Introduction

Swallowing disorders, or dysphagia, affect 10%–30% of people aged 65 and above (Barczi et al. 2000) due to factors such as degenerative diseases, side effects of medication or simply age‐related impairment of physiological oropharyngeal function. Age‐related dysphagia is rarely curable and managed by texture‐modified foods and thickened beverages (Ekberg 2019). The increased viscosity of the beverage induces slower flow, which allows sufficient time also for an age‐impaired physiology to swallow safely (Newman et al. 2016). A specific factor to avoid fluid entering the airway, that is, aspiration, is to assure cohesivity of the bolus so it does not break up into droplets.

Dysphagia caused by dysfunction is clinically diagnosed mainly by two analytical methods: (i) X‐ray video‐fluoroscopy allows real‐time assessment of the anatomy and physiology of the swallowing apparatus during bolus transport (Ekberg 2019). (ii) Fiber Optic Endoscopic Evaluation of Swallowing (FEES) is used to optically examine abnormal swallowing function. FEES is a useful bedside swallowing examination to visualize the anatomy of the pharynx and larynx without X‐ray exposure and is highly sensitive for detection of aspiration after swallowing (Benjapornlert et al. 2020). Both methods are used clinically as the gold standards for diagnosing dysphagia. In addition, other methods such as questionnaires and observation swallow of different fluids are used clinically.

When a person is diagnosed with dysphagia, the management of drinking is mainly through thickened beverages where the level of thickening is expressed through the IDDSI‐level (International Dysphagia Diet Standardization Initiative) (Cichero et al. 2017). For beverages, the scale goes from 0 (not thickened) to 4 (extremely thickened), where IDDSI 1 and 2 are common. The levels 1–3 are clinically tested by letting the beverage drip out of a syringe with specified dimension for 10 s (Steele et al. 2024). The rheology of this test has been thoroughly evaluated and translated to Power law parameters n and K obtained by viscometry (Lecanu et al. 2024). The study shows that several fluids with a wide range of shear‐thinning can still fall into a specific thickness level. However, when considering other fluid properties such as elasticity, the classification becomes much more complicated (Lecanu et al. 2026). It should also be noted that overly viscous or elastic fluids can impair swallowing due to fragmentation with remaining pieces in the pharynx or excessive muscle force required to initiate swallowing.

Cohesiveness has been discussed by several authors and is easy to understand intuitively but has not, to our knowledge, been defined in physical terms (Burbidge et al. 2016; Chen and Lolivret 2011; Gallegos et al. 2023; Hadde et al. 2019; Nyström et al. 2015; McFarland et al. 2026; Wang et al. 2026). By modeling, it has been shown for Newtonian fluids that fragmentation (inverse cohesiveness) increased with bolus volume and velocity and decreased with viscosity (McFarland et al. 2026). Burbridge and co‐workers have also analyzed fluid mechanics of swallowing for Newtonian fluids (Burbidge et al. 2016). They concluded that viscosity is important for both the oral phase to avoid spill into the pharynx (manipulation of the bolus before swallowing) and for slowing down the flow during the pharyngeal phase. They also highlight the inertial effects (i.e., density) especially for fast flow. Water commonly exhibits turbulent flow, whereas thickened fluids flow in a laminar manner. Several authors indicate that fluid elasticity is important for cohesivity even though it remains to be proven in terms of fluid dynamics (Chen and Lolivret 2011; Gallegos et al. 2023; Hadde et al. 2019; Nyström et al. 2015; Wang et al. 2026). In anticipation of a stricter definition, cohesiveness will here be used in a loose sense meaning how well a bolus keeps together in time and space.

Fluid rheology can be measured non‐invasively orally over short distances (Sepehri et al. 2023) but this method is still in the experimental stage. As the complete oropharyngeal flow is of interest, model fluids have been used to capture the influence of different rheological parameters on swallowing. Newtonian fluids are commonly compared to Boger fluids to identify effects of fluid elasticity (Nyström et al. 2015). In a Boger fluid, a small amount of polymer is added to a high‐viscous Newtonian fluid which shields the shear‐thinning, thus keeping an almost constant shear viscosity in addition to being elastic (Boger 1977). The effect of elasticity is often contradicted by shear‐thinning, and such fluids are also interesting to include as model fluids. The shear viscosity can then only be compared for one shear rate, and for swallowing, 50 s−1 is often chosen although real shear rates in the swallowing process have been reported in the range 10–900 s−1 (Gallegos et al. 2023; Qazi et al. 2019).

Edible Boger fluids were first developed for sensory analysis (Koliandris et al. 2011). Edible model fluids can be formulated from viscous maltodextrin solutions with added xanthan for elasticity (Nyström et al. 2015; Qazi et al. 2020). For high viscosity, the effect of shear‐thinning of xanthan can be masked by the Newtonian fluid, whereas for lower viscosity, the addition is a balance between adding sufficient xanthan to induce elasticity and at the same time keeping the addition low enough to suppress shear‐thinning (Nyström et al. 2015). For a Power Law fluid, this means keeping the shear thinning index n as close to n = 1 as possible. The use of polyacrylamide instead of xanthan produces Boger fluids with more constant shear viscosity, but polyacrylamide is not edible (James 2009).

The aim of the present study was to determine which effect of thickening is most important for bolus cohesivity and to avoid aspiration. Model fluids were used to specifically study the effects of Newtonian, elastic, and shear‐thinning mechanisms.