Gelling systems

Extrusion-cooking of cell-wall rich products (e.g. wheat bran, apple pomace, citrus peels, sugar-beet pulp, pea hulls.) led to an important solubilisation of polysaccharides of various types without extensive degradation of the polymeric structure. The possibility of obtaining gelled systems directly with the extruded pectin-rich materials was demonstrated. 

The phenomenon of gelation is best understood by examining some typical gelling systems. Inasmuch as each gel presents peculiarities of its own, it is impossible to discuss every type available. The systems chosen for discussion show properties that illustrate important gel characteristics. 

This approach does not preclude gels having special, additional properties of their own due to peculiarities of their molecules. Such an example was in the thixotrophy of borax gels caused by the dynamic equilibrium of the bonds. Even gels that are formed by two apparently different mechanisms have the same fundamental gelling characteristics. For a timely and exhaustive review on gelling systems the reader should refer to the recent work of Whistler and BeMiller (if). 

Kucera, C.H., Smith, C.F. and Braunlich, F.H. "New Oil Gelling Systems Prevent Damage in Water Sensitive Sands," SPE paper 3503, 1971 SPE Annual Meeting, New Orleans, October 3 6. 

Jeong and coworkers have reported peptide-based thermo-gelling systems using PEG-b-polyAla as an injectable cellular scaffold [315]. The polymer aqueous solution undergoes sol-gel transition as temperature increases. The fraction of the p-sheet structure of the poly Ala dictated the population and thickness of fibrous nanostructure in the hydrogel, which affected the proliferation and protein... 

Pons, M., and Fiszman, S. M. (1996). Instrumental texture profile analysis with particular reference to gelled systems. /. Texture Stud. 27,597-624. 

The points in Figure 4 do not show a tendency for p - e tend to zero as p tends to zero. That is, even in the limit of a perfect gelling system, inelastic loops are formed post-gel. Extrapolation to Pr,c = 0 gives Pr,e> extent of reaction leading to inelastic loops at complete reaction in the perfect gelling system. The values of p grange from about 9% to 18% for the system studied. As expected from considerations of pre-gel intramolecular reaction, the values of p g are smaller for f=3 compared with f=4 and they increase as v decreases, there being less opportunity for intramolecular reaction at lower functionalities(14). 

Table I. Values of parameters characterising pre-gel intramolecular reaction (v,b,(f-2)/(vb ) ) (5-7,12) and the extents of post-gel intramolecular reaction which, in the limit of ideal gelling systems, lead to inelastic loop formation at complete reaction (p g). The values of pj g define the indicated values of Mg/M° and the reductions in shear moduli of the dry networks relative to those of the perfect networks (G/G° = Mc/Mc). The values of Pr g in the limit of reactants of infinite molar mass (v = < ) are denoted p°>°° in the text...

Figure 7. Dependence of post-gel intramolecular reaction leading to inelastic loops in the limit of ideal gelling systems (p° g) on the parameters affecting pre-gel intramolecular reaction ((f-2)/(vb ) ).

The case of RAf polymerisations has been evaluated and the experimentally deduced values of Pg g interpreted in terms of one-membered loop formation within structures of defined size. The knowledge of such structures is important for theories of non-linear polymerisations and network formation. Theories have to simplify in some way the infinite numbers of structures which actually occur and it is important that those significant for intramolecular reaction are retained. In this context, it should be noted that present theories assume that ideal gelling systems lead to perfect networks. The approximation needs to be removed as it is inconsistent with the structure of the growing gel, a molecule with unlimited pairs of groups for intramolecular reaction. 

In conclusion, it is now possible to reproduce the kinetic behavior of this sreroid/cyclohexane gelling system, including the shape ( defined by model (3) and its kinetic rate constants k, k, k ) and also the absolute values of C p in zone A of the phase diagram. In Figure 13, we demonstrate the reversibility of the gelation process and the additivitv of p values for different... 

Besides, comparisons with other non-macromolecular gelling systems are in progress. Specially, we can co.mpare with a square planar copper complex, which aggregates in linear chains to gelify the cyclohexane (l ). It is immediatly noticed that characteristic times of the aggregation kinetics are correlated to the complexity of the molecular aggregation mechanism involved. 

Kilb, R. W. Dilute gelling systems. I. The effect of ring formation on gelation. J. Phys. Chem. 62, 969 (1958). 

Davis. P A. and I. Gordon "Microslrueiural Analyses of Gelling Systems." food Technrdogy. 38(5t. 99-109 11984). hllpj/wsvw.di.org/... 

Stainsby, G. (1980). Proteinaceous gelling systems and their complexes with polysaccharides. Food Chem. 6 3-14. 

The introduction in the early 1980s of the concept of in situ gel systems demonstrated that a considerable prolongation in duration of action could be obtained. In situ gelling systems have unique properties, which can make a liquid change phase to a gel or sohd phase in the culde-sac upon its instillation into the eye. Three methods have been employed to induce phase transition on the eye surface change in pH and temperature as well as activation by ions. 

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