# Using a natural salivary substitute to improve the swallowability of semi-solid foods: an *in vitro* and *in vivo* study

Yurixy BUGARIN-CASTILLOª, Pascale BOU FADELª, Anne SAINT-EVEª, Marco RAMAIOLIª, Vincent MATHIEUª,*,

*Corresponding Author

Vincent Mathieu

Email: vincent.mathieu@inrae.fr

Paris-Saclay Food and Bio-product Engineering Research Unit (SayFood UMR 782)

University Paris Saclay, INRAE, AgroParisTech

22, place de l'Agronomie

91120 Palaiseau, France

**Declarations of interest:** None.

**Keywords:** Swallowing, Salivary substitute, Flaxseed extracts, Soft robotic, Sensory evaluation.

## Abstract

Flaxseed extracts are solutions and suspensions of polysaccharides with extensional and shear rheological properties close to those of saliva, making them promising candidates as natural salivary substitutes (NSS) to manage xerostomia. Both *in vitro* and *in vivo* approaches were considered to investigate the lubricating performance of such NSSs during swallowing of Greek yogurt and apple puree. The choice of these two semi-solid foods made it possible to compare colloidal systems with contrasting properties of composition, structure and rheology. The oral phase of swallowing was simulated *in vitro* with an innovative soft robotic model while sensory evaluations were performed with a healthy cohort (n=16) to assess the potential of the NSSs for reducing the amounts of post swallowing food residues into the mouth. Flaxseed extract with 0.40 % dry matter showed promising performance, with low amounts of oral residues reported both *in vitro* and *in vivo*, and with a bolus that was not prone to undesired fragmentation. The original approach presented in this research offers perspectives to prescreen other salivary substitutes or food textures, in order to help addressing the needs of growing populations affected by swallowing disorders, and in particular those caused by xerostomia. The development of natural saliva substitutes should make it possible to recover a state of oral lubrication which ensures oral comfort, safe swallowing, and which limits the problems of malnutrition resulting from dysphagia.

<footnote>ªUniversité Paris-Saclay, INRAE, AgroParisTech, UMR SayFood, 91120, Palaiseau, France</footnote>

<footnote>ᵇINRAE, Institut Agro, STLO, 35042 Rennes, France</footnote>
## Introduction

It is expected that by 2050, 21.5 % of the population will be over 60 years old and that age-related diseases will therefore be more prevalent (Lorieau et al., 2018; Schwartz et al., 2018). Swallowing disorders, also known as dysphagia, concern more than 40 % of people over 65 years old (Rofes et al., 2011). Dysphagia affects quality of life by increasing the risk of choking or aspiration in the lungs, and may lead to the avoidance of some food and drink categories, causing dehydration and malnutrition. Three leading causes can generate it: (i) the perturbation of muscle coordination (like with neurological impairments), (ii) the weakness and rapid fatigue of muscles (such as in the case of sarcopenia), and (iii) the decrease in salivary flow rate related to a dry mouth sensation (xerostomia) (Cassolato & Turnbull, 2003; Clavé & Shaker, 2015).

Salivary substitutes (SS) are effective treatments for xerostomia and work by coating oral surfaces and imitating saliva lubrication (Hu et al., 2021). Studies have appraised SS concerning oral comfort, wettability, rheology, and lubricity (Bugarin-Castillo et al., 2024; Hu et al., 2021; Pailler-Mattei et al., 2015; See et al., 2019; Shahdad et al., 2005; Spirk et al., 2019; Vinke et al., 2020). These characterizations are essential to imitate saliva lubrication, thickness, and moisturizing properties. The assessment of SS is generally achieved *in vivo* using questionnaires (Hahnel et al., 2009). Besides, instrumental approaches have been proposed to evaluate lubricity and wettability using *ex vivo* tissues to approach human conditions (Vinke et al., 2018).

However, the performance of salivary substitutes is not frequently evaluated during food oral processing (FOP) (Piaton et al., 2021). It is well established that a deficiency in saliva during FOP can affect bolus formation and lead to a measurable increase in post-swallowing oral residues (Wirth et al., 2016). Post-swallowing oral residues are described as the amount of food in the oral cavity after the main swallow and the willingness to clean it with multiple swallows (Hadde & Chen, 2021). The accumulation of residues promotes oral discomfort, decreases food palatability, and can cause post-swallow aspiration (Lavoisier et al., 2021). In addition, in the cases of neurological impairments or sarcopenia, poor handling of oral residues might increase the risk of choking (Gallegos et al., 2021). Furthermore, bolus formation is an essential and complex step in the preparation of the oral phase of swallowing. It includes mixing and hydrating food particles with saliva to increase cohesion until a compact bolus is able to flow through the pharynx (Avila-Sierra et al., 2024; Fujiso et al., 2018; Gallegos et al., 2021; Nishinari et al., 2024). During bolus formation, saliva also impacts the perception of food texture and taste. In particular, the bolus extensional properties have been shown in both *in vitro* (Marconati & Ramaioli, 2020) and *in vivo* conditions (Hadde et al., 2019) to foster bolus compactness. Given that the extensional properties of food bolus are the direct consequence of those of saliva, xerostomia could, in some cases, impair the compactness of the bolus.

The use of *in vitro* swallowing simulators constitutes a promising approach to study the influence of physiological disturbances on the food bolus flow during swallowing. In particular, they enable to screen different novel food products and assess their oral residence time, *in vitro* oral pressures and post-swallow residues without risks for patient safety. The different physical measurement modalities that are embedded in these devices, along with the control of experimental conditions that they allow, create an ideal environment for gaining a deeper understanding of the
mechanisms that govern the flow of food bolus. One example is the soft actuator developed by Marconati et al., (2020) known as the soft-robotic tongue (SRT). The *in vitro* set up is inspired by tongue movements during swallowing, including bolus retention and propulsion through the oropharynx. However, the model does not replicate saliva lubrication, which is critical for enhancing the realism of the SRT. Several issues arise when using real human saliva to lubricate swallowing simulators (technical, ethical, regulatory and microbiological). Moreover, one of the major characteristics of saliva is its strong intra- and inter-individual variability. As a consequence, it would be beneficial to explore the use of synthetic saliva substitutes, as long as their properties can realistically mimic the complex rheological and mucoadhesive properties of natural saliva.

Lately, several natural extracts, including flaxseed extracts, have been proposed as potential salivary substitutes (Bugarin-Castillo et al., 2024; Pabois et al., 2023; Wagner, 2015). Moreover, recent formulations show shear and extensional rheological properties similar to human saliva (Bugarin-Castillo et al., 2024).

The present study aims at testing these natural salivary substitutes in the swallowing simulator, and investigating how (i) walls and tongue lubrication and (ii) food properties may influence the formation of oral residues during swallowing. Two different semi-solid foods (apple puree and Greek yogurt) were considered. The measurements carried out on the *in vitro* swallowing simulator were also compared to sensory evaluations carried out by a trained panel. Coupling *in vitro* and *in vivo* approaches thus allows a better understanding of the mechanisms controlling the formation of oral residues during swallowing, so as to be able to identify strategies to reduce them.

## 2. Materials & Methods

### 2.1. *Natural Salivary Substitute (NSS)*

Natural Salivary Substitutes (NSS) based on flaxseed extracts were prepared and used to simulate human saliva lubrication. Brown flaxseed (Markal, Saint-Marcel-lès-Valence, France) containing 20 % proteins was used to formulate these NSS. The NSS preparation, rheological & tribological characterization and sensory properties are described in detail by Bugarin-Castillo et al., (2024). Two different formulas for NSS were considered, based on their similarity to human saliva in terms of sensory perceptions (like mouth hydration, tongue-palate slipperiness and adhesion) and rheology (including extensional relaxation times and shear viscosity). The solutions were prepared at concentrations of 0.40 % and 1.0 % dry matter, labeled as F₀.₄₀ and F₁.₀, respectively.

### 2.2. *Characterization of Semi-Solid Foods*

Apple puree (Andros, Biars-sur-Cère, France) and Greek yogurt (Yaourt à la grecque, Cora, Croissy-Beaubourg, France) were used for both *in vitro* and *in vivo* tests due to their thick consistencies generally associated with ease of swallowing (Penman & Thomson, 1998). Both products were commercially available and ready for tasting without modification in the laboratory, facilitating the organization of sensory evaluations. Moreover, both of them are generally appreciated and widely consumed. Additionally, apple puree and yogurt have been previously used in research involving a pediatric swallowing soft-actuator (Lavoisier et al., 2022).

Greek yogurt (labeled as GY hereafter) contained 8.80 % of fats and 2.80 % of proteins, while apple puree (AP hereafter) contained 1.20 % of fibers, 0.10 % of fats and 0.30 % of proteins.
Both products were purchased at a local store (Cora, Croissy-Beaubourg, France) and kept at 4 °C for storage.

The rheological behavior of the semi-solid foods was characterized using a Physica MCR-702 rheometer (Anton Paar GmbH, Graz, Austria). For apple puree, shear viscosity was measured with a Couette geometry between 1 and 500 s⁻¹ (d = 18 mm; gap = 5 mm) at 20 °C. For Greek yogurt, shear viscosity was determined with a sand-blasted plate-plate geometry PP50S (d = 50 mm; gap = 1 mm) ranging between 1 to 500 s⁻¹ at 20 °C. An amplitude sweep was performed for both products at a constant angular frequency of 10 rad/s, varying the strain from 0.01 to 1000% at 20 °C. In addition, the yield stress was obtained as the stress at the crossing point of G' and G'' based on the method of Espinosa-Muñoz et al., (2013). The measurements were performed in triplicates.

The lubricity of apple puree and Greek yogurt were studied by tribology with a biomimetic setup. Srivastava et al., (2021) described the tribometer geometry and design details. To test the semi-solid foods, 1 g of each was placed on a tongue mimicking sample (TMS). The TMS was made out of Ecoflex-030 (Smooth-On, Macungie, USA) and 5 % Span80 (sorbitan mono-oleate from Sigma-Aldrich, St. Louis, USA). The roughness of the TMS was inspired by the human tongue topography using sandpaper P40 (Saint-Gobain Norton, Paris, France), as suggested by Srivastava et al., (2021). An aluminum plate (rectangular, 45 mm × 25 mm) plays the role of the palate and applies an initial normal force of 5 N on the TMS (around 5 kPa). Friction measurements were then conducted during five cycles of back and forth shear motions operated with an amplitude of 10 mm and a constant velocity of 10 mm·s⁻¹. More information about the methodology is provided in Bugarin-Castillo et al., (2024).

