>0.83±0.01</td></tr></tbody></table>

Droplet size ranged between 21 µm and 82 µm. The size range is surprising considering that the ideal particle-to-droplet size ratio is of one order of magnitude [43]. However, stable emulsions can be obtained with higher particle-to-droplet ratio. For example, R. Gupta et al. observed a ratio of around 33% with solid lipid nanoparticles [44], which is close to the 28% obtained here (with 7 µm for particle size and 25 µm for droplet size).
<figure><img src="image_5.png" /><figcaption>Figure 3 – Inverse of the initial droplet size ($D_{max}$) as a function of the concentration in soluble species in the aqueous phase ([soluble species]$_{aq}$)</figcaption></figure>

Figure 3 shows the evolution of the inverse of oil droplet size as a function of soluble matter concentration for the different emulsions. The reference main droplet population displayed a diameter of 25 µm which is consistent with previous work [26]. The coarser emulsion was obtained with the lowest concentration of soluble compounds. Indeed, as the soluble content increased, the average droplet size decreased from 82 µm to a plateau value around 21-27 µm (Figure 3). This latest threshold might correspond to the maximum shear rate of the emulsifying tool in these conditions. Two main hypotheses could explain the droplet size decrease between E-7WP, E-1WP and E-NP: 1) low surfactant concentration have been proved to favour droplet breakup during emulsification and prevent fast recoalescence, hence giving enough time for particles to adsorb, leading to finer emulsions [16]; 2) as the anchoring ratio decreased between E-NP, E-1WP and E-7WP, less particles are anchored to the interfaces inducing larger droplets for E-7WP and E-1WP [29]. However, E-NP and E-NP+S5% exhibited the same droplet sizes although E-NP+S5% anchoring ratio was halved compared to E-NP. The droplet size dependence to the water-soluble components' concentration (Figure 3) thus shows that the emulsion initial droplet size is driven by the water-soluble species. Further evidence is gathered by comparing E-NP, E-S and E-r. Indeed, they show the same droplet size whereas E-S does not contain any particles and E-r contains washed particles. However, they all contain the exact same concentration in water-soluble species. This droplet size control by the soluble species in the presence of particles is consistent with literature studies led on starch particles mixed with different types of common surfactants [45] and with low-purified orange fibres powder [38].

### 3.2.1.3. Microstructure
In order to better understand the microstructure, representative light and confocal microscopy images of the emulsions are displayed in Figure 2. A double staining of cellulosic particles and proteins was made to capture confocal microscopy images to identify possible co-localization of both components. Confocal microscopy images display cellulosic particles in pink, and proteins were stained in green through the whole continuous phase which makes it impossible to distinguish specific adsorption sites of proteins at the interface (Figure 2.d-f). E-NP and E-7WP display similar microstructures: a dispersed oil phase in a continuous water phase containing suspended particles which form a network in which oil droplets are embedded (Figure 2b-c;e-f). Contrary to emulsions with particles, oil droplets are aggregated in E-S (Figure 2d-f). This droplets aggregation – or flocculation – is often encountered in proteins stabilized emulsions [22]. Two very different droplet size classes appear on E-S confocal microscopy images (Figure 2a;d): the first around 1 µm and the second around 20 µm which is confirmed by the bimodal distribution size curve (Figure 2h). The bigger droplets are surrounded by slight pink circles, suggesting that some very fine particles were still present in the supernatant (Figure 2d). The droplet size observed on micrographs are consistent with droplet size summarized Table 3 except for E-S light microscopy image as coalescence had already occurred during the small delay before light microscopy images were taken (Figure 2). In particles-stabilized emulsions, particles are clearly identified as being both adsorbed onto the interfaces and present in the continuous phase (Figure 2b-c;e-f). Free particles presence is highlighted by the small peak around 6-8 µm in both E-NP and E-7WP size distributions (Figure 2i-j) which corroborates anchoring rate results. As observed by A. Tamayo Tenorio et al.[36], particles anchored at the interfaces are the smaller, whereas bigger particles form a network in the continuous phase. Droplet surfaces are not completely covered with particles which is consistent with low coverage rates calculated above.

