BLBS102-c29 BLBS102-Simpson March 21, 2012 13:27 Trim: 276mm X 219mm Printer Name: Yet to Come
29 Biochemistry of Vegetable Processing 579and enhance sedimentation. Under ideal conditions, obtaining a
colloidal particle size will enhance the stability of a juice prepa-
ration.Physical StabilityPhysical stability results from the property of a polydisperse
system that inhibits the agglomeration of suspended particles.
In a system with low physical stability, suspended particles ag-
glomerate to form heavier particles (> 5 × 10 −^4 cm in diameter)
that easily sediment. Physical stability depends on two opposing
forces, attracting and repulsing forces.
Attracting forcesbetween molecules are referred to as van
der Vaals forces, which reduce the physical stability of het-
erogeneous colloidal system. The intensity of attractive forces
increases as the distance between suspended particles decreases.
Repulsing forcesbetween particles are caused by the charges
surrounding the particles designated by theirζ- potential. When
ζ-potential is zero, the net charge surrounding the particle is also
zero, and the suspended particles are said to be at their isoelec-
tric point. Agglomeration of particles begins at a given value of
ζ-potential called as the critical potential. This is the point at
which equilibrium is reached between van der Vaals forces (at-
traction) and repulsing forces. Different heterogeneous colloidal
disperse systems have different values of critical potential. When
theζ-potential (repulsive forces) of a particle is higher than the
critical potential, hydrophilic colloids are stable due to the re-
pulsion between particles, whereas at a lower potential than the
critical potentialζ, the particles tend to aggregate. The effec-
tive energy of interaction between particles of a heterogeneous
colloidal system is expressed by:E=EA+ER, (6)where EAis the attracting energy, and ERis the repulsing energy.
Attracting energy is actually the integration of the sum of all
attracting forces between molecules of two colloidal particles.
For two particles with radius r, the potential energy is expressed
by:EA=−Ar/ 12 h (7)where,his the distance between surfaces of the two particles,
and A is the Hamaker constant (10−^1 –10−^21 J). Consequently,
attracting forces decrease as the distance between particles in-
crease.
On the other hand, particles can come closer to one another
up to a certain distance after which they start repulsing each
other because of theirζ-potential. When two particles are very
close, their similar ionic charge (positive or negative) layers
create a repulsive force, which keeps them apart. The repulsing
energy between two particles with the same radius r and the
same surface potential 0 is expressed by:ER=− 2∫
Pdδ, (8)where Pis the increase in osmotic pressure;δis thickness of
the double ionic layer; and the number 2 relates to the energy
changes between 2 particles.
P=
nKT (9)where nis the increase in the ionic charge between the
2 particles; K is the Boltzman’s constant; andTis the tem-
perature in degrees Kelvin (◦K).
Assuming that the two particles are spherical, the formula (8)
can be expressed by:ER≈[(
εrψ 02 exp{−χh})]/
2 , (10)whereεis the dielectric constant;ris the radius of particles;
his the distance between the two particles;ψ 0 is the surface
potential, andχis a constant which characterizes the double
ionic layer.
The inferences from the Equation 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 radiusrand a constant surface potential, ER
depends onχ. Figure 29.2 shows the interaction between two
particles (1 and 2) under constantly increasingχ. As a result of
decreasing the thickness of the double ionic layer and a decrease
inζ-potential, the distance between the two particles decreasesERERE(A)(B)EAEA0 2211MM+E0+EhhEmaxFigure 29.2.Interactive energy between two particles.