Fluid foods are popular in modern life. They are not only enjoyable to consume and
provide nutrition, but are also beneficial to special populations, such as those with
dysphagia and temporomandibular joint disease or who are edentate. Food rheological
properties have an important influence on food oral processing and swallowing. Tongue
movement plays a vital role during oral processing of liquid, semi-solid and soft-solid
foods. The purpose of this research was to investigate the boundary criteria for
categorising liquid, semi-solid and soft-solid foods; identify relationships between food
properties and oral processing behaviours; and characterize tongue and lower jaw
behaviours during food oral processing, in particular shear stresses generated between
the tongue, lower jaw and hard palate.
Constant weight samples were served to subjects who were instructed to consume them
naturally, whilst movements of the tongue and lower jaw were measured via
articulography and masseter and submental muscle activities were measured via
electromyography. Food rheological properties (viscosity, flow curve, stretch-ability,
storage modulus and loss modulus), pH and moisture content were characterized for
each food sample.
The oral residence time was found to be an important oral processing behaviour, which
is affected by the original food viscosity, viscoelastic properties, moisture content, and
stretch-ability. Tongue movements dominate the oral processing of semi-solid and softsolid
foods instead of mastication which occurs for hard-solid food. The shear stress of
the tongue and lower jaw is the main power during oral processing of semi-solid and
soft-solid foods. The maximum shear stress of Greek yoghurt on tongue tip was 123 ±
31 Pa and 151 ± 59 Pa for two subjects; for custard, it was 144 ± 46 Pa and 192 ± 20 Pa.
These results agree with estimated data which is currently available for the same food
types. Overall, the shear stress tends to increase with increasing food viscosity.
The method developed for measuring shear stresses applied in the oral cavity during
oral processing was novel and is the closest to measuring real, in – mouth, shear stresses,
which has not been possible to date.