Neetu GoudaMsc. 1st yr

The word colloids has actually derived from Greek

word “kola” where it means glue like.

The science of colloids was founded about 1861 by

Thomas Graham.

In his summary of work , he classified substances into

two groups viz: crystalloids and colloids ,depending on

their ability to diffuse through parchment membrane.

According to Graham , crystalloids readily passed

through parchment membranes while colloids do not.it

possess a size of diameter greater than 1 mu and less

than 100 mu.

Colloids are the particles that are too large to dissolve but remain dispersed in

the liquids. The term colloid doesn’t indicate a kind of matter, but a state of

matter. All colloidal systems have two phases:

A continuous phase extends throughout the system, surrounding all parts of

the other phase of the system.

The discontinuous phase or the discontinuous phase is, as the name implies,

distributed in isolated or disconnected fashion throughout the entire colloidal

system.

COLLOIDS

Name of colloidal system Dispersed phase Continuous phase Example in foods

EMULSION LIQUID LIQUID SALAD DRESSING

SOL SOLID LIQUID GRAVY

GEL LIQUID SOLID BAKED CUSTARD

FOAM GAS LIQUID EGG WHITE FOAM

SUSPENSOID GAS SOLID CONGEALED WHIPPED CREAM

Colloidal systems in foods:

Name of

colloidal

system

Disperse

d phase

Continuo

us phase

Example

in foods

EMULSI

ON

LIQUID LIQUID SALAD

DRESSIN

G

SOL SOLID LIQUID GRAVY

GEL LIQUID SOLID BAKED

CUSTARD

FOAM GAS LIQUID EGG

WHITE

FOAM

SUSPENS

OID

GAS SOLID CONGEA

LED

WHIPPE

D

CREAM

Properties of colloids:

Particles are visible only by ultra microscope or electron microscope – Brownian

motion.

They do not sediment, and pass through common filters ( but not through

semipermeable membrane).

Dispersion of passing light (Tyndall efect).

Produce osmotic pressure.

Colloids are everywhere

In the human body

Washing powder, soup, tooth paste, etc.

Many foods ( yogurt, butter, milk)

Nanotechnologies are based on chemistry of

colloids

TYNDALL EFFECTS: In a colloidal solution, the dispersed particles are

sufficiently large to scatter and polarize the incident light to same extent. This is

known as the tyndall effect. The effect is more pronounced in lyophobic sols than

in lyophilic sols.

NATURAL TYNDALL EFFECT

WHEN THE SUNRAYS ENTER THE FOREST CANOPY, TYNDALL

EFFECT CAN BE OBSERVED DUE TO THE DUST PARTICLES IN

THE ATMOSPHERE WHICH HELPS IN SCATTERING OF LIGHT.

BROWNIAN MOTION: When viewed through an ultra microscope , colloidal

particles appear to be in a state of rapid and irregular motion called the Brownian

movement. The movement is caused by the constant bombardment of the dispersed

particles by the molecules of the dispersion medium .It is zigzag vibration of the

dispersed particles .The smaller the size of the colloidal particles. The more vigorous

is its Brownian motion.

CLASSIFICATION OF COLLOIDS:

Colloids are classified into two groups :

1.] Lyophobic colloids: Includes those colloids

which have affinity for water. Eg. Inorganic

colloids.

Negative charge of S2- or HS- is

compensated by opposite charge from

the (H+).

2.] Lyophilic colloids: Includes organic colloids

such as protein , starch , which have great

affinity to water.

CHARGE ON COLLOIDAL PARTICLES:

All colloids possess an electrical charge which

may be either positive or negative. In a given

colloidal system, all particles have like charges

and hence tend to repel one another and remain

in suspension. To be in suspension state, the

colloidal particles distribute themselves

uniformly throughout the liquid in which they

are dispersed.

Colloidal systems with two phases can occur in

eight different combinations so colloidal

systems are not restricted to the dispersion of

solid in liquid only. Each of the three states of

matter can be dispersed in a medium which may

be gaseous, liquid or solid.

1.EMULSION: A mixture of two or more immiscible liquids. one liquid

(the dispersed phase) is dispersed in the other (the continuous phase). Three

substance are required to produce an emulsion: two immiscible liquids and

an emulsifying agent.

Prepared by shaking – oil/water (milk), water/ oil (butter).

Structure of emulsions: When a liquid e.g. oil is dispersed in

another immiscible liquid e.g. water(continuous phase) by mechanical

agitation , an emulsion is obtained.

Most of the dispersed droplets in food emulsions have diameters

between 0.1 and 10 micrometer.

