The document discusses food gels, which are semi-solid structures with significant moisture content, critical in food processing. It categorizes gels based on their cross-linking mechanisms and provides details on different types of gels such as alginates, agar, carrageenans, gelatin, whey protein, starch, and pectin, explaining their gelation processes and properties. Additionally, it covers the structure-property relationships of gels, including transport and mechanical properties, and highlights techniques for studying gel microstructure and microrheology.

1. DEPARTMENT OF FOOD ENGINEERING
UNIVERSITY OF AGRICULTURE, FAISALABAD

2. Food Gels
o A gel is a form of matter intermediate between a solid and a liquid
o Gels are soft solids present in high moisture foods of almost all origins (jams,
jellies, confectionery, dairy products etc.)
o Gels have ability to structure water in semi solid structures, having immense
technological importance in food processing
o Gelled agar may contain as much as 998 parts of water and only two parts of
polymer and yet stand against gravity
o Continuous network of interconnected material becomes swollen with a high
proportion of liquid
o Uptake of water is affected by interaction energy and polymer entropy
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3. Food Gels
o The reverse phenomena “syneresis” is the expulsion of liquid from gel
generally regarded as defect in food gels
o Gels in which liquid phase is an aqueous solution are called hydrogels; those
in which liquid has been removed are called aerogels
o Researchers around the world have developed some gels known as intelligent
gels which absorb or expel water in response to temperature, pH, electric
fields etc.
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4. Classification of gels
o Gels are classified according to cross-linking mechanism
o Cross-links may be strong covalent bonds or weak bonds including
electrostatic, ion bridging, hydrophobic, hydrogen bonds and van der Waal’s
forces
o Fishing nets or branched three dimensional networks: built from linear flexible
chains linked by covalent bonds; exhibit rubbery consistency
o Thermo-reversible physical gels: formed by partial crystallization of chains or
conformational coil to helix transitions
o Range from soft and highly deformable to hard and brittle
o Egg box structures: formed by junction zones linked by ionic complexation in
which a divalent cation bridges two strands of polymer 4

