Optical anisotropy. The best known example is that where the
refractive index depends on direction. Such a material is said to
bebirefringent: when linearly polarized light passes through it, its
velocity depends on direction (on the angle between the plane
through the light beam and the polarization plane). Interference of
the emerging rays then causes the light to become elliptically
polarized. Such a material generally appears bright when viewed by
a polarizing microscope. The origin is at molecular scale. Most
crystals are birefringent (e.g., all sugar crystals) and fibrous
molecules in a (partly) parallel orientation also. Good examples
are plant cell walls and native starch granules.
Permeability (see Section 5.3.1). Transport of liquid through the
material can be greatly dependent on direction. The prime example
is wood, which derives from vascular tissue that consists of long
tubular cells (trachea), needed for transport of water. Some natural
food materials also show this dependency. It finds its origin at a
scale of, say, a micrometer. Anisotropy with respect todiffusionof
small molecules is rarely encountered in foods. For instance, the rate
of diffusion of salt into muscle tissue is not direction dependent
(along or across the fibers).
Mechanical properties, i.e., rheological and fracture phenomena. This
anisometry is widespread among natural foods and among
fabricated foods made by spinning or extrusion. It is mostly caused
by relatively large structural elements (micrometer to millimeter
scale).
The most complicated structures are found in natural foods, but even
a superficial discussion would take too much space. Just one example is
given in Figure 9.4. Even from this simplified picture, it is obvious that an
apple has numerous structural elements and that these can be arranged in a
hierarchical order. At every level, specific aspects are of importance. To
obtain a full understanding of the properties of the system, the structure and
the interactions between structural elements would have to be studied at
several length scales, from molecular to macroscopic. This brings us to the
next section.
Question
You have a so-called low-fat spread, i.e., a system containing about equal volumes of
triglycerides and an aqueous phase. The system is not a liquid, but it is spreadable.
Can you think of a very simple way to find out whether it is oil or water continuous?