Texture contrast: ultrasonic characterization of stacked gels’ deformation during compression on a biomimicking tongue

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Instrumental Methods

As discussed previously for the sensorial methods, the texture attributes are subjective and sometimes could be abstract, in the sense that these attributes do not necessarily provide a clear understanding of the physical and mechanical phenomena behind them. Sensorial methods are also time consuming and expensive. Therefore the need for instrumental analysis of food (or bolus) in order to comprehend the sensory perception based on its mechanical and structural properties becomes evident. The characterization of the mechanical properties of food can be done at two levels, including (i) the bulk properties of the food and of the bolus (shear viscosity, yield stresses and moduli), and (ii) the properties resulting from the interactions between the food and oral tissues (like friction).
The correlation between instrumental characterizations and sensory analysis data could be complimentary, and should be encouraged for the understanding of the mechanical aspects of food breakdown with resultant textural perception. This point and associated challenges are further discussed in the next section.

Bulk Rheology

Rheological methods are among the first instrumental methods which have been used to study the physical mechanisms behind texture perceptions. Rheology allows to link force (or stress) to the deformation of materials. Therefore, it is suitable to describe the breakdown of food to mechanical stresses encountered during oral processing. With time, classical rheological methods along with other applied methods have been developed to take into account the experimental challenges related to food bolus (Stokes et al., 2013).

Mechanical characterization of solid foods

For instrumental characterization of the texture of solid or semi-solid foods, compression tests could be carried out. Using compression/traction equipments such as so-called Texture analyzers, food is deformed uniaxially between two rigid surfaces. A controlled force or displacement is applied to the product and its response in terms of force and deformation is measured as a function of time. This makes it possible to characterize different types of products and to analyze their mechanical properties. Single or double compression, penetration, spreading, back extrusion and tensile tests can be performed.
Single uniaxial compression is also very often used to measure fundamental mechanical parameters. The results of a uniaxial compression can be presented in the form of a stress-strain curve to determine the Young’s modulus, yield strength, fracture point etc.
One of the most widely used compression tests to characterize texture is Texture Profile Analysis (TPA) (Szczesniak, 1963). This method aims to establish a direct link between mechanical and sensory properties of the product. The TPA protocol requires double compression of the food, to collect a curve tracing the evolution of the force over time (Figure 2.10). The double compression was chosen to mimic the action of the teeth during chewing or the action of the action of the tongue during tongue-palate compression. Friedman, Whitney, & Szczesniak (1963) illustrated a TPA curve and discussed the various resulting quantitative parameters related to food mechanical properties. In Figure (2.10), hardness was defined as the peak force during the first compression cycle. Adhesiveness on the other hand was associated with negative force area A3. And cohesiveness was defined as the ratio of positive force area during the second compression to that during the first compression (A2/A1).
For liquid or semi-solid foods (which are not suitable for compression testing), shear measurements are preferred. The shear rotational tests allow to estimate the viscosity of the product, while oscillatory tests provide insights about its viscoelastic properties (Figure 2.11).
Viscosity is measured by applying a tangential stress to a fluid located between two solid surfaces with varied geometries such as coaxial cylinders; cone-and-plate; rotating disc; parallel plates.
The shear stress (τ) is defined as the ratio between the tangential force (F) measured between the solid surfaces and the area under the plate.
Figure 2.11: Different rheological tests for food texture assessment (Tabilo-Munizaga et al., 2005).
The flow curves represent the tangential stress versus velocity gradient (τ versus )̇. The viscosity curve represents the dynamic viscosity as a function of the velocity gradient (η as a function of )̇. Figure 2.12 presents an example of variation of viscosity of different food products on application of shear stress.
The behavior of the fluid could be Newtonian or non-Newtonian. The viscosity of Newtonian fluids is independent of stress or shear rate. In contrast to Newtonian fluids, fluids with a non-constant velocity gradient behavior have a viscosity called « apparent viscosity ». The non-Newtonian behavior can be further classified into (i) Shear thinning: when the viscosity decreases with applied shear (ii) Shear thickening: when the viscosity increases with applied shear. The shear thinning behavior is the most common in food as seen in figure 2.12. Many foods have rheological properties that depend on time and the thermomechanical history of the material. This is called thixotropy.
Research studies have also used sensory analysis methods (discussed previously) for complementing the instrumental measurements (Table 2.2):
As food undergoes major transformations during the oral processing, one can assume that mechanical properties of the swallowed bolus would be quite different from the initial food ingested, and so will be the texture perception. Therefore, it becomes very necessary to characterize the rheological properties of bolus as well. This information can also be useful for understanding the rheological behavior of the bolus when swallowing and hence could bring key insights when designing products for populations with special needs, like for elderlies or for people with dysphagia.
As mastication during the processing of solid foods leads to fragmentation, important studies have been performed on natural products (peanuts, olives, chicken ham etc.) to understand the particle size distribution in the bolus and its impact on deglutition (Jalabert-Malbos, Mishellany-Dutour,Woda, & Peyron, 2007; Mishellany-Dutour, Woda, Labas, & Peyron, 2006; Peyron, Mishellany-Dutour, & Woda, 2004). These studies manly involved the subjects with strict dental criteria like complete dentition
and no known masticatory disorder. Furthermore, towards more controlled in vitro approach, artificial mastication simulators have also been developed (Mielle et al., 2010; Salles et al., 2007; Woda et al., 2010).
Semi solid gels have also been used as model food to understand the rheology of boli with viscoelastic properties. Ishihara et al. (2011a & b) developed a mechanical simulator to investigate the rheology of a
Figure 2.13: Artificial mastication simulator for preparation of model bolus (Ishihara et al., 2011b).
bolus formed from instrumental mastication of gels in presence or absence of artificial saliva. The gels were compressed and sheared in a reciprocating manner using a flat plunger mimicking the action of a human jaw (Figure 2.13). The authors studied the frequency dependence of the viscoelastic properties of different gels (with homogeneous and heterogeneous structure). The role of saliva on the viscoelastic properties of gels was also explained with respect to the hydrophilic nature of the obtained bolus.
In the similar way, the rheological properties of the bolus and its interaction with saliva also becomes important during swallowing. Number of studies have come up with custom-made setups and ranges of model foods to comprehend the subject better (Preciado-Méndez et al., 2017; Salinas-Vázquez et al. 2014).
Mathieu et al (2018) developed an in vitro elastohydrodynamic model (Figure 2.14) for in-line measurement of the thickness of the coatings resulting from the flow of bolus through the pharynx. The resultant coating on the pharyngeal mucosa were found to be dependent on the food properties (viscosity and density) as well as on physiological variables like lubrication by saliva, velocity of the peristalsis, and to a lesser extent, the deformability of the pharyngeal mucosa.

