Physical Chemistry of Foods

involved. If the junction strength is not too high, it may lead to breaking of a
strand; so loose ends are formed, which can form new junctions. In this way
the gel tends to become more compact and ‘‘tries’’ to expel liquid. These
events are illustrated in Figure 17.17.
Consequently, a particle network in which junctions can occasionally
be broken will build up a pressure on the liquid, calledendogenous syneresis
pressure,psyn. If the liquid can indeed flow out, the Darcy equation can be
used to obtain the rate of the process. We write it in the form

Q¼A

B

Z

psyn

L

ð 17 : 17 Þ

whereQis the volume flow rate,Athe surface area of the (piece of) gel, and
Lthe distance over which the liquid has to flow through the gel. Application
of the equation is not simple, becauseA,B,psyn, andLchange with ongoing
syneresis. Moreover, even if no liquid can flow out, as in the middle of a
piece of gel or in a constrained gel, bothBandpsynchange with time: see
Figure 17.16. This always leads to the formation of larger holes (pores) in
the gel, a process calledmicrosyneresis. Because syneresis can only occur if a
gel has formed,psynincreases with the buildup of the gel. The decrease
afterwards is explained below. Syneresis rate greatly increases with
increasing temperature and hardly occurs below 20 8 C.
In an acid casein gel (such as set yoghurt), syneresis hardly occurs, and
the value of Bremains virtually constant during storage. Presumably,
breaking of junctions is hardly possible in these gels. Only for gels made at
high temperature, say 40 8 C, some syneresis is often observed.

FIGURE17.17 Schematic representation of strands of particles in a gel forming
new junctions, leading to shrinkage and to breaking of a strand.