with the occurrence ofsyneresis, i.e., the expulsion of liquid from the gel,
which is undesirable in most gels. On the other hand, it is an essential
process in cheese making, where most of the liquid (whey) in the rennet gel
has to be removed. The process can be explained as follows.
Since the particles forming a fractal gel are ‘‘reactive’’ for aggregation
over their total surface, the free energy of the system would decrease when
the coordination number of the particles increases, or in other words when
more junctions between particles would be formed. However, since the
particles are more or less immobilized in the network, this is hardly possible,
especially if the gel is constrained in a vessel and sticks to the vessel wall.
However, ifjis not very high, some strands in the gel have a certain
freedom to move (Brownian motion), so that a new junction between
particles may occasionally form. This causes tension in the strand(s)
FIGURE17.16 Changes in rennet gels at various pH values (indicated) as a
function of time after rennet addition, at 30 8 C. (a) PermeabilityB. (b) Endogenous
syneresis pressurepsyn. (After results by H. J. C. M. van den Bijgaart. Ph.D. thesis,
Wageningen University, 1988.)