Swelling and Syneresis. Again, little systematic study has been
made of these phenomena. According to Flory, the equilibrium volume of a
covalently cross-linked polymer gel would be proportional to (m1)0.6,
where m is the average number of cross-links in which each polymer
molecule is involved. Increasing the number of cross-links after a gel has
been made would thus lead to shrinking. However, when a solution of
gelatin or some polysaccharide is made to gel, the whole volume turns into a
gel, and this generally does not change upon aging, even though the
modulus of the gel may markedly increase, and probably the number of
junctions as well. This means that soon after a gel is formed, its shape is
more or less fixed.
On the other hand, by altering the solvent quality, i.e., the parameterw
orbdiscussed in Section 6.2.1, swelling—i.e., taking up of solvent by the
gel—or shrinkage—i.e., syneresis or the expulsion of liquid from the gel—
can occur. For polyelectrolytes, the same phenomena can happen when
altering the pH or the ionic strength. However, these processes tend to be
very slow, because of the very small permeability of the gels: generallyB¼
10 ^17 10 ^16 m^2. This causes a very strong resistence to the flow of liquid
through the gel; see Section 5.3.1.
Question
Classical jam manufacture is achieved by mixing fruit pulp with a large proportion of
sucrose and then evaporating water by boiling until the mass ratio of sucrose to
water equals about 2:1. As mentioned, the pectin of the fruit can form a typical
‘‘weak’’ gel at these conditions, leading to a jam of desirable consistency, i.e., with a
significant, but not overly large, yield stress. Currently, jam is often made with far
less sugar, to reduce ‘‘calories.’’ The gel obtained then is too weak, for the most part
because the solvent quality for pectin is too good. What measures can you think of to
improve the consistency of such a jam?
Answer
Several possibilities can be considered.
- Add more pectin (or use fruit varieties with a higher pectin content) and
lower the pH to neutralize carboxyl groups on the pectin. This helps to some extent,
but many consumers consider the jam too acid. - Lower the solvent quality by addition of poorly digestible saccharides, like
mannitol. This gives a good consistency, but the mannitol reaching the large intestine
attracts large amounts of water to decrease osmotic pressure, often leading to
diarrhea.