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Isolation and gel formation of polysacchrides and strach
1. ISOLATION AND GEL FORMATION
OF POLYSACCHRIDES AND
STRACH RETROGRADATION
M. S. ESHWAR
School of Food Technology
JNTU K
2. INTRODUCTION
Gel 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.
3. Different Constituents of gel formation
• Gelatin
• whey protein
• Egg protein
• Alginates
• Pectin
• Agar
• starch
• carragennan
4. 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
5. 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
6. EGG PROTEIN
Both 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.
7. Alginates
Gels 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++
8. 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.
9.
10. 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.
11.
12. 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
13. 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
14. 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
15. 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+
22. 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.
23. 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.
24. TYPES OF GEL
• Hydro gel
• organo gel
• xero gel
• Aero gel
• Temperature sensitive gel
• Weak gel
• Fluid gel
25. 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.
26. 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.
