crystallites of stretched chain sections are present in gels of some
galactomannans and possibly xanthan.
Another categorization distinguishes ‘‘strong’’and‘‘weak’’ gels. A
strong gel, e.g., agar or alginate, is characterized by a relatively high
modulus and a low value of tand(generally<0.1). The deformation is
linear up to a strain of about 0.25 and at a somewhat larger strain the gel
will fracture. This means that the gel type may perhaps better be named
‘‘brittle.’’A weak gel, e.g., xanthan, has a lower modulus and a higher tand
value; its deformation is linear up to a far smaller strain, but it will yield
rather than fracture when the strain is increased.
Pectinis somewhat special, since it can make two types of gel. A high-
methoxyl, high-molar-mass pectin, present in a highly concentrated sugar
solution of low pH (to neutralize most carboxyl groups), can form a typical
weak gel, where the junctions contain stacked helices. This is what is
obtained in traditional jam. A low-methoxyl pectin, i.e., with relatively
many carboxyl groups, can form egg-box junctions in the presence of
calcium ions. A typical brittle gel results. In both cases, only nonhairy
regions on the pectin chain participate in the junctions.
Themagnitude of the moduluscan be affected by several variables:
Gelation time. As mentioned for carrageenans, it may take a while
(e.g., an hour) before the gel is fully developed after the gelation
temperature has been reached. An increase of the modulus with time
is observed for many polysaccharide systems (and also for gelatin),
but the time needed varies widely among systems and with process
conditions (temperature, concentration, pH, etc.). All of this means
that ‘‘gelation time’’ is a poorly defined property. ‘‘Weak gels,’’ as
formed, e.g., by xanthan, may show yielding upon mild agitation. It
will then take some time, e.g., 15 min, before the yield stress is
restored.
Concentrationof gelling material. Examples are given in Figure 17.13a
and b, and it is seen that the relations can vary widely. The
minimum concentration at which a gel is formed is in principle given
by the chain overlap concentration jov[Eq. (6.12)] or c*: see
Sections 6.4.2 and 6.4.3. The value can vary significantly among
polymers, as illustrated in Table 6.5. In practice, the lowest
concentration giving a gel tends to be somewhat higher thanc*.
Molar mass, or degree of polymerization, of the polysaccharide. This is
because for a higher molar mass, the value ofc* will be smaller.
Consider a series of samples of the same material but varying in
molar mass. If the concentration range is well above c* for all
samples, the effect of molar mass is in principle zero; if the