Investigation into the polymerization and changes of physicochemical properties of sugar beet pectin through controlled dry-heating

Abstract

As a polyhydroxy carboxylic polymer, pectin is prone to physicochemical reactions under dehydration stress. This study investigated the molecular polymerization, structures and emulsifying properties of sugar beet pectin (SBP) after dry-heating at 45, 60, 75 and 110 °C for different lengths of time. When thermally treated, SBP molecules gradually cross-linked into insoluble and irregular-shaped flocs in a time- and temperature-dependent manner. The polymerized SBP was confirmed by high-performance size exclusion chromatography and dynamic light scattering techniques, and then morphologically characterized by a scanning electron microscope, which exposed a compact texture. Both soluble and insoluble SBP cross-linked polymers (named ISBP) were formed, depending on the dry-heating conditions. ISBP110 °C–2 h had a notably high protein content (16.2%), which evidenced the role of proteinaceous materials in constructing the cross-linked structures. Moreover, the FTIR and NMR spectroscopic bands relative to the amidation between amino and carboxyl groups suggested that SBP molecules might be covalently cross-linked with protein as the linker. The emulsifying properties of dry-heating treated SBP (75 °C for 12 h or 110 °C for 2 h) were significantly improved with the soluble cross-linked SBP with increased surface hydrophobicity. In that case, controlled dry-heating treatment could be employed as a simple method to improve the emulsifying activities of SBP. Altogether, this work demonstrates that the endogenous protein moiety plays an essential role in the polymerization of SBP under dry-heating, possibly via cross-linking SBP chains through the amidation with carboxyl groups of galacturonic acid residues.