A laser diffraction analyzer was used to measure the particle size distribution of apple puree and Greek yogurt (Mastersizer 3000, Malvern Panalytical, London, UK). The samples were diluted 1:10 (w/w) with purified water. To better compare with literature, particle size at the 90th percentile *d*(0.9) was calculated from three replicates.

## 2.3. *In Vitro* Soft-Robotic Tongue (SRT)

The *in vitro* setup utilized in this study was proposed by Marconati et al., (2020). This *in vitro* soft-robotic tongue has been adapted to simulate pediatric swallowing (Avila-Sierra et al., 2023; Lavoisier et al., 2022). The 3D model is based on CT scans of the oral airway of a healthy adult (Xi & Longest, 2007). A comprehensive description of the actuation patterns was reported by Lavoisier et al., (2022). The adult size SRT presented in this study was derived from the original model (Marconati et al., 2020), with minor modifications described in the following sections.

### 2.3.1. 3D Oral Cavity

The oral cavity was 3D printed using a hard material (VeroClear® resin from Sculpteo, Villejuif, France) chosen for its transparency, allowing image analysis. A schematic representation of the oral cavity design is shown in Figure 1.a&b. A hole (4 mm diameter) located 6 mm from the tongue tip makes it possible to inject the semi-solid foods and the NSS in the cavity, using a syringe. In addition, two holes with 1/8" metallic nuts glued on the top were used to position pressure transducers. Hence, palatal pressure of the food products during the swallowing sequence were recorded. The central hole was closed with a rubber stopper during *in vitro* swallowing. Conversely, when feeding the oral cavity, the central hole was open to avoid any undesired pressure increase and the incorporation of air bubbles.
<figure><img src="image_1.png" /><figcaption>Figure 1. Diagram and dimensions of the 3D oral cavity and of the soft-robotic in vitro tongue (SRT). a) Sagittal plane of the 3D oral cavity, b) Coronal plane of the 3D oral cavity, c) 3D view of the soft-robotic tongue proposed by Marconati et al., (2020) and used in this study.</figcaption></figure>

### 2.3.2. Soft-Robotic Tongue (SRT)

The soft-robotic tongue by Marconati et al., (2020) was used in this study. Marconati et al., (2020) designed a geometry possessing two air chambers that can inflate and deflate independently (see Figure 1.c). The tongue is obtained by molding a solution of Ecoflex-030 (Smooth-On, Macungie, USA) mixed with 5 % Span80 (sorbitan mono-oleate from Sigma-Aldrich, St. Louis, USA).

An important modification was introduced in this study, which involved patterning the top surface of the SRT. The roughness imitates the topography of the human tongue, which is mostly driven by the presence of the filiform papillae. To achieve this, sandpaper P40 (Saint-Gobain Norton, Paris, France) was used to cover the mould in which the silicon was poured, following the technique used by Srivastava et al., (2021).

### 2.3.3. Wettability and Papillae Dimensions of the SRT

Mechanical properties of the SRT were similar to those from the SRT developed and characterized in the previous study (Marconati et al., 2020). Due to the addition of roughness on the top surface wall, its wettability and topography were evaluated.

The contact angle of water and NSS were measured by depositing a 20 µL droplet on a portion of the SRT surface; images of the drop were captured for 15 seconds after 1 minute of deposition. A camera (Vision Research, Wayne, USA) with a 50 mm lens (Opto Engineering, Mantova, Italy) was used to record (50 fps) the shape evolution of the droplet, which was processed
with ImageJ® and the contact angle plugin (Avila Sierra et al., 2023). All measurements were repeated in triplicate.

The topography of the SRT was characterized by optical microscopy using an Olympus BX51 (Olympus, Tokyo, Japan) and a 10x objective. The samples were sliced on a thin layer to measure the height and diameter of the *in vitro* papillae. All measurements were repeated five times. Results are detailed in Appendix B and Appendix A.

#### 2.3.4. Swallowing Sequence

The design of the *in vitro* tongue allows the independent displacement of the posterior and anterior chambers by applying different pressures, determined based on *in vivo* data. The *in vitro* swallowing sequence established by Marconati et al., (2020) was used in this study. The schematic representation of the timeline of this sequence is provided in Figure 2. The bolus is injected in the oral cavity through the feeding hole located in its front, while a pressure of 10 kPa is imposed in the posterior chamber in order to allow bolus holding (consistently with *in vivo* glossopalatal pressures) (Fujiwara et al., 2014). The swallowing sequence then starts ($t = 0$ s) with two simultaneous actions: the inflation of the anterior chamber (25 kPa) and the deflation of the posterior chamber with an imposed vacuum of -5 kPa. The pressure in the anterior chamber (25 kPa) was set based on tongue-palate maximal pressure values available in the literature (around 60 kPa), attenuated by a factor of 40% as the foods considered here have a low viscosity and do not require intense mechanical efforts to flow (Fei et al., 2013; J.-S. Park et al., 2016). At $t = 0.6$ s, the maximum pressure in the anterior chamber is reached, consistently with *in vivo* timings of the oral phase of swallowing for liquids (Dodds et al., 1990; Zhang et al., 2012). After $t = 1.1$ s, pressure was no longer applied to either chamber, causing both chambers to return to their resting positions.

<figure><img src="image_2.png" /><figcaption>Figure 2. Schematic timeline of the swallowing sequence. Bolus injection represents bolus holding and swallowing sequence illustrates the *in vitro* swallowing displacement inspired by the oral phase of swallowing of liquids. Pressure evolution with time, corresponding to the pressures in each chamber.</figcaption></figure>
### 2.3.5. *In Vitro Swallowing Protocol*

Two *in vitro* conditions were assessed for evaluating the lubricating performance of the NSS: (i) “Lubricated Swallowing” (referred to as “LS”) and (ii) “Unlubricated Repeated Swallowing” (referred to as “URS”). Figure 3 illustrates the steps involved in each of these two *in vitro* protocols.

The *in vitro* device used in this study is not suitable for performing swallowing in completely dry conditions, due to excessive amounts of residues observed in the first dry swallow. To avoid the positive bias that would result from comparing a dry swallow with a swallow in presence of oral lubrication, it was decided to focus on the repeated feeding and swallowing of a product in absence of salivation: a condition that is representative of the repeated consumption by a xerostomic person. The protocol used for simulating this condition was referred to as “Unlubricated Repeated Swallowing” (“URS”).

The “Lubricated Swallowing” (“LS”) protocol comprised an initial feeding of NSS (2 g) to coat the *in vitro* oral cavity. The URS protocol involved an initial coating of semi-solid food (2 g). The two conditions thus started with the same initial mouth coating mass (2 ± 0.3 g), to avoid the bias that would result from a different initial degree of filling of the *in vitro* oral cavity. Following the coating step, two consecutive semi-solid food swallows were completed (5 g each) in each sequence. The masses of the semi-solid food injected in the system and of bolus expelled from the system after *in vitro* swallowing were carefully weighted using a precision scale of 1 mg (Sartorius PRACTUM513-1S, Göttingen, Germany).

<figure><img src="image_3.png" /><figcaption>Figure 3. *In vitro* protocol for “Unlubricated Repeated Swallowing” (URS) and “Lubricated Swallowing” (LS) conditions. Firstly, the mouth coating step consisted in coating the swallowing simulator either with Natural Salivary Substitute (NSS) or with semi-solid food (apple puree or Greek yogurt). Then, two repeated swallows (called #1 and #2) with semi-solid food (5 g) were completed.</figcaption></figure>

### 2.3.6. *Measured Variables*

#### 2.3.6.1. *Post-Swallowing In Vitro Residue (g)*

Human saliva has the ability of reducing the residues in the throat (Wirth et al., 2016). Based on this, the performance of the salivary substitutes was evaluated by quantifying the amount of semi-solid food residues after two successive *in vitro* swallows. The quantity of residue could be quantified by measuring the difference between the masses injected into the instrument and those expelled after the swallowing sequence. To quantify semi-solid food residues, it was necessary to adapt the method according to the lubrication condition. Hence, two distinct approaches were followed for LS and URS protocols.

For URS, the mass of semi-solid food residues induced by a swallowing sequence simply resulted in the difference between injected and expelled masses.

For LS condition, the method had to be adapted because residues collected after each swallow consist of a mixture of semi-solid food and NSS. The NSS concentration in the samples
collected at the exit of the device was quantified using a refractometer (ATAGO®, Tokyo, Japan).
260
The refractometer was calibrated to confirm a linear relation between the obtained Brix degrees
261
and the dilution rate of semi-solid foods with flaxseed extracts. The calibration curves are reported
262
in Appendix C at concentrations between 0 to 0.86 w/w. The Brix degree measured on samples
263
expelled after swallows #1 and #2 were then converted into the mass of semi-solid food ejected
264
during each swallow. The mass differences between semi-solid food injected and bolus expelled
265
could thus be used to characterize the residual mass of semi-solid food in the oral cavity at the end
266
of each swallow.
267

268
269
### 2.3.6.2. *In Vitro* Bolus Transit Time (s)

The *in vitro* bolus transit time was defined as the time required for the front of the bolus to
270
exit the oral cavity (bolus FO, for "Front Out"). Two cameras were recording the bolus transit and
271
were positioned at different views. A high-speed camera (Vision Research, Wayne, USA) recorded
272
the sagittal plane, and a Basler acA2000-165uc (Basler AG, Ahrensburg, Germany) recorded the
273
side view, both of them at 100 fps. To obtain the bolus transit time, the image analysis consisted
274
in identifying manually the frame where the sequence started (frame "0") and the frame where the
275
front of the bolus exits the SRT (frame "bolus FO"). The difference in frames and the acquisition
276
rate of the tests allowed to quantify the time required for the front of the bolus to exit the oral
277
cavity (bolus FO).
278

279
280
### 2.3.6.3. Palatal Pressure (kPa)

The anterior and posterior palatal pressures were measured by two pressure sensors
281
operating from 0 to 0.2 MPa (model PX2AG2XX002BAAAX, from Honeywell, Charlotte, USA)
282
located in the 3D oral cavity (see Figure 1). The pressures were acquired at a sampling rate of 294
283
data points per second by using an Arduino Uno board (Arduino AG, Chiasso, Switzerland) and a
284
control algorithm through Arduino IDE v1.8.16 (Arduino AG, Chiasso, Switzerland). These
285
results are expressed as a relative pressure (in kPa) and as a function of time during the course of
286
each swallowing sequence. The maximum palatal pressure recorded for each swallowing sequence
287
was used to compare different *in vitro* conditions (LS and URS).
288
289