### 3.2.1.4. Rheological properties
<figure><img src="image_6.png" /><figcaption>Figure 4 - Elastic (full symbols) and loss (empty symbols) modules as a function of angular frequency at D0 (a) and G'/G' ratio as a function of angular frequency at D0 (b) of E-NP (●), E-r (●), E-S (●), E-1WP (■), E-3WP (■), E-7WP (■), E-NP+S3% (▲) E-NP+S5% (▲)</figcaption></figure>

Rheological properties of emulsions were investigated through apparent viscosity (results summarized Table 3) and dynamic viscoelasticity (Figure 4) measurements. Firstly, all emulsions exhibited a shear-thinning behaviour for emulsions with Ostwald n values comprised between 0.29 and 0.40 (Table 3**Erreur ! Source du renvoi introuvable**.) for all particles containing emulsions. This typical behaviour of emulsions can be explained by the disrupt of the interactions between the different dispersed compounds under the external shear force [46]. The emulsion devoid of particles (E-S) behaviour was closer to a Newtonian fluid (n=0.83) as the dispersed phase and soluble components were not highly concentrated. It can be concluded that particles are mainly responsible for the shear-thinning behaviour of these emulsions. The apparent viscosity (at 10 s⁻¹) of particles-stabilized emulsions is a linear function of the concentration in soluble species (R²=0.989). The emulsions devoid of particles (E-S) display the lowest viscosity of 165 mPa.s. When particles are added while keeping the concentration in soluble compounds constant (E-R and E-NP) the viscosity is 10-times higher (respectively 1510 and 1635 mPa.s). In addition, when particles are used alone (E-7WP) the viscosity is divided by 4 (451 mPa.s) compared to E-NP. It highlights the synergic effects of particles and soluble species on the network strength. E-r shows a viscosity similar to E-NP (respectively 1510±28 mPa.s⁻¹ versus 1635±81 mPa.s⁻¹). This suggests that the soluble compounds removal from particles surface induced by washing has only a slight impact on the flow behaviour.
Secondly, dynamic measurements showed that G' > G'' and G' almost constant and tending to a plateau value at low frequencies (Figure 4a) in the explored frequency-range. Thus, all particles-stabilized emulsions exhibited a viscoelastic solid behaviour in the explored frequency range. This rheological property is common for emulsions charged in oil as a closely packing system is reached [11]. However, E-S which also contained 50% of dispersed oil exhibited a viscoelastic liquid behaviour with G' > G'' in almost all the frequency range and G' tending to very small values at low frequencies (Figure 4a). It shows that the particles network enables to produce an elastic behaviour even with a volume fraction of dispersed phase below the close-packing limit for spheres. G' values at low frequency spanned from 0.03±0.01 Pa for E-S to 100.7±5.0 Pa for E-NP+S5%. G' and G'' gradually increased with the increasing concentration in soluble species, suggesting a progressive strengthening of the network by soluble polymers of the powder (probably pectins). E-NP has a G' slightly higher than E-r, suggesting that soluble compounds attached to the particles surface have a slight effect on the network strength. E-1WP, E-3WP and E-7WP have similar G' at low frequency (Figure 4a).

Figure 4b shows the evolution of tanδ=G''/G' ratio) as a function of the angular frequency. The higher tanδ the less structured the studied system. E-S is not represented because as a viscoelastic liquid, its tanδ were very high (comprised between 2.2 and 4.5). If the corresponding curve was represented it would hinder other curves comparison. Four groups can be distinguished according to curves shapes (Figure 4b).E+S3% and E+S5% curves are almost superimposed and significantly positioned above all the other curves. These emulsions are less structured than the others. E-r and E-NP are very similar which confirms that soluble compounds attached to the particles surface have almost no effect on the viscoelastic behaviour of the particle and the emulsion networks. E-1WP curve is below E-r and E-NP curves and E-3WP is even more below. The decrease in soluble compounds concentration increases the particle network structuring. Finally, E-7WP curve is below all the other curves confirming the role of particle for structuring the emulsions. Moreover, it seems to reach a plateau at low frequencies which is characteristic of a reinforced solid-like behaviour.

