Food Biochemistry and Food Processing

548 Part V: Fruits, Vegetables, and Cereals

Where viscosity of the medium, rradius of
the particle in meters, and vvelocity of the parti-
cle (m/s).
Friction between the particles increases depend-
ing on the density of the medium: the higher the
density, the higher the friction. When f
fsedi-
mentation of the particles occur.
Kinetic stability of a heterogeneous dispersion
system also depends on Brownian motion. The most
dispersed particles have a very complex motion due
to collisions from molecules in the dispersion medi-
um. Because of this, the suspended particles are sub-
jected to constant changes in their velocity and
trajectory. Molecular kinetics shows that dispersed
particles with colloidal size change their path 10^20
times/s. These particles may also acquire a rotation-
al Brownian motion. This is why colloidal particles
have a higher sedimentation stability than larger par-
ticles. With an increase in the mass of the suspended
particles, their momentum also increases. Particles
smaller than 5  10 -4cm in diameter that oscillate
around a point do not sediment, whereas larger par-
ticles that do not experience as much Brownian mo-
tion as smaller particles easily sediment. Thus, when
particles have reduced motion, they tend to aggre-
gate and enhance sedimentation. Under ideal condi-
tions, obtaining a colloidal particulate size will en-
hance the stability of a juice preparation.

Physical Stability Physical stabilityis a result of
the ability of a polydisperse system to inhibit 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), which easily sediment. Physical stabil-
ity depends on two opposing forces, attracting and
repulsing.
Attracting forcesbetween molecules are referred
to as VanDerVaals forces, and they reduce the phys-
ical stability of a heterogeneous colloidal system.
The intensity of attractive forces increases as the dis-
tance between suspended particles decreases.
Repulsing forcesbetween particles are caused
by the charges surrounding the particles designated
by their -potential. When -potential is zero, the net
charge surrounding the particle is also zero, and the
suspended particles are said to be at their isoelectric
point. Agglomeration of particles begins at a given
value of -potential called the critical potential. This
is the point at which equilibrium is reached be-

tween VanDerVaals (attracting) forces and repuls-

ing forces. Different heterogeneous colloidal dis-

persion systems have different values of critical po-

tential. When the -potential (repulsive forces) of a

particle is higher than the critical potential, hydro-

philic colloids are stable due to the repulsion be-

tween particles, whereas, at a lower potential than

the critical potential , the particles tend to aggre-

gate. The effective energy of interaction between

particles of a heterogeneous colloidal system is ex-

pressed by

EEAER, (2.6)

where EAis attracting energy, and ERis 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

rthe potential energy is expressed by

(2.7)

where his the distance between surfaces of the two

particles, and A is the Hamaker constant (10^1 to

10 ^21 J). Consequently, attracting forces decrease as

the distance between particles increases.

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 their -potential.

When two particles are very close, their similar ion-

ic charge (positive or negative) layers create a repul-

sive 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 
 Pd, (2.8)

where Pis the increase in osmotic pressure, is

thickness of the double ionic layer, and the number 2

relates to energy changes between two particles.

P nKT (2.9)

where nis the increase in the ion concentration

between the two particles, K is Boltzman’s constant,

and Tis temperature in degrees Kelvin (K).

Assuming that the two particles are spherical,

Equation 2.8 can be expressed by

E (2.10)

rh

R

exp { Ki

≈

[( − })]

,

εψ 02

2

E

Ar

A h

=

−

12