290
291
## 2.4 Sensory Evaluation

### 2.4.1 Panel

The present sensory study followed the ethical standards defined by the Declaration of
292
Helsinki. A panel of 16 healthy volunteers (9 women and 7 men, 23-60 years old) was recruited
293
based on motivation, availability and food allergies. The study comprised four sessions (including
294
two for training and two for evaluation) of 40 min each. The study aimed to evaluate the impact
295
of different oral lubrication conditions on the amount of oral residues perceived after swallowing
296
AP or GY. At the end of the sensory study, the panelists were compensated for their participation.
297
Panelists were instructed to refrain from eating, drinking or smoking for at least an hour prior to
298
the sessions. The study took place in a sensory laboratory featuring 16 individual tasting booths,
299
each equipped with a sink. The booths were air-conditioned (20°C) and illuminated with red
300
lighting to reduce visual biases. For the evaluation sessions, panelists were presented with different
301
combinations of oral coating conditions and of semi-solid foods, which were arranged in a
302
balanced order across participants using a Latin square and divided into three replicate blocks.
303

The panel was provided with a tablet and trained to use a data acquisition interface called Fizz
304
software (version 2.7; Biosystemes, Couternon, France). For each test, the panelists were asked to
305
rate the intensity of the attribute by moving a slider on a scale from 0 to 10, where 0 indicated "no residues" and 10 indicated "a lot of residues". During training sessions, contrasting samples were presented to the panel, allowing them to become familiar with the ends of the notation scale: a full spoon of water to indicate "no residues" and a full spoon of mashed potatoes to indicate "a lot of residues".

### 2.4.2 Mouth Coating Protocol

To evaluate the mouth coating conditions, up to six distinct conditions were tested in vivo. The first condition, referred to as "S" for saliva, involved taking a reference measurement without altering the natural salivary coating in the volunteers' mouths. To achieve this, volunteers were asked to rinse their mouths with water, wait for a minute to reach natural oral lubrication, and then swallow the semi-solid food. The other conditions consisted of drastically altering the natural lubrication, by performing a series of mouth rinses (Nayak & Carpenter, 2008) with five different products: water (referred to as “W”, Evian Danone, Évian-les-Bains, France), the two types of NSS formulas (“$F_{0.4}$” and “$F_{1.0}$”) and the two types of foods (“$A_P$” and “$G_Y$”).

The products were served in 30 mL graduated plastic cups with a lid. The tray carrying the cups was organized into rows and columns, with each row corresponding to a specific test composed of four cups (Figure 4.a&b). The test code was labeled at the beginning of each row and on each cup lid, ensuring easy identification. On each row, the first three cups were identical and intended for the mouth coating step, while the fourth cup contained the food sample for which post-swallowing oral residues were investigated. The volunteers were instructed to pour the entire content of the first cup into their mouth and swirl it around for five seconds, making sure it came into contact with all the oral mucous membranes, before spitting it out. They were then asked to repeat this step with the next two cups (Bugarin-Castillo et al., 2024).

<figure><img src="image_4.png" /><figcaption>Figure 4. Sensory protocol. a) Description of a testing sequence and rinsing protocol after each evaluation. The testing sequence includes three mouth rinsing steps and swallowing one full spoon of food. Abbreviations: (MC) Mouth Coating, (SO) Spit Out. b) Schematic representation of the organization of the trays on which the samples were presented to the panelists. Each line corresponds to a test, with cups #1 to #3 dedicated to mouth rinsing, followed by cup #4 for sensory evaluation of oral residues.</figcaption></figure>
After that, the panelists were asked to pour into their mouth the entire content of the fourth cup, which contained either apple puree or Greek yogurt. They were then asked to evaluate the amount of food residues in their mouth immediately after swallowing. The process was the same for all rows on the tray. To prepare for the next test on the following row, the trained panelists were instructed to rinse their mouths with water for 10 seconds and wait a minute to sufficiently attenuate residual perceptions and recover their natural state of oral lubrication as much as possible (Figure 4.a). For more clarity, all the mouth coating combinations are illustrated in Appendix D.

### 2.4.3 Post-Swallowing In Vivo Residue

The objective of the sensory study was to investigate and quantify the perception of food residues remaining in the mouth after swallowing, under various conditions of oral lubrication. The lubrication conditions investigated were designed to be as similar as possible to those used in *in vitro* swallowing experiments.

To reproduce specific lubrication conditions, the panelists were instructed to rinse their mouths with different samples (details provided in 2.4.2). The volunteers tasted various semi-solid foods and evaluated a single attribute - the amount of post-swallowing oral residues. This attribute was defined based on oral sensations of food remaining in the oral cavity after the first swallow, as well as sensations of requiring multiple swallows to clear the mouth after the first main swallow (Hadde & Chen, 2021).

### 2.5 Statistical Analysis

The data were analyzed using XLSTAT (v.2023.1.1 Addinsoft, Paris, France) for each variable in the *in vitro* and physico-chemical measurements and for each testing condition in the sensory evaluation. To determine if there were significant differences for each factor, analyses of variance (ANOVA) (α=0.05) were applied. The study used the Newman-Keuls multiple comparison test to compare the means of different conditions. If significant differences were found, the variables used in the analysis are useful in explaining the variability of the dependent variable. However, if no significant differences were found, it may indicate that the variables used do not provide significant information to the model or that some covariates that help explain the variability are missing. The performances of the panel (repetition, discrimination and homogeneity) were evaluated by investigating three different independent variables (condition, panelist identity, and replicate) and one order of interaction (panelist*condition).

## 3. Results and Discussion

The results of this study are divided in three sections: (i) the characterization of the semi-solid foods such as rheology, tribology, and particle size, (ii) the influence of different Natural Salivary Substitutes (NSS) on swallowing apple puree, and (iii) the influence of the properties of apple puree and Greek yogurt on the swallowing performance of a NSS. The second and third parts contain both *in vitro* and *in vivo* approaches.

### 3.1. Rheology, Tribology, and Particle Size of Semi-Solid Foods

Apple puree (A<sub>P</sub>) and Greek yogurt (G<sub>Y</sub>) were used as semi-solid foods to assess the performance of various oral lubrication conditions under *in vitro* and *in vivo* configurations. These commercial products have a thick consistency and can be easily swallowed without chewing.
Following the classification tests proposed by the International Dysphagia Diet Standardization (IDDSI), $A_P$ is classified as Level 4, while no IDDSI Level applies to $G_Y$ due to its stickiness. These products show different compositions and matrices that can affect their sensory perceptions (Mossaz et al., 2010). Fruit purees have soft and deformable insoluble particles (cell walls of the fruit), which are dispersed into an aqueous solution of sugars, organic acids, and pectic substances called serum (Cepeda and Gómez, 2002; Espinosa-Muñoz et al., 2013). Yogurt is achieved by acidification of milk with lactic acid bacteria (inducing the coagulation of the protein network), also containing fat and additional stabilizers (Gilbert & Turgeon, 2021).

Figure 5.a illustrates the average flow curves of apple puree and Greek yogurt. The two food products exhibit shear-thinning behavior, with apple puree approximately twice more viscous than Greek yogurt.

The two semi-solid foods demonstrated higher values in storage modulus (G') than in loss modulus (G'') when low shear stress is applied (Figure 5.b). Apple puree shows higher values of G' and G'' than Greek yogurt (approximately four times higher). Moreover, in Figure 5.c, the yield stress (shear stress at the crossover point between storage and loss modulus) was found to be higher for $A_P$ (19.8 ± 0.6 Pa) than for $G_Y$ (7.9 ± 0.1 Pa). This minimal stress required to initiate flow of the products may be related to their structural network (Sun & Gunasekaran, 2009). Comparable orders of magnitude were found in the literature when studying apple puree, establishing that the viscoelastic properties of this food product strongly depend on the insoluble solid content (Espinosa-Muñoz et al., 2013; Leverrier et al., 2016). Greek yogurt's yield stress has been found to fluctuate between 6 and 25 Pa (Costa et al., 2019; Fangary et al., 1999; Mokoonlall et al., 2016).

In addition, the particle size of the products was also assessed and it is illustrated in Figure 5.d. The mean and standard deviation values of $d(0.9)$ were of 1693.3 ± 77.7 µm for $A_P$ and of 44.5 ± 7.7 µm in $G_Y$. For apple puree, the peak was centered at 555.3 µm, resembling an almost monomodal size distribution. In contrast, Greek yogurt showed three peaks: one around 0.5 µm, the second centered at 18.6 µm and the third between 150 µm and 190 µm. The small peak could be the representation of fat globules, which are usually between 0.2 µm and 1.4 µm (Truong et al., 2014). Moreover, the central peak might be the cluster aggregates of fat droplets interacting with the protein network (mainly whey protein/casein micelle complexes) (Moussier et al., 2019). These clusters can range from 1 to 40 µm (Mellema et al., 2002). Lastly, the third peak showed high variability, probably due to the reorganization between the cluster aggregates such as sedimentation or syneresis, usually fluctuating from 40 µm to macroscale (Mellema et al., 2002; Moussier, 2019).

Friction coefficients were 0.52 ± 0.1 for $A_P$ and 0.65 ± 0.1 for $G_Y$. These tribological tests were carried out at constant normal force and shearing velocity. In these conditions, the lubrication regime may then be influenced by various factors related to the rheology and to the structure of food, which plays a role of lubricant at the interface between the tribopairs. The average dimensions of the asperities of the artificial tongue used on the tribometer (around 200 µm in height and 400 µm in diameter – see Appendix B) are above particle size distribution quantified in $G_Y$, whereas they fall in the range of particle size distribution measured in $A_P$. For this reason, the shape and rigidity of the particles in $A_P$ are more likely to influence the contact surfaces and the friction phenomena. In addition, some components of the two food products (such as
polysaccharides in A<sub>P</sub> or fat in G<sub>Y</sub>) are likely to affect wettability and spreading on the surfaces of the tribopairs, with consequences on friction. Liu et al., (2016) stated that the lubrication properties of foodstuffs containing fat reduce friction due to “fat film patches” formation. This fat film may lead to hydrophobic interactions between the TMS and the Greek yogurt.

