Food Biochemistry and Food Processing (2 edition)

BLBS102-c40 BLBS102-Simpson March 21, 2012 14:23 Trim: 276mm X 219mm Printer Name: Yet to Come

766 Part 7: Food Processing

Electrodialysis

The electrodialysis process is based on the use of ion-selective

membranes, which are sheets of ion-exchange resin. The pro-

cess permits the separation of electrolytes from nonelectrolytes

in solution and the exchange of ions between solutions. The elec-

trodialysis process is used to separate ionic species in the food

and biotechnology industries. In the electrochemical separation

process, a gradient in electrical potential is used to separate ions

with charged, ionically selective membranes. The electrodialysis

process is used widely to desalinate brackish water to produce

potable water. The process is also used in the food industry

to deionize cheese whey and in a number of pollution-control

applications.

Ion-Exchange

The ion-exchange process is defined as the selective removal of

charged molecules from one liquid phase by transfer to a second

liquid phase by a solid ion-exchange material. The mechanism

of absorption is electrostatic opposite charges on the solutes

and ion-exchanger. The feed solution is washed off, followed by

desorption, in which the separated species are recovered back

into solution in a purified form. The main areas using it are

sugar, dairy (separation of protein, amino acids), and water pu-

rification. Ion-exchange processes are also widely used in the

recovery, separation, and purification of biochemicals, mono-

clonal antibodies, and enzymes.

Solid/Solid Separation

Solid/solid separation is a mechanical separation such as in

a milling separation facility. Solid/solid separation can be

achieved on the basis of particle size from the sorting of large

food units down to the molecular level. Shape, moisture content,

electrostatic charge, and specific gravity may affect the sepa-

ration process. Screening of materials through perforated beds

(wire mesh, cloth screen) produces materials of more uniform

particle size. For example, screening is usually used to sort and

grade many food materials such as fruits, vegetables, and grains.

A wide range of geometric designs of flat bed and rotary fixed

aperture screens are used in the food industry.

Sedimentation (Precipitation or Settling)

The sedimentation process is based on gravitational settling

of solids in liquids. Sedimentation processes are slow and are

widely used in water and effluent treatment processes.

Magnetic Separation

Magnetic separation relies on the behavior of individual parti-

cles under the influence of magnetic forces. When exposed to a

magnetic field, ferromagnetic materials are attracted along lines

of magnetic force from points of lower magnetic field intensity

to points of higher magnetic field intensity.

Equilibration Separation Processes

The following processes are the unit operations commonly ob-

served in the food industry that involve equilibrium between

solid and liquid phases.

Adsorption

Absorption is a process whereby a substance (absorbate or sor-

bate) is accumulated on the surface of a solid. Absorption process

is used to recover and purify the desired products from a mixture

of liquids. Adsorption is used for decolorization and enzyme and

antibiotic recovery. Liquid-phase adsorption is usually used for

the removal of contaminants present at low concentrations in

process streams.

Evaporation

Evaporation is the concentration of a solution by evaporating

water or other solvents. The process is largely dependent on

the heat sensitivity of the material. Boiling temperatures can

be lowered under vacuum. Most commercial evaporators work

in the range 40–90◦C and minimize the residence time in the

heating zone, for the concentration of heat-sensitive liquid foods

and volatile flavor and aroma solutions. The final product after

an evaporation process is usually in the liquid form.

Crystallization

Crystallization processes can be used to separate a liquid mate-

rial from a solid. Crystallization separates materials and forms

solid particles of defined shape and size from a supersaturated

solution by creating crystal nuclei and growing these nuclei to

the desired size. Crystallization is often used in a high-resolution,

polishing, or confectioning step during the separation of biolog-

ical macromolecules. Crystallization can be affected by either

cooling or evaporation to form a supersaturated solution in which

crystal nuclei formation may occur. Sometimes, it is necessary

to seed the solution by addition of solute crystals. Batch and

continuous operations of crystallization processes are used in

commercial food production.

Dewatering (Dehydration or Drying)

Dewatering is the process applied to separate water (or volatile

liquids) from solids, slurries, and solutions to yield solid

products.

Extraction

Solvent extraction is a method for a solvent–solvent or

solvent–solid contacting operation.

Solid–Liquid Extraction, e.g., Organic–Aqueous

Extraction

The solid is contacted with a liquid phase in the process called

solid–liquid extraction or leaching in order to separate the de-

sired solute constituent or to remove an unwanted component

from the solid phase. Solid–liquid extraction or leaching is a