ISOLATION AND GEL FORMATION

OF POLYSACCHRIDES AND

STRACH RETROGRADATION

M. S. ESHWAR

School of Food Technology

JNTU K

INTRODUCTIONGel is an intermediate between solid and liquid, consists of both flow and elastic

characteristics

Ex:- jams, jellies, desserts and quick set gels etc

Gelling agents that are used in gel forming products that are hydrocolloids.

These hydrocolloids are derived from the natural sources of polysaccharides.

These hydrocolloids are used as a thickening, gelling dispersions, stabilizing

foams, emulsions.

Mechanism of gelation depend upon on the conditions of gel formation like

temperature, pH, concentration of gelling agents used.

Gel formation depend upon the gelling agents and nature of synergistic effects of

different constituents.

Different Constituents of gel formation

• Gelatin

• whey protein

• Egg protein

• Alginates

• Pectin

• Agar

• starch

• carragennan

Gelatin

• Melts when heated and solidifies when it is cooled.

• Gelatin occurs by a polypeptide chain takes an orientation to

induce a reactive site.

• Later, condensation of two other chains

occurs giving rise to triple helix formation

WHEY PROTIEN

• Heat induced during the gelation of whey proteins proceeds

some transitions

• denaturation (unfolding) of native proteins.

• aggregation of unfolded molecules results in the association of

stands and forms network

EGG PROTEINBoth albumen and yolk of liquid eggs have the capacity to form

gels upon heating. Gel formation is a two-step process of

Denaturation and followed by

aggregation of denatured proteins.

AlginatesGels are formed on the addition of polyvalent cations at a low pH <4.

Guluronic acid residues give a buckled conformation providing an

effective binding site for the cations .

The gel strength also depends on the nature of the divalent cation with

the order like Ba++> Sr++> Ca++> Mg++

PECTIN

The gelling characteristics of pectin strongly depend on the

degree of esterification.

Gelation of High methoxyl pectin occur in presence of sugars

Gelation of low mehoxyl pectin occurs only in the presence of

divalent cations like calcium.

AGAR

Agar requires het to bring into dispersion, on cooling hot dispersion

sets into gel.

STRACH

When starch grains are heated above a critical temperature

(gelatinization temperature) results in removal of excess of water.

gelatinization occurs, which implies that amylose leaches out from

the granules and later swelling results in solubilization of amylose

and upon cooling gel is formed.

GALLAN GUM

Gelation mechanism of gellan gum is based on the domain model. In

aqueous solutions at high temperatures, gellan polymers are in a

disordered single coiled state. Cooling of the gellan promotes the

formation of a threefold double helix, stabilized by internal hydrogen

bonding. gellan double helixes can be associated in the presence of

cations to form junction zones, which can aggregate and lead to the

formation of an interconnected three-dimensional gel network.

During this step, the sol is converted into a gel. The gelation

process is dependent on the type of cation, ionic strength,

temperature, and polymer concentration (Sanderson, 1990), and the

network structure of the gellan gels can also be modified

GAUR GUM

Gels are formed on cooling in the presence of salts. Molecules

undergo a coil helix transition followed by aggregation of helices.

Guar gum is readily soluble in coldwater. It has amannose to

galactose ratio of around 2:1, and exhibits weak synergism

XANTHAN GUM

Gels are obtained in the presence of electrolytes over a broad pH range

and at high temperatures. Gels are formed on cooling. Xanthan and

polymannan chains associate following the xanthan coil-helix

transition

CARRAGENNAN

Gelation of carragennan involves by association of polymer chains by

formation of intermolecular double helices to form domains

Gelation occurs by aggregation of domain mediated specific binding

gel promoting cations like K+, Rb+, Cs+, and NH4+

SOME HYDROCOLLOIDS USED AS A GELLING AGENTS

SOME HYDROCOLLOIDS USED AS A GELLING AGENTS

PROTIENS USED AS GELLING AGENTS

RHEOLOGICAL MEASURMENT OF GEL CHARECTERIASTICS

OTHER MEASURMENT OF GEL CHARECTERIASTICS

MULTICOMPONENT GEL

CONDITIONS FOR GEL FORMATION

(a) Temperature

occurs in 2 steps

unfolding of molecules due to high energy input and

Aggregation of unfold molecules to form complex structure.

(b) Pressure

Pressure causes water to dissociate and pH becomes more acidic

under pressure conditions and results in decrease in volume of the

system.

(C) Ionic Strength

Monovalent (Na) and divalent (Cl) cations increase the ionic strength

of gel and electro static repulsive forces between molecules are

reduced and gelation occur.

pH

Changes in pH due to addition of acids change the net charge of

molecule and alters the

attractive and repulsive forces between the molecules

attractive and repulsive forces between the molecules and solvent

Results in the gel formation.

TYPES OF GEL

• Hydro gel

• organo gel

• xero gel

•Aero gel

• Temperature sensitive gel

• Weak gel

• Fluid gel

ISOLATION OF CELLULOSE FROM VEGETABLES

Cellulose in rich in whole grains and in all vegetables. These were

spread onto isolation media Carboxyl Methyl Cellulose (CMC) agar

medium and incubated at 37 degree C for 24 hours after serial

dilution of 10-1 to 10-6. The isolates were screened for cellulase

activity. This was done by inoculating the organisms on the Carboxy

Methyl Cellulose (CMC) agar medium plates containing 1gm

Carboxy Methyl Cellulose, 0.02gmFeSO4, 0.3gm K2HPO4, 0.01 gm

MgSO4.7H2O, 0.04gmCaCl2 and 2.5 gm agar in 100 ml, the initial

pH of medium was adjusted to 7 and incubated at 370C for 24 h. The

plates were flooded with 0.1% Congo red for 15 to 20 min, washed

with 1 M NaCl for 15-20 min, and incubated for 15 min at 37°C. A

clear zone around the growth of the bacteria was indicated to

cellulase activity.

STARCH RETROGRADATION

Retrogradation is a reaction that takes place in gelatinized starch

when the amylose and amylopectin chains realign themselves,

causing the liquid to gel.

When native starch is heated and dissolves in water, the crystalline

structure of amylose and amylopectin molecules is lost and they

hydrate to form a viscous solution.

If the viscous solution is cooled or left at lower temperature for long

enough period, the linear molecules, amylose, and linear parts of

amylopectin molecules retrograde and rearrange themselves again to

a more crystalline structure.

Retrogradation can expel water from the polymer network. This is a

process known as syneresis.