<figure><img src="image_5.png" /><figcaption>Figure 5. Rheology, tribology and particle size of apple puree and Greek yogurt. a) Shear viscosity of apple puree and Greek yogurt, b) Storage (G') and Loss (G'') Modulus, c) Yield stress, obtained from the overlap between G' and G'', d) Particle size distribution, e) Friction coefficient. The capital letters above the histogram bars correspond to the groups resulting from multiple comparison tests, indicating statistically significant differences (p < 0.05).</figcaption></figure>

## 3.2 Influence of different Natural Salivary Substitutes on swallowing Apple Puree

### 3.2.1 In vitro Swallowing Assessment

To further explore the mechanisms governing the formation of food residues in the oral cavity after swallowing, an *in vitro* simulator of swallowing was employed. The physiological relevance of such simulators requires appropriate consideration of oral lubrication. Previous works
investigating *in vitro* swallowing of Newtonian (Glycerol) and non-Newtonian (THICKEN UP®) CLEAR, Nestle) fluids have shown that using water to lubricate the oral cavity of the SRT simulator was equivalent to not lubricating at all (Marconati, 2019). Due to water's Newtonian properties, attempts at lubrication failed to produce a regular and homogeneous film. To achieve a mouth-coating effect in the SRT and to prevent any leakage from the cavity, a viscoelastic fluid is preferable. With their interesting extensional and shear rheological properties, the flaxseed extract solutions studied in our previous article (Bugarin-Castillo et al., 2024) are interesting candidates for overcoming these aforementioned limitations.

Therefore, two concentrations of the NSS were evaluated *in vitro* to determine their ability to reduce post-swallowing *in vitro* residues. To avoid any bias, the objective was to use the same mass of initial coating (2 g) in both the “Lubricated Swallowing” and “Unlubricated Repeated Swallowing” conditions.

As illustrated in Figure 6.a&b, the masses of *in vitro* residues remaining after swallows #1 and #2 were determined. The statistical analysis of post-swallowing *in vitro* residues showed *P-values* < 0.01 for the variables “condition” (LS and URS) and “swallow” (swallow #1 and #2), and *F*-values of 94.95 and 220.31, respectively. After the first swallow (see Figure 6.a), the URS condition led to a higher mass of *in vitro* residues (4.2 ± 0.1 g) compared to both LS conditions (3.0 ± 0.1 g and 2.9 ± 0.1 g for F₀.₄₀ and F₁.₀, respectively). The effect of lubrication with NSS is also observed in the residues after swallow #2 (Figure 6.b), with a higher level also observed in URS, when compared to LS with F₀.₄ and F₁.₀. This observation confirms that the different conditions of oral lubrication resulting from swallow #1 have an influence on the residues obtained after swallow #2. Whatever the lubrication condition, the quantity of residues after swallow #2 was significantly greater than the corresponding one measured after swallow #1.

Moreover, Figure 6.c&d display the bolus FO of the different conditions during swallows #1 and #2. The statistical results from bolus FO revealed no significant difference between URS and LS when comparing both swallows. It has been studied in an *in vitro* model that bolus flow velocity might be mitigated by bolus viscosity, yield stress, and firmness of the products (increasing transit times) (Patel et al., 2020). The similar results obtained in bolus transit times under the three conditions are thus consistent with the close rheological behaviors of the expelled boli provided in Appendix E.

Figure 6.e&f display the palatal pressures recorded from the anterior sensor of the SRT during swallows #1 and #2. No significant differences could be observed between the URS and LS(F₁.₀) conditions. However, a slightly lower pressure is remarked in the LS(F₀.₄) condition in both swallows. Corresponding *in vivo* pressures were estimated in the literature (Moritaka et al., 2018; Nicosia et al., 2000; Steele et al., 2010) using flat pressure sensors, during swallowing of foods with wide ranges of rheological properties: nectar-thick apple juice (0.497 Pa·s at 50 s⁻¹), semi-solid barium, and carrot puree (0.2 Pa·s at 50 s⁻¹). For healthy volunteers, *in vivo* palatal pressures from the anterior flat sensor ranged from 11.2 to 20 kPa with a duration of ~1 s. These orders of magnitude are consistent with the data obtained *in vitro* employing the SRT (Figure 6.e&f).
<figure><img src="image_6.png" /><figcaption>Figure 6. In vitro swallowing using the Soft-Robotic Tongue under different mouth coatings when testing apple puree as semi-solid food. Average and standard deviation values of post-swallowing in vitro residues after swallow #1 (a) and #2 (b). Average and standard deviation values of Bolus FO time from swallow #1 (c) and #2 (d). Average curves of palatal pressures from swallow #1 (e) and #2 (f). The capital letters above the histogram bars correspond to the groups resulting from multiple comparison tests, indicating statistically significant differences (p < 0.05). Abbreviations are: (URS) unlubricated repeated swallowing, (LSF<sub>0.40</sub>) lubricated swallowing using NSS at 0.40 % dry matter, and (LSF<sub>1.0</sub>) lubricated swallowing using NSS at 1.0 % dry matter.</figcaption></figure>

### 3.2.3 *In vivo* Assessment

The influence of the NSS in the formation of post-swallowing residues was also assessed *in vivo*. The ANOVA results from the sensory profile showed F = 4.615 and P-value < 0.01 for the conditions tested, F = 4.381 and P-value < 0.01 for the panelists and F = 2.197 and P-value < 0.01 for the interaction panelist*condition. In this study, all repetitions were not significantly different,
participants differentiated all conditions and were consistent in their evaluation (panelist*condition). Overall, the results showed confidence in the panel performance.

The results of this evaluation are presented in Figure 7. It is observed that no significant difference in perceived residues were reported between condition S (close to natural saliva coating state) and conditions $F_{0.4}$ and $F_{1.0}$ (oral coating with the two NSS candidates). Contrastingly, lubrication with water (W) and with $A_P$ led to significantly higher levels of residues (greater for $A_P$ than for W).

The strong perception of post-swallowing residues when the pre-coating step was with apple puree may be related to the accumulation of particles on the mouth surfaces. Previous studies have put in evidence a relationship between the increase in post-swallowing residue perception and the increase of particle size in apple puree and mashed potatoes (Espinosa-Muñoz et al., 2012; H. W. Park & Yoon, 2018).

Moreover, the results obtained after coating the mouth with water can be related to a study conducted by Nayak & Carpenter, (2008), where rinsing the mouth with water reduced the mobile phase of the salivary film. Additionally, Bugarin-Castillo et al., (2024) showed that rinsing the mouth with water results in a reduction of perceptions such as slipperiness and adhesion between the tongue and the palate, when compared to a natural oral lubrication state. This might facilitate food adhesion during swallowing, and affect the perception of post-swallowing residues.

In addition, Bugarin-Castillo et al., (2024) showed comparable oral perceptions (mouth hydration, tongue-palate adhesion and slipperiness) between the natural coating state and the coating with the two NSS, which could be related to the significant similarities in post-swallowing *in vivo* residues. However, shear viscosity values and thickness perceptions of $F_{0.4}$ were closer to human saliva than $F_{1.0}$. Therefore, $F_{0.4}$ was chosen for the subsequent sections of the study to avoid biases in the perception of mouth coating thickness.

<figure><img src="image_7.png" /><figcaption>Figure 7. Sensory scores of post-swallowing in vivo residues when testing apple puree as semi-solid food. Evaluation of five mouth coatings conditions (panel means). The capital letters above the histogram bars correspond to the groups resulting from multiple comparison tests, indicating statistically significant differences (p < 0.05). Sensory scores on a scale from 0 to 10, where 0 meant "no residues" and 10 meant "a lot of residues". Abbreviations are: (Ap) apple puree, (W) water, (LSF<sub>0.40</sub>) natural salivary substitute at 0.40 % dry matter, (S) saliva, and (LSF<sub>1.0</sub>) natural salivary substitute at 1.0 % dry matter.</figcaption></figure>

## 3.3 Influence of the properties of apple puree and Greek yogurt on swallowing performance of a Natural Salivary Substitute
### 3.3.1 *In vitro* Swallowing Assessment

To further understand the mechanisms of food residue formation during swallowing with *in vitro* and *in vivo* approaches, attention was focused on lubrication by NSS F₀.₄, seeking to better understand how its performance (in terms of quantity of oral residues) evolves depending on the type of food studied. Greek yogurt was thus also considered, making it possible to account for high changes in composition, structure and rheology when compared to apple puree. The differences between *in vitro* swallowing of Greek yogurt and apple puree were evaluated by ANOVA both for post-swallowing *in vitro* residues and for bolus FO (see Table 1). The variables considered were the type of food product (Gᵧ or Aₚ), the lubrication condition (URS or LS), the swallowing number (#1 or #2), and the interaction between lubrication condition and product type. It is observed in Table 1 that there are significant differences between products, lubrication conditions, and swallowing number when comparing post-swallowing *in vitro* residues. Moreover, significant differences are observed for product type and swallowing number in bolus FO.

<table><caption>Table 1. ANOVA results (food product+condition+swallow+panelists* food product). P-values are in bold (α<0.05). Abbreviations: Post-Swallowing (P-S)</caption><thead><tr><th rowspan="2">ANOVA</th><th colspan="2">Product</th><th colspan="2">Condition</th><th colspan="2">Swallow</th><th colspan="2">Condition*Product</th></tr><tr><th>F</th><th>Pvalue</th><th>F</th><th>Pvalue</th><th>F</th><th>Pvalue</th><th>F</th><th>Pvalue</th></tr></thead><tbody><tr><td>P-S in vitro residues</td><td>68.803</td><td><strong>&lt;0.01</strong></td><td>134.503</td><td><strong>&lt;0.01</strong></td><td>66.27</td><td><strong>&lt;0.01</strong></td><td>4.196</td><td><strong>0.06</strong></td></tr><tr><td>Bolus FO</td><td>20.948</td><td><strong>&lt;0.01</strong></td><td>0.902</td><td>NS</td><td>17.906</td><td><strong>&lt;0.01</strong></td><td>7.167</td><td><strong>0.02</strong></td></tr></tbody></table>

Figure 8.a&b display the averages and standard deviations of post-swallowing *in vitro* residues obtained for Greek yogurt (Gᵧ) and apple puree (Aₚ), under lubricated swallowing (LSF₀.₄) and unlubricated repeated swallowing (URS) conditions. In the URS condition, higher amounts of food residues were reported for Gᵧ than for Aₚ. Significant differences were also observed in the LSF₀.₄ condition between the two products, with slightly higher amounts of residues of Gᵧ than of Aₚ. Swallow #2 follows similar trends as swallow #1, but with higher amounts of residues, depicting accumulation. Literature suggests that thicker liquids leave higher levels of residues in the pharynx (Marconati, Engmann, et al., 2019). However, in the present study, higher amounts of residues were found for Gᵧ, which had the lowest yield stress and viscosity.

