Physical Chemistry of Foods

Some important functionalities are mentioned in Tables 17.1 and 2,
and the physical properties needed to realize these qualities are specified. It
is seen that widely varying qualities may be desired, often in combination, as
illustrated above. It also follows that a gel can generally not be characterized
by its modulus: for hardly any of the functionalities mentioned is it an
essential parameter. The message of these tables is that one should always
first find the properties needed and then find methods to determine those
properties; then it should be possible to develop formulations and processes
to realize the qualities desired. The tables are not exhaustive: special
products often require special properties. Moreover, it is often desirable that
a property should depend in a given way on external conditions. For
instance, a food, or parts of it, may have to be ‘‘solid’’ at ambient
temperature but melt in the mouth.
Table 17.1 gives mechanical properties, and these have already been
discussed for the most part.Shapingmay need some elaboration. In the
table, shaping of a gel is what is meant, i.e., a gel is present and it has to be
forced into another shape. To that end it must exhibit yielding; if yielding
then leads to a low apparent viscosity, the time needed before the solid
character is restored is of importance. Shaping more commonly involves
pouring of a (thick) liquid into a mold and then letting it gel, for instance by
lowering temperature (gelation of gelatin), or by changing pH (some
spinning operations).
Table 17.2 indicates measures to be taken against physical instability
of various kinds. To prevent particle motion, a very weak gel generally
suffices. This is discussed in Section 13.3 under ‘‘Immobilization of
Particles.’’ In several systems, the particles themselves can form the gel;

TABLE17.1 Desired Mechanical Characteristics of Gels Made for a Given Purpose

Property desired Essential parameters Relevant conditions

‘‘Stand-up’’ Yield stress (or modulus, if
quite low)

Time scale

Firmness Modulus, or yield stress, or
fracture stress

Time scale, strain

Shaping Yield stress, restoration time Several
Handling, slicing Fracture stress and work of
fracture

Strain rate

Eating characteristics Yield and/or fracture
properties

Strain rate

Strength, e.g., of a film Fracture properties
or strain hardening

Stress;þtime scale

or strain rate