Food Biochemistry and Food Processing

23 Biochemistry of Vegetable Processing 549

Where is the dielectric constant, ris the radius of
particles, his the distance between the two particles,

0 is the surface potential, and Ki is a constant that
characterizes the double ionic layer.
The inferences from the Equation 2.10 are that

• Repulsing energy exponentially decreases as the
  distance between the particles increases, and


• Repulsing energy quadratically increases as the
  surface potential increases and as the radius
  linearly increases.

For particles with radius rand a constant surface
potential, ERdepends on Ki. Figure 23.1 shows the
interaction between two particles (1 and 2) under
constantly increasing Ki. As a result of decreasing
the thickness of the double ionic layer and a de-
crease in -potential, the distance between the two
particles decreases, leading to an increase in the
repulsing energy according to Equation 2.10. For a
given h,the residual energy reaches the maximum
value (Emax), which must be overcome by particle 1
in order to aggregate with particle 2. Continual
increase of Ki and a decrease in the distance be-
tween particles h,reduces the effect of repulsing
energy ERover the residual energy E(EREAor
Emax), which reaches its minimal value M. At this
stage, the attracting energy EAdominates, and the
particles aggregate. The speed of aggregation is an
important physicochemical parameter for the stabil-
ity of a microheterogeneous system. Emaxis called
coagulation energy and determines the speed of
aggregation. WhenEmax0 (Fig. 23.1A), the ag-
gregation process is slow, whereas whenEmax
0
(Fig. 23.1B), the process is quick, since the particles
of the system do not have to overcomeEmax(when
EAER,EREAis
0; Fig. 23.1A,B). The critical
concentration of coagulation is reached atEmax0,
when the attractive and repulsive energies are equal.
In the real situation of a polydisperse colloidal
system such as juice, suspended particles have dif-
ferent sizes and different
0 and -potential. When
two particles with different sizes and different sur-
face -potential approach each other, an opposite
charge is induced upon the particle with lower -
potential, which activates the agglomeration. An-
other important factor in the stability of juices is the
hydrated layer of suspended colloidal particles. The
formation of the hydrated layer is due to the orienta-
tion of water molecules towards the hydrophilic
groups, such as —COOH and —OH, situated on the

surface of the particles. A decrease in -potential

leads to a decrease in the hydrated layer of suspend-

ed particles, and therefore increases the susceptibili-

ty to aggregation. The ability of the hydrated layer

to increase stability is explained by Deryagin the-

ory, which stipulates that when suspended particles

are close enough, their hydrated layers are reduced,

leading to an increase in repulsing forces between

them. Only strong hydrated layers can affect the

aggregative stability of a polydisperse system. The

probability of the formation of such hydrated layers

around vegetal particles in juices is very low, be-

cause of the weak energetic relationship between the

particles and the disperse phase. In this case, hydrated

Figure 23.1.Interactive energy between two particles.