Figure 8.c&d display the averages and standard deviations of bolus FO in the same conditions. Longer times were reported for Gᵧ than for Aₚ to transit through the cavity in swallow #1 (both in URS and LS conditions). Patel et al., (2020) studied yogurt using an *in vitro* model and suggested that yogurt showed a greater dependency over G' (negative slope), creating a weaker structure and affecting the boli flow. Additionally, the product adhesion to the SRT might impact bolus transit time and post-swallowing *in vitro* residues. Lavoisier et al., (2022) evaluated the adhesion force of apple puree and yogurt, both diluted with artificial saliva (with the incorporation of mini tablets) on a Pediatric Soft Robotic Tongue (PSRT). Despite the absence of significant differences, a trend was observed where yogurt (1.11 mN) showed higher adhesion force than apple puree (0.72 mN). Interestingly, during swallow #2 the bolus transit time was similar for all conditions, additionally, the bolus FO tended to decrease between swallow #1 and swallow #2.

Figure 8.e&f show the variations of pressure recorded from the anterior sensor during swallow #1 and #2, respectively. No difference in pressure was observed between LSF₀.₄ or URS for Gᵧ (solid and dashed grey lines), while some could be reported for Aₚ (solid and dashed green
lines). The palatal pressures from Aₚ showed higher pressures for URS during 700 ms when compared to lubricated conditions LSF₀.₄. Moreover, differences are observed depending on the type of semi-solid food. Food products with higher viscosities were shown to require higher swallowing pressures to be propelled from the oral cavity to the pharynx (Marconati, Engmann, et al., 2019). The results in Figure 8.e&f are consistent with these trends, higher values of palatal pressures being observed for Aₚ (13.2 ± 0.7 kPa) than for Gᵧ (10.3 ± 0.3 kPa) (higher values of shear viscosity and yield stress in apple puree). Overall, post-swallowing *in vitro* residues were impacted by oral lubrication, the type of food product and the number of swallows.

<figure><img src="image_8.png" /><figcaption>Figure 8. In vitro swallowing using the Soft-Robotic Tongue under different mouth coatings when testing apple puree and Greek yogurt as semi-solid foods. Average and standard deviation values of post-swallowing in vitro residues after swallow #1 (a) and #2 (b). Average and standard deviation values of Bolus FO time from swallow #1 (c) and #2 (d). Average curves of palatal pressures from swallow #1 (e) and #2 (f). Dotted lines and dashed bars represent LS condition and solid lines and bars represent URS condition. The capital letters above the histogram bars correspond to the groups resulting from multiple comparison tests, indicating statistically significant differences (p < 0.05). Abbreviations are: (URS) unlubricated repeated swallowing, (LSF₀.₄₀) lubricated swallowing using NSS at 0.40 % dry matter, (Bolus FO) bolus front out.</figcaption></figure>
The shape of the boli expelled from the *in vitro* simulator at the end of a swallowing sequence was also observed. The impact of the product composition, and of the coating conditions could thus be studied. Cohesiveness is a term that has been employed in sensory and food instrumentation approaches, referring to all the forces which participate in maintaining the food bolus compact, in the form of a single mass without fragmentation. Lately, cohesiveness might be defined as the food product ability to maintain its structure during stretching (Gallegos et al., 2021). Conversely, the absence of cohesiveness can in fact lead to fragmentation of the food bolus, which at the time of swallowing increases the risk of aspiration.

Figure 9 aims at providing a qualitative illustration of the evolution of the shape of the boli expelled from the swallowing simulator during swallow #1 of A<sub>P</sub> and G<sub>Y</sub> under LSF<sub>0.40</sub> and URS conditions. Images from swallow #2 could not be analyzed due to food accumulation which could introduce bias in comparing conditions. Figure 9 displays representative cases of four distinct experimental conditions: swallowing of A<sub>P</sub> and G<sub>Y</sub> combined with URS and LSF<sub>0.4</sub> coating conditions “Unlubricated Repeated Swallowing” and “Lubricated Swallowing”. For each test, a sequence of images was taken under similar shooting conditions, all of them at the back of the simulator and according to its coronal plane.

<figure><img src="image_9.png" /><figcaption>Figure 9. Coronal view of the posterior part of the Soft-Robotic Tongue, where the bolus is expelled out of the oral cavity. Images were taken from bolus Front Out (bolus FO) until bolus Back Out (bolus BO).</figcaption></figure>
Respective times are indicated in red (average and standard deviations in seconds, calculated over all the repetitions). Blue arrows indicate bolus fragmentation and red arrows indicate bolus BO.

To appreciate visually the cohesiveness of the food boli, the images provided for each test were taken at a constant frequency (100 fps). The time corresponding to each first image refers to the bolus FO, where the front of the bolus exits the SRT. Additionally, the time corresponding to the last image describes the moment the bolus stops flowing out the *in vitro* cavity breaking in two parts; one flowing down and the other one sticking to the SRT, labeled as bolus BO (bolus Back Out) and represented in red arrows (see Figure 9). Intermediate frames corresponding to 1/4, 2/4 and 3/4 of the duration between FO and BO were displayed.

The two first rows of images in Figure 9.a&b represent URS and LS conditions when swallowing apple puree. Under URS (Figure 9.a), the bolus separates in two parts during $t = 2/4$ and $t = 3/4$ (dark blue arrows), one part getting suspended to the device and climbing up due to the elasticity, while the second part flows down. In comparison, under LS (Figure 9.b) the bolus continues elongating, preventing fragmentation. This shows that using $F_{0.40}$ provides cohesiveness to the food bolus, preventing bolus fragmentation. The properties of elastic rheology of the NSS may thus have been transmitted to the food bolus during *in vitro* tongue-palate compressions (Bugarin-Castillo et al., 2024). Similar trends were observed in Figure 9.c&d for swallowing experiments with Greek yogurt, with bolus fragmentation in URS compared to continuous flow in the LS. Overall, when coating or not with NSS ($F_{0.40}$), the difference in bolus cohesiveness is visible.

It has been stated in the literature that a bolus is perceived safe for swallowing when the cohesive forces between the food and saliva are strong enough to prevent the detachment of particles from the food bolus, preventing fragmentation (Gallegos et al., 2021). In this study, the observations of the shape of the bolus resulting from high-speed camera images indicate that in the presence of $F_{0.40}$, bolus cohesiveness was improved. Such analysis should be extended to the study of other NSS candidates, as well as to the effect of the conditions of mixing between the food and the salivary substitute. To better identify and understand the mechanism by which salivary substitutes promote this cohesiveness of the bolus, two options could be considered, consisting of (i) changing the characteristics of the bolus introduced into the simulator (pre-prepared mixtures with controlled food/saliva ratios), or (ii) varying the amount of salivary substitutes initially coating the internal walls of the simulator. In this way, it would be possible to distinguish the respective contributions (i) of the bulk rheology of the bolus (food diluted with saliva) and (ii) of the film of salivary substitute which envelops the bolus during its flow, on the ability of the bolus to keep a cohesive form.

### 3.3.3 *In Vivo* Performance

To reflect actual food consumption conditions, the influence of oral coating conditions was as well evaluated *in vivo* using Greek yogurt as food to swallow. The ANOVA results from the panel performance showed $F = 7.683$ and $P$-value < 0.01 for the conditions tested, $F = 3.666$ and $P$-value < 0.01 for the panelists and $F = 2.845$ and $P$-value < 0.01 for the interaction panelist*condition. These results showed confidence in the panel performance.
Figure 10 illustrates a comparison between (a) *in vivo* and (b) *in vitro* assessments of post-swallowing oral residues of food, for the four combinations of the two foods (apple puree (A<sub>P</sub>) and Greek yogurt (G<sub>Y</sub>)) with the two lubrication conditions (Unlubricated Repeated Swallowing (URS) and Lubricated Swallowing (LS)). *In vivo* sensations of post-swallowing residues and *in vitro* assessments of the amount of residues were both lower under LS than URS lubrication conditions. However, a difference is observed when the scores of G<sub>Y</sub> and A<sub>P</sub> are compared *in vitro* and *in vivo*. During *in vitro* measurements, A<sub>P</sub> showed lower amounts of oral residues than G<sub>Y</sub> in URS and LS. During *in vivo* measurements, the inverted trend was observed in URS, whereas no difference could be reported in LS. The differences between *in vivo* and *in vitro* results may be due to the quantification method. Throughout *in vitro*, post-swallowing residues were determined by objective quantifications of masses. *In vivo*, the perception of post-swallowing residues might be influenced by various factors. For instance, a possible reason for the higher scores obtained *in vivo* from A<sub>P</sub> than for G<sub>Y</sub> under URS condition could be due to the positive correlation between oral residues and particle size (Aguayo-Mendoza et al., 2021; Espinosa-Muñoz et al., 2012; Marconati, Lopez, et al., 2019). Participants accumulated apple puree particles from the three pre-coating steps performed, potentially enhancing their ability to detect particles in the mouth (graininess) and increasing their perception to post-swallowing residues. An additional cause for the lower scores of G<sub>Y</sub> under *in vivo* URS might be due to its sourness, which is characteristic of fermented dairy products (Desai et al., 2013). Acid stimulation of saliva is due to its buffer capacity, where saliva increases its volume to reduce acidity in the mouth (Lugaz et al., 2005). The perception of sourness has been related to the type and concentration of acid used, as sourness is not only related to pH, but also to titratable acidity (Muñoz-González et al., 2018). Literature shows that A<sub>P</sub> and G<sub>Y</sub> pH values are within similar ranges (A<sub>P</sub>=3.4-4.8 and G<sub>Y</sub>=4.4-4.9). Nevertheless, the percentage of lactic acid in G<sub>Y</sub> (3.3-5.7%) is higher than that of malic acid in A<sub>P</sub> (0.4-0.82%) (Brisset, 2012; Desai et al., 2013; Lan et al., 2022; Picouet et al., 2009; Víquez-Barrantes et al., 2023). The repeated steps of mouth rinsing performed by the participants for G<sub>Y</sub> in URS conditions might have led to an increase in saliva flow stimulation (when compared to A<sub>P</sub> case). One could thus have expected lower perceptions of residues due to dilution effects of saliva.