Classification of emulsions – emulsions can be broadly

divided into two groups :

1) Oil in water type – Oil in water type emulsions

consist of fat dispersed in water e.g. milk.

2.) Water in oil type – water in oil type emulsions are

cream , butter , mayonnaise etc.

Some examples of food emulsions and uses:

Milk is an emulsion of oil in water type and consists of fat dispersed in

water; the emulsifying agent being albumin and casein .

Cod liver oil is an emulsion of oil in water type in which egg yolk ,

casein , gums have been added as emulsifier.

Butter is an emulsion of water dispersed in fat.

Properties of emulsions: • The opacity of an emulsion is determined by droplet sizes , droplet

concentration and difference in the refractive indices of the two phases.

• Emulsion is transparent when the refractive indices of the two liquids are the same or when droplet diameter is 0.05micrometer or less.

• When the dispersed phase is increased to a volume greater than that of the continuous phase the viscosity of the emulsion increases.

Formation of emulsions: • A stable emulsions is not formed by a mere mixing of the liquids .

• Because when it will be left for sometime the droplets in the dispersed phase coalesce due to surface tension.

• Therefore emulsifier is added to the emulsions,which reduces the interfacial tension and enhances the coalesce.

• Ex: Eggs yolk is incorporated in the emulsions since it contains lecithin , a natural emulsifying agents.

Stability of emulsions: • Stability changes in food emulsions can occur through the processes of creaming

, flocculation and coalescence.

• Emulsions can be stabilised against creaming , flocculation and coalescence by

introducing a strong interfacial film around each droplets , adding electric

charges to the droplets surfaces and increasing the viscosity of the continuous

phase.

• E.g Hydrocolloids, plant gums and gelatin , act as stabalizers in oil-in-water

emulsions by increasing the viscosity of the continuous phase.

• sometimes by forming a strong interfacial film around droplets of thr dispersed

phase.

• Pectins , alginates , plant and seed gums, and cellulose derivatives, such as

carboxymethayl, hydroxypropyl and methylcellulose gums are used as stabilizers.

Norevo- Acacia gum

1.SOLS : • Sols are the colloidal systems in which solid of colloidal dimensions is

dispersed throughout a liquid .

• This type of system has flow properties , which may range from rather

fluid to extremely viscous , barely flowing .

• Sol possessing a continuous aqueous phase , the colloidal particles have

an electrically charged surface.The ionized groups of protiens and

phospholipids can be sources of this charges.

• The resulting potential difference between the surface of the particles and

the solution is known as zeta potential.

• In lyophobic sols, there is little interaction between the dispersed phase

and the dispersion medium.

• The viscocity of the lyophilic sols is greater than that of the dispersion

medium and it increases in particle concentration and decreases in

increasing of teperature.

GELS: • Sols that have fairly high concentration of dispersed solids

change spontaneously into gels.

• The liquid phase is entrapped in the interstitial ares of this

structure, due to which the sols loses its fluidity and

becomes a gel.

• Bonds present in the junction zones of gel are electrostatic,

hydrophobic ,covalent and hydrogen bond.

• Some of the free liquid ma be released if the gel structure

is cut.Drainage of free liquid from a gel is termed as

Syneresis.

Characteristics of gels:

• The concentration of the jelling agent ,the salt content ,

the pH and temperature determine the strength , elasticity

and brittleness of the gel.

• All gels can be destroyed by mechanical action ,but some

of them will set again after the agitation stops,such gels

said to be thixotropic.

FOAMS: • Foams are dispersions of gas bubbles in a liquid which is in the

continuous phase .

• The diameters of the foam bubbles range from about 1 micrometer to

several centimeters. Depending on the bubble size and wall thickness,

dense or light foams are formed.

• Food foams contain large amounts of entrapped gas.

Formation of foams: • Liquid capable of forming foams have low surface tension and

thus can spread or stretch easily and so not coalesce readily.

• For the formation of a foam there must be a foaming agent in

the continuous phase prior to dispersion of gas.

• The foaming agent must be absorbed at the surface to reduce

surface tension and provide a distinct surface layer which

resists the coalescence of gas bubbles.

• Surface active lipids, glucosides, cellulose derivatives and

protein are used as foaming agents.

• Defoaming agents : Several liquids will destroy foams ; ether,

toulene, the aliphatic alcohols , and some oil breaks most

foams.

Foam stability: • Foam stability can be enhanced by increasing the elasticity of the

bubble walls, by increasing the viscosity of the solution.

• This can be done by introducing gums and proteins, such as

gelatin. Foaming agents also helps in the stability of foam.

• Protein coagulum has a high water binding capacity and thus

contributes a decrease in the rate of drainage from foam.