|Ph.D Student||Ventura Irit|
|Subject||Nanostructure of Low Methoxyl Pectin Solutions and Gels|
|Department||Department of Chemical Engineering||Supervisor||Professor Havazelet Bianco-Peled|
|Full Thesis text|
The first part of this thesis focused on calcium-low-methoxyl-pectin gels. The nanostructure of calcium-low-methoxyl-pectin gels is still not fully understood despite its wide use in food and biomedical devices. In this thesis we present a comprehensive small angle X-ray scattering (SAXS) analysis of calcium-pectin gels with various calcium concentrations. Several modeling approaches were examined taking into account the contribution from both junction zones and chains between cross-links. The model of semiflexible chains with excluded volume effects was found to be most suitable for describing calcium-pectin gels. The SAXS analysis suggests that both rodlike junction zones and point-like cross-links between neighboring chains are formed in calcium-pectin gels. Moreover, as the calcium content increases, the number of the rod-like junction zones decreases while the number of the point-like cross-links increases. The SAXS findings indicate a cross-linking mechanism that is quite different from the common description of Ca2-LM pectin gels. Apparently, the cross-linking scheme is mainly governed by the branched nature of pectins, as opposed to the linear nature of alginate. The SAXS studies was supplemented by determination of mechanical properties and swelling behaviors of the gels, which were analyzed using known swelling theories originally developed for chemically cross-linking polymers. The second part of this thesis explores gelation of low-methoxyl pectin in the presence of chitosan, which is a family of linear cationic copolymer. Hot solutions containing certain compositions of pectin and chitosan in acidic pH demonstrate various phenomena of gelation occurring upon cooling to room temperature. To date, the origin of the gelation is not well established. We explore the origin of this gelation using SAXS as the main experimental tool. Modelling SAXS patterns revealed rigid rod formation in chitosan solutions, whereas pectin formed a self-avoiding chain. In addition, nanometric aggregates were detected in pectin solutions. Pectin and chitosan in mixed solutions adopted a similar semiflexible conformation. The only sample that created a visually stable gel displayed a different pattern, characteristic to polymers gels. Manipulating the composition by adding urea and salts highlighted the important role of hydrogen bonding, which governs the gelation mechanism. Nevertheless, electrostatic interactions were also found to take part in the gelation.