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Table of contents :

I. Introduction
II. State of the art
2.1. Food oral processing
2.2. Food texture perception
2.3. Food texture characterization
2.3.1. Sensory Methods
2.3.2. Instrumental Methods
2.3.2.1 Bulk Rheology
2.3.2.2 Tribology
2.3.2.3 A novel ultrasound based technique
2.4. Objective of the thesis
III. Ultrasound monitoring of a deformable tongue-food gel system during uniaxial compression–an in vitro study
3.1. Introduction
3.2. Materials and methods
3.2.1. Preparation and characterization of food gels
3.2.2. Preparation and characterization of the artificial tongue models
3.2.3. Ultrasound measurements
3.2.4. Signal processing
3.3. Results and discussion
3.3.1. Characterizing the deformation of the artificial tongue models—ToF
3.3.2. Analysis of the tongue model-food gel interface—R*
3.4. Conclusions
IV. Texture contrast: ultrasonic characterization of stacked gels’ deformation during compression on a biomimicking tongue
4.1. Introduction
4.2. Materials and methods
4.2.1. Preparation of the stacked gels
4.2.2. Preparation of the artificial tongue model
4.2.3. Test sequence and ultrasound measurements
4.2.4. Signal analysis
4.2.5. Data Processing
4.3. Results and discussion
4.3.1. Impact of gel properties on the deformation of the artificial tongue model
4.3.2. Distribution of deformation: the tongue-food system
4.4. Conclusions
V. Towards a new biomimetic setup
V (A). Setup development
5a.1. Requirements of the system
5a.2. Setting up the device
5a.2.1. Mechanical design
5a.2.2. Electronic elements
5a.2.3. Development of custom-made parts
5a.2.4. Design of a user-machine interface
5a.3. Proof of concept study
V (B). A new biomimetic set-up to understand the role of the kinematic, mechanical, and surface characteristics of the tongue in food oral tribological studies
5b.1. Introduction
5b.2. Materials and methods
5b.2.1. Model food system
5b.2.2. Development of the bio-mimicking tongue-palate set-up
5b.2.3. Test protocol
5b.2.4. Data processing and set-up validation
5b.3. Results and discussion
5b.3.1. Role of food matrix viscosity and heterogeneity
5b.3.2. Role of surface roughness
5b.3.3. Role of operational parameters: normal stress and shearing velocity
5b.3.4. Role of TMS rigidity
5b.4. Conclusions
VI. General Discussion
6.1. Towards more realistic in vitro setups
6.2. Finding the sensory links
VII. Conclusion & future prospects

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