In addition, under LSF<sub>0.40</sub> participants perceived lower levels of post-swallowing residues as compared to those under URS. Also they perceived similar results for both apple puree and Greek yogurt. Under LSF<sub>0.40</sub>, participants pre-rinsed their mouths three times with NSS and had only one exposure to the food product at the time of swallowing, suggesting that less saliva was produced due to acidic stimulation. Despite the lower dilution effect that saliva could have created, the condition LSF<sub>0.40</sub> still exhibited lower post-swallowing residues than URS. This suggests that saliva's ability to reduce post-swallowing food residues depends not only on its quantity but also on its physical-chemical properties. It has been stated that stimulated saliva has lower viscosity, viscoelasticity, and mucin content and leads to higher friction than unstimulated saliva (Sarkar et al., 2019). Thus, these results might be influenced by the higher elasticity resemblance of the NSS to unstimulated saliva observed in our previous study (Bugarin-Castillo et al., 2024).

These findings suggest further work on testing the NSS in an environment where saliva stimulation will not interfere in the perception of post-swallowing residues. This work emphasizes
the importance of *in vitro* devices able to simulate unhealthy conditions to encourage new ways of developing personalized food for specific swallowing disorders.

692

<figure><img src="image_10.png" /></figure>

693

Figure 10. Comparison in vivo and in vitro of post-swallowing residues when testing apple puree and Greek yogurt. a) "In vivo" sensory scores of perception of residues in mouth on a scale from 0 to 10, where 0 meant "no residues" and 10 meant "a lot of residues", b) "In vitro" results from Soft-Robotic Tongue. The capital letters above the histogram bars correspond to the groups resulting from multiple comparison tests, indicating statistically significant differences (p < 0.05). Abbreviations are: (URS) unlubricated repeated swallowing and (LSF₀.₄₀) lubricated swallowing using NSS at 0.40 % dry matter.

694

695

696

697

698

699

## 4. Conclusion and Next Steps

700

701

702

703

704

705

706

707

708

709

710

711

712

713

714

715

716

717

In this study, *in vitro* and *in vivo* approaches were employed to investigate the performance of natural salivary substitutes obtained from flaxseed extracts, to help swallowing food. The *in vitro* method employed a soft actuator, able to reproduce human tongue surface properties, displacements and pressures. For *in vivo* evaluations, healthy volunteers followed a new protocol to imitate lubricated and unlubricated conditions. From this study, oral food residues were influenced by lubrication and food matrix composition—shear viscosity and yield stress were related to higher palatal pressures *in vitro*. Apple puree and Greek yogurt were considered. For both of them, *in vitro* and *in vivo* results show that oral coating with flaxseed extracts at 0.40 % dry matter makes it possible to reduce the amount of post-swallowing oral residues; suggesting an increase in bolus moisture and a more compact bolus. Such promising reduction of oral residues suggests that this salivary substitute should be tested clinically to confirm whether it can help promoting food oral processing for xerostomic patients. Apple puree and Greek yogurt were found to be noticeably different in terms of composition, rheology, tribology, or even of structure. In future work, the emphasis may be placed on model semi-solid foods produced or adapted in the laboratory. A reduced number of controlled targeted properties could thus be selected and varied. Such an approach could help understanding the role of these properties on the interactions between semi-solid foods and salivary substitutes during swallowing.

718

## Acknowledgements
719 We wish to acknowledge David Forest, Anne Regourd and Pascale Reichl for their technical
720 support. Also our thanks to the volunteers who agreed to participate in the sensory study.
721
722

PREPRINT
723

References

724

725

726

727

Aguayo-Mendoza, M. G., Chatonidi, G., Piqueras-Fiszman, B., & Stieger, M. (2021). Linking oral processing behavior to bolus properties and dynamic sensory perception of processed cheeses with bell pepper pieces. *Food Quality and Preference, 88*, 104084. https://doi.org/10.1016/j.foodqual.2020.104084

728

729

730

Andablo-Reyes, E., Bryant, M., Neville, A., Hyde, P., Sarkar, R., Francis, M., & Sarkar, A. (2020). 3D Biomimetic Tongue-Emulating Surfaces for Tribological Applications. *ACS Applied Materials & Interfaces, 12*(44), 49371-49385. https://doi.org/10.1021/acsami.0c12925

731

732

733

Avila Sierra, A., Bugarin, Y., Voisin, J., Mathieu, V., & Ramaioli, M. (2023). *Toward a More Realistic 3D Biomimetic Soft Robotic Tongue to Investigate Oral Processing of Semi-solid Foods* (pp. 131-140). https://doi.org/10.1007/978-3-031-39504-8_9

734

735

736

737

Avila-Sierra, A., Decerle, N., Ramaioli, M., & Peyron, M.-A. (2024). Effect of salivary fluid characteristics on the physical features of in vitro bread bolus: From the absence of saliva to artificially simulated hypersalivation. *Food Research International, 175*, 113753. https://doi.org/10.1016/j.foodres.2023.113753

738

739

740

741

Avila-Sierra, A., Lavoisier, A., Timpe, C., Kuehl, P., Wagner, L., Tournier, C., & Ramaioli, M. (2023). Paediatric solid oral dosage forms for combination products: Improving in vitro swallowability of minitablets using binary mixtures with pellets. *European Journal of Pharmaceutical Sciences, 187*, 106471. https://doi.org/10.1016/j.ejps.2023.106471

742

743

744

Brisset, L. C. E. M. (2012). *Texture de la purée de pomme: Influence de la structure sur les propriétés rhéologiques et la perception sensorielle - effet du traitement mécanique*. [Phdthesis, AgroParisTech]. https://pastel.hal.science/pastel-00749695

745

746

747

748

Bugarin-Castillo, Y., Bou-Fadel, P., Mohamed-Ismail, S., Huang, N., Saint-Eve, A., Mathieu, V., & Ramaioli, M. (2024). On the rheological and sensory properties of a novel natural salivary substitute. *European Journal of Pharmaceutical Sciences, 199*, 106802. https://doi.org/10.1016/j.ejps.2024.106802

749

750

Cassolato, S. F., & Turnbull, R. S. (2003). Xerostomia: Clinical Aspects and Treatment. *Gerodontology, 20*(2), 64-77. https://doi.org/10.1111/j.1741-2358.2003.00064.x

751

752

753

Cepedaand, E., & Gómez, R. (2002). Rheological Characteristics of Pimento Purée: Theoretical Interpretation. *Journal of Food Science, 67*(7), 2734-2738. https://doi.org/10.1111/j.1365-2621.2002.tb08806.x

754

755

756

Clavé, P., & Shaker, R. (2015). Dysphagia: Current reality and scope of the problem. *Nature Reviews Gastroenterology & Hepatology, 12*(5), 259-270. https://doi.org/10.1038/nrgastro.2015.49

757

758

759

760

Costa, M. F., Pimentel, T. C., Guimaraes, J. T., Balthazar, C. F., Rocha, R. S., Cavalcanti, R. N., Esmerino, E. A., Freitas, M. Q., Raices, R. S. L., Silva, M. C., & Cruz, A. G. (2019). Impact of prebiotics on the rheological characteristics and volatile compounds of Greek yogurt. *LWT, 105*, 371-376. https://doi.org/10.1016/j.lwt.2019.02.007

761

762

763

Desai, N. T., Shepard, L., & Drake, M. A. (2013). Sensory properties and drivers of liking for Greek yogurts. *Journal of Dairy Science, 96*(12), 7454-7466. https://doi.org/10.3168/jds.2013-6973

764

765

766

Dodds, W. J., Stewart, E. T., & Logemann, J. A. (1990). Physiology and radiology of the normal oral and pharyngeal phases of swallowing. *American Journal of Roentgenology, 154*(5), 953-963. https://doi.org/10.2214/ajr.154.5.2108569
767 Espinosa-Muñoz, L., Renard, C. M. G. C., Symoneaux, R., Biau, N., & Cuvelier, G. (2013).
768 Structural parameters that determine the rheological properties of apple puree. *Journal of
769 Food Engineering*, 119(3), 619-626. https://doi.org/10.1016/j.jfoodeng.2013.06.014

770 Espinosa-Muñoz, L., Symoneaux, R., Renard, C. M. G. C., Biau, N., & Cuvelier, G. (2012). The
771 significance of structural properties for the development of innovative apple puree textures.
772 *LWT - Food Science and Technology*, 49(2), 221-228.
773 https://doi.org/10.1016/j.lwt.2012.06.020

774 Fangary, Y. S., Barigou, M., & Seville, J. P. K. (1999). Simulation of Yoghurt Flow and Prediction
775 of Its End-of-Process Properties Using Rheological Measurements. *Food and Bioproducts
776 Processing*, 77(1), 33-39. https://doi.org/10.1205/096030899532231

777 Fei, T., Polacco, R. C., Hori, S. E., Molfenter, S. M., Peladeau-Pigeon, M., Tsang, C., & Steele,
778 C. M. (2013). Age-related Differences in Tongue-Palate Pressures for Strength and
779 Swallowing Tasks. *Dysphagia*, 28(4), 575-581. https://doi.org/10.1007/s00455-013-9469-6

781 Fujiso, Y., Perrin, N., van der Giessen, J., Vrana, N. E., Neveu, F., & Woisard, V. (2018). Swall-
782 E: A robotic in-vitro simulation of human swallowing. *PLOS ONE*, 13(12), e0208193.
783 https://doi.org/10.1371/journal.pone.0208193

784 Fujiwara, S., Ono, T., Minagi, Y., Fujiu-Kurachi, M., Hori, K., Maeda, Y., Boroumand, S.,
785 Nitschke, I., Ursula, V., & Bohlender, J. (2014). Effect of Supraglottic and Super-
786 supraglottic swallows on Tongue Pressure Production against Hard Palate. *Dysphagia*,
787 29(6), 655-662. https://doi.org/10.1007/s00455-014-9556-3

788 Gallegos, C., Turcanu, M., Assegehegn, G., & Brito-de la Fuente, E. (2021). Rheological Issues
789 on Oropharyngeal Dysphagia. *Dysphagia*. https://doi.org/10.1007/s00455-021-10337-w

