Gelation ability

John, G., Jung, J.-H., Masuda, M. and Shimizu, T. (2001) Self-assembly of a sugar-based gelator in water its remarkable diversity in gelation ability and aggregate structure. Langmuir, 17 (23), 7229-7232. 

We have seen earlier in this chapter how the self-assembly of casein systems is sensitively affected by temperature. Another thermodynamic variable that can affect protein-protein interactions in aqueous media is the hydrostatic pressure. Static high-pressure treatment causes the disintegration of casein micelles due to the dismption of internal hydro-phobic interactions and the dissociation of colloidal calcium phosphate. This phenomenon has been used to modify the gelation ability of casein without acidification as a consequence of exposure of hydrophobic parts of the casein molecules into the aqueous medium from the interior of the native casein micelles (Dickinson, 2006). High-pressure treatment leads to a reduction in the casein concentration required for gelation under neutral conditions, especially in the presence of cosolutes such as sucrose (Abbasi and Dickinson, 2001, 2002, 2004 Keenan et al., 2001). 

The existence of thermodynamically favourable interactions between two biopolymers influences their gel-forming ability in aqueous media. As an example, let us refer here to the effect of low-methoxyl amidated (LMA) pectin (DE = 30 %) on the gelation ability of sodium caseinate (Matia-Merino, 2003 Matia-Merino et al., 2004 Dickinson, 2008a,b). 

For qualitative determination of gelation ability, 3 ml aliquots of the WPC dispersions were dispensed In screw capped test tubes and heated In an oil bath at lOO C. Tubes were removed from the bath at 30-s Intervals and placed Into an Ice bath. Gel strength was evaluated on a visual rating scale of 0 to 5,0 (13, 1 ), The time required at lOO C for the formation of a gel with a rating of 4.0 or higher was reported as gel time. 

Molecules of widely different structures may produce such aggregation behavior. Hence compounds as diverse as anthracene alkyl ethers [69a], cyclic depsipeptides [69b], gluconamides [69c], steroid analogues [69d], and polar compounds containing long apolar moieties [69e], all show gelation abilities in different solvents. 

The heat treatment of the Alcalase-treated hydrolysates does not cause any changes in the viscosity, in contrast to the Neu-trase-treated samples which show an irreversible increase in viscosity, in particular at DH = 1.5%. No gel formation was observed in these experiments, whereas Pour-el and Swenson (26) were able to introduce gel formation capacity in soy protein hydrolysates. This indicates that the removal of the iso-electric soluble phase as carried out in their work (26), is necessary for obtaining gelation ability. Pour-el also proposed that the presence of small peptides would hinder the three-dimensional cross-linking necessary for gel formation (27). 

Fig. 28. Two families of sugar-based gelators. Substitution and conformation have a remarkable effect on the gelation abilities of these compounds...

Luboradzki, R. et al.. An attempt to predict the gelation ability of hydrogen-bond-based gelators utilizing a glycoside library. Tetrahedron, 2000. 56(49) 9595-9599. 

However, for most other gelators. the supramolecular arrangement in the gel fibers is still not known in detail and in many cases, the explanation for the presence or absence of gelation ability remains speculative. It should be noted that the above-mentioned examples are only a selection of the many known gelling agents, and for an extensive overview, the reader is referred to the review by Terech and Weiss. 

The thermally-reversible gel returns to the solution because the droving forces are noncovalent bonds such as hydrogen bonding, hydrophobic interaction, n-n interaction and electrostatic interaction. These noncovalent bonds are broken down by heating, so the gel reverts to the sol. Since the thermally-irreversible gel forms a network structure by firm chemical covalent bonds, the gel formed never changes to the sol. This is why the thermally-irreversible gel is called physical gel and the thermally-irreversible gel is referred to as chemical gel. Moreover, the gelation ability is found in not only biopolymers but also low molecular weight compounds. 

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