Optical Measurement of the stress and flow profile of foams in an idealized deglutition

Various physicochemical properties play a decisive role in the evaluation of foods, influencing
taste, odor, texture and mouthfeel when the food is distorted. Therefore, rheological investiga-
tions of foods are used in product development to specifically improve the texture or mouthfeel
of a product. Since mouthfeel describes the physical interaction between the food and various
haptic sensors in the mouth during the chewing and swallowing process, it is advantageous to
perform rheological measurements in geometries and under conditions that reflect the flow
conditions present in the mouth. Up to now, such investigations have been limited primarily to
viscous or lumpy foodstuffs. Here, foam, as a multiphase system consisting of a (highly) vis-
cous liquid and dispersed gas, exhibits complex rheological behavior due to its compressibility.
In addition, the foam undergoes partial destruction of its structure during the swallowing pro-
cess, which can change its rheological properties over time.
For the imaging of the swallowing process, an experimental setup was developed consisting
of a two-dimensional replica of the palate and a movable tongue based on dental impressions.
Foam with different properties such as the mean bubble size and the liquid content or the
degree of polydispersity can be generated. Furthermore, two tongue geometries with different
roughness are available. The flow as well as the deformation of the foam is evaluated by optical
methods such as PIV and particle tracking. The resulting velocity, shear rate and (wall) shear
stress distributions can provide information about the haptic perception in the mouth during the
swallowing process.