### 3.2.2. Influence of powders composition on emulsion stability

The storage stability was assessed by monitoring E-NP, E-S, E-1WP, E-7WP and E-NP+S5% over a month, every 7 days. The visual appearance, droplet size, flow behaviour and viscoelastic properties were followed. Results are respectively reported in Figure 2a-c and Figure 5a, Figure 5b.
<figure><img src="image_7.png" /><figcaption>Figure 5 – Droplet size evolution overtime (a) and apparent viscosities at 10s⁻¹ as a function of aging time (b) of E-NP (●), E-S (♦), E-1WP (■), E-7WP (■) and E-NP+S5% (▲)</figcaption></figure>

### 3.2.2.1. Stability against coalescence

As can be seen in Figure 5a and Figure 3a, E-NP and E-1WP droplet size were stable over a month. For E-7WP and E-NP+S5%, the droplet size increased from D0 to D7, and remained then stable. Light backscattering measurements showed that this increase happened in the first 45 minutes after emulsification for E-7WP and in the first 24h for E-NP+S5% (results not shown). Besides, no macroscopic oil leakage was observed at the top of the recipients. This initial droplet increase is thus attributed to limited coalescence. These results showed that the washing degree of powders, i.e. their soluble content, had no effect on the emulsion stability overtime. The same phenomenon was observed with cocoa powder [13]. Moreover, whereas the emulsion solely stabilized by the powder soluble fraction (E-S) displays the same initial droplet size as the E-NP (Table 3), it exhibited a significant increase of its $D_{max}$ overtime with a 103% increase of its initial size (Figure 2a). On the contrary, E-7WP, devoid of soluble species, remained stable overtime. Thus, solid particles are responsible for the emulsion stability against coalescence. These results are in line with those obtained for spinach leaves, matcha tea, cocoa and rapeseed cake powders [13], [29]. This stabilization comes from both the droplets coverage by particles and their entanglement in the free particles network [38].

### 3.2.2.2. Stability against creaming

Drainage of the continuous phase quickly appeared after emulsification for E-S (visible serum layer on E-S D28 image of Figure 2a). For all other emulsions, no drainage was detected. The viscosity increase of the continuous phase by soluble polymers from apple powder (mainly pectins) is not sufficient to protect the emulsion from drainage. E-7WP being stable against
drainage, it can be deduced that the creaming stability is provided by the droplet trapping in the particles network which was particularly structured as shown by tanδ results.

### 3.2.2.3. Stability of rheological properties

Rheological properties were monitored over 28 days. Figure 5b shows the apparent viscosity evolution over time. The apparent viscosity significantly increased as a function of the aging time for all emulsions. For E-S, the viscosity increase can mainly be attributed to the drainage phenomena. Indeed, the measured sample is collected in the emulsified phase which has been concentrated in dispersed phase because of drainage. It leads to higher viscosity [26]. E-NP viscosity continuously increased over the first three weeks, to reach a 80±13% increase of its initial value at D28. E-1WP, E-7WP and E-NP+S5% also exhibited significative viscosity increase: respectively 69±5%, 51±1%* and 34±8%. d This hypothesis was checked by comparing the viscosity of the continuous phase of a suspension of powder in water at D0 and after 7 days of powder contact with water. It was indeed doubled from 7 mPa.s to 15 mPa.s at 50s⁻¹ (results not shown). It would explain that the viscosity increase follows the order E-NP > E-1WP > E-7WP as the more the powder is washed, the less soluble polymers can be released. However, the E-NP+S5% viscosity increase was much lower than for E-NP while they both contain the same powder amount. This could be explained by the high soluble polymers load in E-NP+S5% which would hinder soluble species release from the powder.