790 Gilbert, A., & Turgeon, S. L. (2021). Studying stirred yogurt microstructure and its correlation to
791 physical properties: A review. *Food Hydrocolloids*, 121, 106970.
792 https://doi.org/10.1016/j.foodhyd.2021.106970

793 Hadde, E. K., & Chen, J. (2021). Texture and texture assessment of thickened fluids and texture-
794 modified food for dysphagia management. *Journal of Texture Studies*, 52(1), 4-15.
795 https://doi.org/10.1111/jtxs.12567

796 Hadde, E. K., Cichero, J. A. Y., Zhao, S., Chen, W., & Chen, J. (2019). The Importance of
797 Extensional Rheology in Bolus Control during Swallowing. *Scientific Reports*, 9(1),
798 Article 1. https://doi.org/10.1038/s41598-019-52269-4

799 Hahnel, S., Behr, M., Handel, G., & Bürgers, R. (2009). Saliva substitutes for the treatment of
800 radiation-induced xerostomia—A review. *Supportive Care in Cancer*, 17(11), 1331-1343.
801 https://doi.org/10.1007/s00520-009-0671-x

802 Hu, J., Andablo-Reyes, E., Mighell, A., Pavitt, S., & Sarkar, A. (2021). Dry mouth diagnosis and
803 saliva substitutes—A review from a textural perspective. *Journal of Texture Studies*, 52(2),
804 141-156. https://doi.org/10.1111/jtxs.12575

805 Lan, W., Jaillais, B., Chen, S., Renard, C. M. G. C., Leca, A., & Bureau, S. (2022). Fruit variability
806 impacts puree quality: Assessment on individually processed apples using the visible and
807 near infrared spectroscopy. *Food Chemistry*, 390, 133088.
808 https://doi.org/10.1016/j.foodchem.2022.133088

809 Lavoisier, A., Avila-Sierra, A., Timpe, C., Kuehl, P., Wagner, L., Tournier, C., & Ramaioli, M.
810 (2022). A novel soft robotic pediatric in vitro swallowing device to gain insights into the
811 swallowability of mini-tablets. *International Journal of Pharmaceutics*, 629, 122369.
812 https://doi.org/10.1016/j.ijpharm.2022.122369
813 Lavoisier, A., Shreeram, S., Jedwab, M., & Ramaioli, M. (2021). Effect of the rheological
814 properties of the liquid carrier on the in vitro swallowing of solid oral dosage forms.
815 *Journal of Texture Studies*, 52(5-6), 623-637. https://doi.org/10.1111/jtxs.12618

816 Leverrier, C., Almeida, G., Espinosa-Mu Noz, L., & Cuvelier, G. (2016). Influence of Particle Size
817 and Concentration on Rheological Behaviour of Reconstituted Apple Purees. *Food
818 Biophysics*, 11(3), 235-247. https://doi.org/10.1007/s11483-016-9434-7

819 Liu, K., Tian, Y., Stieger, M., van der Linden, E., & van de Velde, F. (2016). Evidence for ball-
820 bearing mechanism of microparticulated whey protein as fat replacer in liquid and semi-
821 solid multi-component model foods. *Food Hydrocolloids*, 52, 403-414.
822 https://doi.org/10.1016/j.foodhyd.2015.07.016

823 Lorieau, L., Septier, C., Laguerre, A., Le Roux, L., Hazart, E., Ligneul, A., Famelart, M.-H.,
824 Dupont, D., Floury, J., Feron, G., & Labouré, H. (2018). Bolus quality and food
825 comfortability of model cheeses for the elderly as influenced by their texture. *Food
826 Research International*, 111, 31-38. https://doi.org/10.1016/j.foodres.2018.05.013

827 Lugaz, O., Pillias, A.-M., Boireau-Ducept, N., & Faurion, A. (2005). Time-Intensity Evaluation
828 of Acid Taste in Subjects with Saliva High Flow and Low Flow Rates for Acids of Various
829 Chemical Properties. *Chemical Senses*, 30(1), 89-103.
830 https://doi.org/10.1093/chemse/bji004

831 Marconati, M. (2019). *In vitro models for the formulation of easy-to-swallow products* [Ph.D
832 Thesis]. University of Surrey.

833 Marconati, M., Engmann, J., Burbidge, A. S., Mathieu, V., Souchon, I., & Ramaioli, M. (2019). A
834 review of the approaches to predict the ease of swallowing and post-swallow residues.
835 *Trends in Food Science & Technology*, 86, 281-297.
836 https://doi.org/10.1016/j.tifs.2019.02.045

837 Marconati, M., Lopez, F., Tuleu, C., Orlu, M., & Ramaioli, M. (2019). In vitro and sensory tests
838 to design easy-to-swallow multi-particulate formulations. *European Journal of
839 Pharmaceutical Sciences*, 132, 157-162. https://doi.org/10.1016/j.ejps.2019.02.026

840 Marconati, M., Pani, S., Engmann, J., Burbidge, A., & Ramaioli, M. (2020). A soft robotic tongue
to develop solutions to manage swallowing disorders. arXiv:2003.01194 [Physics, q-Bio].
http://arxiv.org/abs/2003.01194

843 Marconati, M., & Ramaioli, M. (2020). The role of extensional rheology in the oral phase of
844 swallowing: An in vitro study. *Food & Function*, 5, Article arXiv:2001.07810.
845 https://doi.org/10.1039/c9fo02327e

846 Mellema, M., Walstra, P., Van Opheusden, J. H. J., & Van Vliet, T. (2002). Effects of structural
847 rearrangements on the rheology of rennet-induced casein particle gels. *Advances in Colloid
848 and Interface Science*, 98(1), 25-50. https://doi.org/10.1016/S0001-8686(01)00089-6

849 Mokoonlall, A., Nöbel, S., & Hinrichs, J. (2016). Post-processing of fermented milk to stirred
850 products: Reviewing the effects on gel structure. *Trends in Food Science & Technology*,
851 54, 26-36. https://doi.org/10.1016/j.tifs.2016.05.012

852 Moritaka, H., Mineki, M., Kobayashi, M., Ono, T., & Hori, K. (2018). Effect of carrot puree in
853 vegetable juice on linguapalatal swallowing pressure. *Journal of Texture Studies*, 49(2),
854 240-246. https://doi.org/10.1111/jtxs.12315

855 Mossaz, S., Jay, P., Magnin, A., Panouillé, M., Saint-Eve, A., Déléris, I., Juteau, A., & Souchon,
856 I. (2010). Measuring and predicting the spreading of dairy products in the mouth: Sensory,
857 instrumental and modelling approaches. *Food Hydrocolloids*, 24(8), 681-688.
858 https://doi.org/10.1016/j.foodhyd.2010.03.006
859 Moussier, M. (2019). *Structuration de la matière grasse dans une matrice protéique laitière en fonction de la composition et du procédé: Influence sur les propriétés du produit* [Phdthesis, Université Paris Saclay (COmUE)]. https://theses.hal.science/tel-02294430

860 Moussier, M., Guénard-Lampron, V., Lachin, K., Moulin, G., Turgeon, S. L., Michon, C., Huc-Mathis, D., & Bosc, V. (2019). What do stirred yogurt microgels look like? Comparison of laser diffraction, 2D dynamic image analysis and 3D reconstruction. *Food Structure*, 20, 100107. https://doi.org/10.1016/j.foostr.2019.100107

861 Muñoz-González, C., Feron, G., & Canon, F. (2018). Main effects of human saliva on flavour perception and the potential contribution to food consumption. *Proceedings of the Nutrition Society*, 77(4), 423-431. https://doi.org/10.1017/S0029665118000113

862 Nayak, A., & Carpenter, G. H. (2008). A physiological model of tea-induced astringency. *Physiology & Behavior*, 95(3), 290-294. https://doi.org/10.1016/j.physbeh.2008.05.023

863 Nicosia, M. A., Hind, J. A., Roecker, E. B., Carnes, M., Doyle, J., Dengel, G. A., & Robbins, J. (2000). Age Effects on the Temporal Evolution of Isometric and Swallowing Pressure. *The Journals of Gerontology Series A: Biological Sciences and Medical Sciences*, 55(11), M634-M640. https://doi.org/10.1093/gerona/55.11.M634

864 Nishinari, K., Peyron, M.-A., Yang, N., Gao, Z., Zhang, K., Fang, Y., Zhao, M., Yao, X., Hu, B., Han, L., Mleko, S., Tomczyńska-Mleko, M., Nagano, T., Nitta, Y., Zhang, Y., Singh, N., Suk Meng, A. G., Pongsawatmanit, R., Gamonpilas, C., ... Su, L. (2024). The role of texture in the palatability and food oral processing. *Food Hydrocolloids*, 147, 109095. https://doi.org/10.1016/j.foodhyd.2023.109095

865 Pabois, O., Avila-Sierra, A., Ramaioli, M., Mu, M., Message, Y., You, K.-M., Liamas, E., Kew, B., Durga, K., Doherty, L., & Sarkar, A. (2023). Benchmarking of a microgel-reinforced hydrogel-based aqueous lubricant against commercial saliva substitutes. *Scientific Reports*, 13(1), Article 1. https://doi.org/10.1038/s41598-023-46108-w

866 Pailler-Mattei, C., Vargiolu, R., Tupin, S., & Zahouani, H. (2015). Ex vivo approach to studying bio-adhesive and tribological properties of artificial salivas for oral dryness (xerostomia). *Wear*, 332-333, 710-714. https://doi.org/10.1016/j.wear.2015.02.020

867 Park, H. W., & Yoon, W. B. (2018). Effect of drying and grinding characteristics of colored potato (Solanum tuberosum L.) on tribology of mashed colored potato paste. *CyTA - Journal of Food*, 16(1), 135-145. https://doi.org/10.1080/19476337.2017.1348394

868 Park, J.-S., Oh, D.-H., & Chang, M. (2016). Comparison of maximal tongue strength and tongue strength used during swallowing in relation to age in healthy adults. *Journal of Physical Therapy Science*, 28(2), 442-445. https://doi.org/10.1589/jpts.28.442

869 Patel, S., McAuley, W. J., Cook, M. T., Sun, Y., Hamdy, S., & Liu, F. (2020). The Swallowing Characteristics of Thickeners, Jellies and Yoghurt Observed Using an In Vitro Model. *Dysphagia*, 35(4), 685-695. https://doi.org/10.1007/s00455-019-10074-1

870 Penman, J. P., & Thomson, M. (1998). A review of the textured diets developed for the management of dysphagia. *Journal of Human Nutrition and Dietetics*, 11(1), 51-60. https://doi.org/10.1046/j.1365-277X.1998.00079.x