5. Classification of gels
o Particle or colloidal gels: consisting of strands of more or less spherical
aggregates ordered into a string of beads or cluster arrangement
o Other classifications can be based on
o Composition of network or phases present; single or mixed
o Role of heat in gel formation; thermotropic or thermoreversible
o Light transmission; transluscent or opaque
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6. Alginates
o Form heat stable irreversible gels in cold systems
o In foods, it is Ca+ mediated mechanism of gelation
o Regions of polyguluronic acid are linked to similar regions in other polymer
giving rise to egg box structure
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7. Agar
o In hot solutions agarose molecules tend to behave as stiffened coils
o Process involves first the formation of bundles of double or single helices in
an order disorder phenomena involving polymer associations
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8. Carrageenans
o Structurally they are closely related to agar
o Linkage pattern introduces a twist in the molecule giving rise to helical
structures
o Ability to gel is based on the association of helical chains into double helices
o Because of anionic nature, require counter ions to gel which may be Ca or K
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9. Gelatin
o Obtained from collagen by controlled acid or alkaline hydrolysis
o Gelling properties are derived from triple helical molecular structure
o Swells in contact with cold or warm water and at temperatures above its
melting point gets dissolved
o On cooling molecules reform into triple helices giving rise to a transparent
and elastic gel
o Groups of triple helices align themselves in a parallel array forming micro-
crystallites
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10. Whey protein
o Gelation is induced by heating and process is reversible
o Mostly are particle gels in which units of forming the network are protein
aggregates (0.5-2 um)
o Gelation appears to involve a series of transitions from the native proteins,
aggregation of unfolded molecules, strand formation from aggregates and
association of aggregates into a network
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11. Starch
o On heating in excess water, swell and imbibe enough liquid to reach several
times their dry weight; gelatinization process
o Crystalline portion of granule melts, amylose leaches out of granule
o On cooling free amylose becomes ordered into microcrystalline regions
surrounding swollen granules; thus forming a gel
o Actually, it is a composite gel of amylose matrices filled with swollen granules
o Gelling mechanism and microstructure become more complicated if both
amylose and amylopectin are present in aqueous phase
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12. Pectin
o Generally classified as low (LMP) and high methoxyl (HMP) pectin each having
its own gelation mechanism
o In HMP gels junction zones are aggregates of chains of various sizes
promoted by hydrogen bonding and hydrophobic interactions
o Gelation is favored by presence of sugar and low pH conditions
o LMP have structure similar to alginates; gel by calcium bridging mechanism
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13. Mixed gels
o Several food gels include more than one type of gel-forming materials in their
formulation
o A blend of gelling polymers sometimes provides superior properties than a
single component
o Most of the high moisture structured foods can be regarded as combination
of gel matrices with embedded microstructural elements
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14. Mixed gels
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15. Structure property relationships-transport properties
o Transport properties of gels are similar to liquids
o Diffusion of small solutes occurs largely in aqueous phase and is affected by
the steric and electrostatic hindrances of strands
o Diffusivity is lower for large solutes and molecules carrying charges opposite
to those of polymeric chains and when small pores are present
o Sieve effect provided by the network is useful in molecular separations
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16. Structure property relationships-transport properties
o Thermal conductivity of gels having low total solids is similar to that of water
o Thermal conductivity of filled gels decreases with volume fraction of dispersed
phase
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17. Structure property relationships-viscoelastic properties
o Since gels are viscoelastic, they are best characterized in terms of mechanical
response to oscillatory shear under small deformations
o Storage modulus G’ is the measure of solid like behavior and is related to
polymeric connectivity of network
o G’ varies as a power function (close to 2 for strong concentrated gels) of
concentration of gelling agent
o The same square dependence is observed for the elastic modulus E (E = kc2)
for gelatin gels
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18. Structure property relationships-mechanical properties
o Mechanically assayed through small and large deformations
o Former mode aims at simulating small forces acting during handling of
products
o The later represent the action of mastication or cutting
o Some gels fail in brittle fracture under small deformations but most of them
show a yield stress; dissipating energy
o In filled gels, gel strength increases above a critical phase volume of the filler;
especially when there is good interaction between filler and matrix
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19. Gel microstructure
o Gels are difficult to study with microscopy because of risks of artifacts during
sample preparation
o The high proportion of water in gels makes dehydration without collapse
difficult
o Molecular nature of some networks requires high resolution
o In-spite of such problems, most food gels are usually examined by both light
and electron microscopy
o Solutions of globular proteins form transparent or opaque heat set gels
depending on ionic strength and pH
o low ionic strength and isoelectric point render gel transparent; due to thinner
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20. Gel microstructure
o Opaque gels are of particulate nature
o The size of pores in fine stranded gels may be on the order of nm
o In particulate gels, size is in the range of 10-100 um
o Microscopy is a powerful tool to study the phase separated gels
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21. Gel microrheology
o A number of techniques are available to characterize the microrheology
o particle tracking rheology,
o diffusing wave spectroscopy,
o laser particle tracking (optical tweezers),
o magnetic tweezers, atomic force microscopy,
o piezorheometer,
o quasi-elastic light scattering
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22. Gel microrheology
o Particle tracking microrheology (also termed as video particle tracking
microrheology) is an emerging passive micro-rheological technique
o Particles (tracers) are incorporated into the gel structure through different
means
o No external driving force is applied; the Brownian motion of embedded
particles (called tracers) is used to probe local dynamics of soft material
o The diffusive motions of these embedded particles can be recorded
simultaneously using either fluorescence or bright field microscopy,
o The information of each of the individual particle trajectories is kept in record
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23. Gel micro-rheology
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24. Gel micro-rheology
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