Food Biochemistry and Food Processing (2 edition)

BLBS102-c29 BLBS102-Simpson March 21, 2012 13:27 Trim: 276mm X 219mm Printer Name: Yet to Come

29 Biochemistry of Vegetable Processing 579

and enhance sedimentation. Under ideal conditions, obtaining a

colloidal particle size will enhance the stability of a juice prepa-

ration.

Physical Stability

Physical 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:

1. repulsing energy exponentially decreases as the distance
   between the particles increases; and


2. 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 decreases

ER

ER

E

(A)

(B)

EA

EA

0 2

2

1

1

M

M

+

E

0

+

E

h

h

Emax

Figure 29.2.Interactive energy between two particles.