<figure><img src="image_8.png" /><figcaption>Figure 6 - Elastic modulus (G') evolution between D0 (light grey) and D28 (dark grey) (a) and evolution over aging time (a) and G''/G' ratio as a function of angular frequency at D0 (b) of E-NP (●), E-1WP (■), E-7WP (■), E-NP+S5% (▲)</figcaption></figure>

Regarding dynamic rheological properties, G' values of D0 are compared with D28 on Figure 6a at 1 Hz as G' values were almost constant over the explored domain of frequencies (Figure 4a). Difference between D0 and D28 were significant only for E-S and E+S5%. E-S difference is explained by the enrichment in dispersed phase by drainage. E-NP+S5% increase might be
also due to the presence of more polymers which strengthen the network. The evolution of tanδ as a function of the angular frequency at D28 (Figure 6b) can be compared to D0 (Figure 4b). Contrary to what was observed at D0, after 28 days only two groups are distinguishable: 1) E-W1, E-NP, E-NP+S5%; 2) E-7WP. Indeed, E-7WP curve is much below the other ones over most of the angular frequency range. After 28 days of aging, the E-7WP network is much more structured than those of the other emulsions. It highlights the difference of network structuring when particles are only composing it. Moreover, the ratio is higher for E-NP and E-1WP than E-NP+S5%, meaning the soluble compounds of E-NP+S5% led to a more organized network after this period. Finally, comparing D0 and D28 results, all curves show a higher tanδ with a smaller variation as a function of angular frequency after 28 days of aging. These differences show the impact of the time on the network connections type and ways to answer to dynamic solicitations.

## 4. Conclusion

We herein proposed a novel method to further study the role of both soluble and insoluble content of vegetal powders in emulsion stabilizing mechanisms by characterizing different fractions of apple pomace powder and their emulsifying properties.

First, the native powder characterization highlighted the key role of micrometric solid particles in its surface activity. Indeed, we showed that native powder decreased the oil-water interfacial tension whereas its soluble content alone did not. It can be concluded that particles are surface active in the presence of soluble compounds, probably due to capillary inter-particular interactions.

Secondly, the powder soluble matter and the particles were found to exhibit synergetic behaviour for emulsifying and stabilizing the oil-in-water emulsions. The soluble compounds lead to fine emulsions by favouring droplet breakup during emulsification whereas particles are responsible for the emulsion stability. Indeed, all particles-stabilized emulsions were stable whether they contain soluble compounds or not, even with coverage rate as low as 1.5%. Cellulosic solid particles protect the emulsions against coalescence by the combination of two mechanisms which remain difficult to decorrelate: 1) by covering droplets interfaces; 2) by embedding the droplets into a network which contributes substantially to the rheological properties of the emulsions. The latter effect also hinders creaming phenomenon.

The impact of apple pomace soluble compounds on particles-stabilized emulsion was studied over a wide range of concentration, far beyond the native concentration of soluble species in the native powder. It highlighted the concentration-dependent effect of soluble compounds on the stabilizing properties of emulsions. Indeed, at low soluble compounds concentration,
particles and soluble compounds play complementary roles in the emulsion structuring and stabilizing mechanism: soluble compounds ease the droplet breakage during emulsification leading to smaller droplets while particles hinder the coalescence phenomena. At higher concentrations in soluble compounds, some competitions appear with solid particles. The anchoring rate decrease with the increasing concentration in soluble compounds show that soluble compounds can either displace particles from interfaces whether hinder their adsorption by reaching first the interfaces.

To sum up, the work presented provides a useful strategy to decorrelate the emulsion stabilizing mechanisms of soluble and insoluble content of complex powders. Future work will provide more insight in the existence of a threshold concentration of soluble compounds from which soluble and insoluble content effect become competitive by surface activity studies on both fractions. Using this novel method on by-products with very different chemical compositions would also be of great interest to gain knowledge about other mechanisms to pave the way towards a possible prediction of by-products potential as emulsions stabilizers from their composition.

## Acknowledgements

The authors thank JRS Rettenmaier for providing the micronized apple powder and the very interesting collaborations. Many thanks go to Mrs Gabrielle Moulin and Dr Giana Almeida-Perre from SayFood for their precious help to get respectively the confocal laser scanning observations and the scanning electron microscopy images that were very useful in the mechanisms understanding. The authors also thank the ANR for financing the CLEVER project (ANR-20-CE21-005).

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