871 Piaton, S., Duconseille, A., Roger-Leroi, V., & Hennequin, M. (2021). Could the use of saliva substitutes improve food oral processing in individuals with xerostomia? A systematic review. *Journal of Texture Studies*, 52(3), 278-293. https://doi.org/10.1111/jtxs.12591

872 Picouet, P. A., Landl, A., Abadias, M., Castellari, M., & Viñas, I. (2009). Minimal processing of a Granny Smith apple purée by microwave heating. *Innovative Food Science & Emerging Technologies*, 10(4), 545-550. https://doi.org/10.1016/j.ifset.2009.05.007
905 Ranc, H., Elkhyat, A., Servais, C., Mac-Mary, S., Launay, B., & Humbert, Ph. (2006). Friction coefficient and wettability of oral mucosal tissue: Changes induced by a salivary layer. *Colloids and Surfaces A: Physicochemical and Engineering Aspects*, 276(1–3), 155–161.
https://doi.org/10.1016/j.colsurfa.2005.10.033

907 Rofes, L., Arreola, V., Almirall, J., Cabré, M., Campins, L., García-Peris, P., Speyer, R., & Clavé, P. (2011). Diagnosis and management of oropharyngeal Dysphagia and its nutritional and respiratory complications in the elderly. *Gastroenterology Research and Practice*, 2011, 818979. https://doi.org/10.1155/2011/818979

910 Sarkar, A., Xu, F., & Lee, S. (2019). Human saliva and model saliva at bulk to adsorbed phases – similarities and differences. *Advances in Colloid and Interface Science*, 273, 102034.
https://doi.org/10.1016/j.cis.2019.102034

915 Schwartz, C., Vandenberghe-Descamps, M., Sulmont-Rossé, C., Tournier, C., & Feron, G. (2018). Behavioral and physiological determinants of food choice and consumption at sensitive periods of the life span, a focus on infants and elderly. *Innovative Food Science & Emerging Technologies*, 46, 91–106. https://doi.org/10.1016/j.ifset.2017.09.008

920 See, L., Mohammadi, M., Han, P. P., Mulligan, R., & Enciso, R. (2019). Efficacy of saliva substitutes and stimulants in the treatment of dry mouth. *Special Care in Dentistry*, 39(3), 287–297. https://doi.org/10.1111/scd.12370

923 Shahdad, S. a., Taylor, C., Barclay, S. c., Steen, I. n., & Preshaw, P. m. (2005). A double-blind, crossover study of Biotène Oralbalance and BioXtra systems as salivary substitutes in patients with post-radiotherapy xerostomia. *European Journal of Cancer Care*, 14(4), 319–326. https://doi.org/10.1111/j.1365-2354.2005.00587.x

927 Spirk, C., Hartl, S., Pritz, E., Gugatschka, M., Kolb-Lenz, D., Leitinger, G., & Roblegg, E. (2019). Comprehensive investigation of saliva replacement liquids for the treatment of xerostomia. *International Journal of Pharmaceutics*, 571, 118759.
https://doi.org/10.1016/j.ijpharm.2019.118759

931 Srivastava, R., Bosc, V., Restagno, F., Tournier, C., Menut, P., Souchon, I., & Mathieu, V. (2021). A new biomimetic set-up to understand the role of the kinematic, mechanical, and surface characteristics of the tongue in food oral tribological studies. *Food Hydrocolloids*, 115, 106602. https://doi.org/10.1016/j.foodhyd.2021.106602

935 Steele, C. M., Bailey, G. L., & Molfenter, S. M. (2010). Tongue Pressure Modulation During Swallowing: Water Versus Nectar-Thick Liquids. *Journal of Speech, Language, and Hearing Research*, 53(2), 273–283. https://doi.org/10.1044/1092-4388(2009/09-0076)

938 Sun, A., & Gunasekaran, S. (2009). Yield Stress in Foods: Measurements and Applications. *International Journal of Food Properties*, 12(1), 70–101.
https://doi.org/10.1080/10942910802308502

941 Truong, T., Bansal, N., Sharma, R., Palmer, M., & Bhandari, B. (2014). Effects of emulsion droplet sizes on the crystallisation of milk fat. *Food Chemistry*, 145, 725–735.
https://doi.org/10.1016/j.foodchem.2013.08.072

944 Vinke, J., Kaper, H. J., Vissink, A., & Sharma, P. K. (2018). An ex vivo salivary lubrication system to mimic xerostomic conditions and to predict the lubricating properties of xerostomia relieving agents. *Scientific Reports*, 8(1), 9087. https://doi.org/10.1038/s41598-018-27380-7

948 Vinke, J., Kaper, H. J., Vissink, A., & Sharma, P. K. (2020). Dry mouth: Saliva substitutes which adsorb and modify existing salivary condition films improve oral lubrication. *Clinical Oral Investigations*, 24(11), 4019–4030. https://doi.org/10.1007/s00784-020-03272-x
Víquez-Barrantes, D., Lau-Lee, N., Cubero-Castillo, E., & Cordero-García, M. (2023). Comparing consumer acceptance, sensory profile, and chemical properties in artisanal and industrial Greek-style yogurts processing: A path to enhancing food safety. *NFS Journal, 33*, 100156. https://doi.org/10.1016/j.nfs.2023.100156

Wagner, C. (2015). *An experimental and theoretical investigation of the rheological properties and degradation of mucin solutions (or why saliva becomes watery when removed from your mouth)* [Ph.D Thesis]. Massachusetts Institute of Technology.

Wirth, R., Dziewas, R., Beck, A. M., Clave, P., Heppner, H. J., Langmore, S., Leischker, A., Martino, R., Pluschinski, P., Rösler, A., Shaker, R., Warnecke, T., Sieber, C. C., Volkert, D., & Hamdy, S. (2016). Oropharyngeal dysphagia in older persons &ndash; from pathophysiology to adequate intervention: A review and summary of an international expert meeting. *Clinical Interventions in Aging*, 189. https://doi.org/10.2147/CIA.S97481

Xi, J., & Longest, P. W. (2007). Transport and Deposition of Micro-Aerosols in Realistic and Simplified Models of the Oral Airway. *Annals of Biomedical Engineering*, 35(4), 560–581. https://doi.org/10.1007/s10439-006-9245-y

Zhang, S., Olthoff, A., & Frahm, J. (2012). Real-time magnetic resonance imaging of normal swallowing. *Journal of Magnetic Resonance Imaging*, 35(6), 1372–1379. https://doi.org/10.1002/jmri.23591
# Appendix / Supplementary Data

## Appendix A - Wettability of the Soft-Robotic Tongue

The contact angle of water, $F_{0.40}$, and $F_{1.0}$ were tested on the roughness of the *in vitro* tongue (RoughSRT). The values are compared with *ex vivo* data trials using pig tongues. The results of two studies are illustrated. Ranc et al., (2006) evaluated the contact angle of cleaned tongues. Half of the samples were assessed dried “uncoated”, and the other half were plunged into fresh human saliva for 45 min, called “coated”. They observed that the contact angle of water reduced when the tongue was “coated”. In addition, Andablo-Reyes et al., (2020) evaluated the static water contact angle on fresh pig tongues. Similar values from *ex vivo* are reached when adding roughness to the SRT and by evaluating water and NSS.

<figure><img src="image_11.png" /><figcaption>Figure A.1. Contact angle of the Soft-Robotic Tongue compared with *in vivo* data.</figcaption></figure>
Appendix B - In Vitro Papillae Dimensions of the Soft-Robotic Tongue

The *in vitro* dimensions of the asperities on the surface of the SRT were tested using optical microscopy. It was obtained 369.7 ± 182.3 µm of diameter and 225.5 ± 88.7 µm of height. An example of the view of the SRT surface is represented. Values are comparable with *in vivo* filiform data from the literature; diameter = 355 µm and height =195 µm (Andablo-Reyes et al., 2020).

<figure><img src="image_12.png" /><figcaption>Figure B.1. In vitro papillae dimensions of the Soft-robotic Tongue.</figcaption></figure>
Appendix C - Degree Brix Calibration Curves

The refractometer was calibrated to ensure a linear relationship between Brix percentage and the mixing ratio of different combinations of flaxseed extracts and semi-solid foods. The calibration curves are in a range of concentrations between 0 to 0.86 w/w. Data shows good agreement with R² = 0.9996 for Ap(F₁₀), R² = 0.9966 for Ap(F₀.₄) and R² = 0.9859 for Gy(F₀.₄).

<figure><img src="image_13.png" /><figcaption>Figure C.1. Brix calibration curves of apple puree and Greek yogurt using natural salivary substitutes at 0.40 % and 1.0 % dry matter.</figcaption></figure>
Appendix D - Mouth Coating Combinations

The study analyzed six different mouth coating conditions and two types of food to be swallowed. In total, nine different combinations were investigated. To evaluate the performance of natural salivary substitutes formulated from flaxseed extracts, only apple puree was used as semi-solid food to swallow. This decision was based on the presence of particles in apple puree which makes easier the evaluation of residues perception. Then, the performance of a particular NSS (F₀.₄) was focused on with swallowing experiments using two semi-solid foods (apple puree and Greek yogurt). Three different mouth coating conditions were tested. The "LS" condition was assessed when using F₀.₄₀ as a mouth coating, while the "URS" condition was evaluated by using Greek yogurt or apple puree.

<figure><img src="image_14.png" /><figcaption>Figure D.1. Mouth coating combinations.</figcaption></figure>
1011 Appendix E - Mouth Coating Combinations - Bolus Viscosity Swallow #1 with Apple Puree
1012 The bolus viscosity after in vitro swallowing was characterized and it is presented in Figure
1013 E.1. These results correspond to section 3.2, where the influence of natural salivary substitutes was
1014 compared when swallowing apple puree.

<figure><img src="image_15.png" /><figcaption>1016
1017 Figure E.1. Bolus viscosity from swallow #1 with apple puree. Abbreviations are: (URS) unlubricated repeated swallowing, (LSF0.40) lubricated swallowing using NSS at 0.40 % dry matter, and (LSF1.0) lubricated swallowing using NSS at 1.0 % dry matter.</figcaption></figure>

1012
1013 E.1. These results correspond to section 3.2, where the influence of natural salivary substitutes was
1014 compared when swallowing apple puree.

<figure><img src="image_16.png" /></figure>