Materials and Methods

Materials

The model fluids used in this study were prepared using maltodextrin Glucidex IT 19 (Roquette, Lestrem, France) and xanthan Grindstedt Clear 80 (IFF, Brabrand, Denmark). The ingredients were food grade to allow use of the same model fluids in sensory studies. The fluids were for the same reason mixed in a sugar‐free lemonade (Fun Light Lemonade, Orkla Foods, Oslo Norway). Polyacrylamide (PAA) 5–6 × 106 Da (Thermo Fischer Scientific, 178042500, Waltham, USA) was used as a non‐edible reference instead of xanthan.

Model Fluids

Fun Light was mixed with water 1 + 9 according to specifications. Stock solutions of 60% w/w maltodextrin and 1% w/w xanthan in diluted Fun Light were mixed and stirred overnight on a magnetic stirrer. The stock solutions were then mixed into model fluids (Koliandris et al. 2011; Nyström et al. 2015) to fulfill two criteria (i) to have IDDSI level 2 (Cichero et al. 2017); and (ii) to have shear viscosity of 0.15 Pa s at 50 s−1. The mixed fluids were stirred overnight. The three fluids were:

Newtonian: 53.7% w/w maltodextrin.

Boger: 50% w/w maltodextrin and 0.045% w/w xanthan.

Shear‐thinning: 0.45% xanthan. The edible Boger fluid was compared to one with PAA instead of xanthan mixed as follows:

BogerPAA: 51.5% w/w maltodextrin and 0.06% w/w PAA.

Swallowing Model

The swallowing simulator, referred to as the Gothenburg Throat, has been described in detail elsewhere (Stading et al. 2019). A brief overview is provided here to introduce it to the reader (see Figure 1).

FIGURE 1: Overview of the swallowing simulator, the Gothenburg Throat model. The expanded schematic shows the pharynx section of the simulator with the ultrasonic transducer in black and pressure sensors in green.

The test fluid is stored in a tank connected to the simulator and is delivered to a syringe pump representing the oral phase of swallowing. A bolus of predefined volume and injection velocity is introduced into the model pharynx, thereby mimicking the propulsive action of the tongue. During filling of the syringe, a slide valve remains closed to prevent gravity‐driven flow. The valve is opened only momentarily during bolus delivery, ensuring that flow is generated exclusively by the thrust applied by the syringe.

Anatomical structures are represented by mechanical components: two valves simulate the openings to the trachea and nasopharynx, a movable epiglottis closes during swallowing, and a clamping valve represents the upper esophageal sphincter (UES). The UES remains closed at the start of each swallow and opens transiently to allow bolus passage, coinciding with the closure of the epiglottis from its initial open position. The epiglottis does not form a hermetic seal over the laryngeal inlet.

As the bolus traverses the pharyngeal model, velocity profiles are measured using ultrasound velocity profiling, UVP, (Incipientus Ultrasound Flow Technologies, Västra Frölunda, Sweden) while pressure transducers record pressures at four predefined locations.

Device Settings

In the present study, the bolus volume was set to 30 ± 2 mL, which corresponds to a substantial gulp as we do when we drink. The ultrasonic transducer was placed above the epiglottis as shown in Figure 1. Two digital cameras (DSC‐RX100, Sony, Tokyo, Japan), positioned in front of and on the side of the pharynx part of the swallowing simulator to record the bolus motion. The piston speed, that is, the initial bolus speed was set to 0.30 ± 0.03 m/s and was kept constant independent of fluid rheology. For healthy swallowing the epiglottis closed immediately on initiation of swallowing, the airways (trachea) were closed and the UES open. For impaired swallowing, the closing of the epiglottis was delayed 1 s and the airways and UES were open. The ultrasonic transducer gives velocities along a beamline through the center of the pharynx as a function of depth and time as shown in Figure 1 (Qazi et al. 2020; Wiklund et al. 2007).

During bolus ejection, image acquisition was performed at 100 frames s−1. The recorded videos were analyzed and edited using Premiere (Adobe, San Jose, USA).

Shear Rheometry

Viscometry was performed using an ARES‐G2 rheometer (TA Instruments, New Castle, DE, USA) equipped with a concentric cylinder system for viscosity (cup diameter 20 mm or 30 mm depending on fluid tested) and a cone‐plate system (40 mm, 2.3° angle) for determination of normal force. The measurements were performed at 20°C.

Extensional Viscometry

The transient extensional viscosity of the thickened solutions was determined by the Hyperbolic Contraction Flow method (HCF) using an Instron 68SC‐5 (Instron Corp., Norwood, USA). The HCF method measures the force on a hyperbolic nozzle subjecting the sample to a constant extension rate and is previously thoroughly described and evaluated (Nyström et al. 2017; Stading and Bohlin 2000; Wikström and Bohlin 1999). The hyperbolic nozzle used for the measurement had an inlet radius of 10 mm and an outlet radius of 0.83 mm, which gives a total Hencky strain of typically 6.6–9.9 for the fluids investigated.

Density and Surface Tension

Density was measured using a Densito 30PX (Mettler Toledo, Schwarzenbach, Switzerland). Surface tension was determined by an OCA40 pendant drop instrument (DataPhysics Instruments, Filderstadt, Germany).

Statistics

Measured values are presented as mean values with error bars denoting the standard deviation. Three or more replicates were measured to calculate the mean values.

Theory

Dimensionless numbers are often used to estimate the influence of different physical mechanisms, here on the flow of fluids. The Ohnesorge number (Oh) is a dimensionless number that relates viscous forces to inertial and surface tension forces. A low Oh indicates droplet formation and fluid breakup for example, when a fluid exits a tube given byThe Reynolds number (Re) is a dimensionless number that relates inertial forces to viscous forces and predicts laminar or turbulent flow as expressed for flow in a tube.The Weissenberg number relates elastic forces to viscous forces in a viscoelastic fluid. In shear flow it can be expressed as follows:In (Equations (1), (2), (3)), η denotes viscosity, ρ density, σ surface tension, v fluid velocity, D the tube diameter, λ the relaxation time and γ˙ the shear rate.

(1) Oh=ηρσD

(2) Re=ρvDη

(3) Wi=λγ˙