handling the food, this may relate to spreading, slicing, grinding, mixing,
etc. The most important is, of course, texture as perceived during eating,
briefly calledmouth feel. Any sensory perception, be it (mainly) by eye, by
ear, by touch, or by the chemical senses, is a highly complex phenomenon.
The result depends on the individual and his or her experience, and on a
number of conditions during the perception. Thus perception of the same
attribute in the same food is subjective and quite variable.
Texture perception is poorly understood. The sensation that a person
experiences when eating is the resultant of an interaction between the
nervous impulses evoked in the mouth and the (almost fully unknown)
processing of the resulting signals in the brain. Here the dynamic aspect is
essential: without motion of jaws or tongue, texture perception is virtually
impossible. Even ‘‘spatial information,’’ i.e., food inhomogeneity, is largely
derived from ‘‘temporal information.’’ Moreover, it is not only the
consistency and the physical inhomogeneity of the food that is responsible
for the perceived mouth feel; other variables can also contribute, for
instance flavor, flavor inhomogeneity, sharpness, coolness, and any sound
emitted due to fracture of a piece of food. All of this is in the realm of
‘‘psychorheology,’’ a discipline that is clearly outside the scope of this book.
However useful it would be to know what makes a food ‘‘crispy’’ or
‘‘creamy’’ or ‘‘sticky,’’ most of it is still uncertain. Food manufacturers have
to rely on correlations between evaluations of trained panels and
experimental test results. A prerequisite then is that these tests should be
done under conditions that mimic what happens in the mouth. This
concerns mainly the following aspects:
Deformation mode. Deformation processes in the mouth vary from
swallowing, to pressing (e.g., between tongue and palate), to
chewing and biting. The phenomena occurring then range from
flow, to yielding, to fracture. This should be mimicked in the test.
Where it concerns flow or yielding, the deformation in the mouth is
predominantly (biaxial) elongation. Especially for viscoelastic
materials, the difference in apparent viscosity as measured in
elongational or in simple shear flow can be large, owing to variation
in the value of the Trouton number (Section 5.1.3). For hard solid
foods, biting may have to be imitated.
Strain rate. For most foods, rheological and fracture parameters are
time scale dependent. Generally, the effective strain rate in the
mouth during biting or chewing is of the order of 2 s^1 , and the
same rate should be applied in the test.
Inhomogeneityof the food poses problems. If it contains ‘‘soft’’ and
‘‘firm’’ regions, this may affect perception; it will also affect