Physical properties of water-soluble pectins in hot- and cold-break tomato pastes

Physical properties of water-soluble pectins (HP and CP) obtained from hot- and cold-break tomato pastes, respectively, and purified by anion-exchange chromatography were compared on the basis of their viscosities. The differences in viscosities between HP and CP solutions depended, not only on the average molecular mass, but also on the chemical structure. Neutral sugar-rich pectin, CP, was more flexible than galacturonic acid-rich pectin, HP. The three-dimensional structure of homogalacturonan was a hollow helix with ionic strength lower than 5 mM NaCl, which was built through hydrogen bonds between intramolecular free carboxyl groups. Such conformation was destroyed by ionic force (>5 mM NaCl). The effects of kinking structures on the physical properties of the rhamnogalacturonan were remarkable, as a consequence of (1 → 2) linkages of the rhamnosyl residues in the pectic backbone, and induced the molecular conformation of the pectin to sphere-like shape, which was unfavourable for Ca2+-gelling.