Ethanol precipitation of sugar beet pectins as affected by electrostatic interactions between counter ions and pectin chains

Abstract

The ethanol precipitation of sugar beet pectins (SBPs) with and without counter ions was investigated. In aqueous solution, when cations bind to SBPs, the negative charges on the pectin chains are reduced and the hydrodynamic radius (Rh) of the whole polymer is decreased. Cation-bound SBP is more prone to precipitation by ethanol because it is less solvated by water molecules. In the absence of cations, partially-ionized SBP interacts extensively with water molecules at the initial precipitation pH (I-pH) range of 3.26–2.83, which makes it difficult to precipitate the pectins by addition of ethanol. However, in the presence of cations, precipitation of SBPs is straightforward due to cation binding and ethanol dehydration. Protonated SBPs and SBP salts were prepared and their conformational features were analyzed by 13C solid-state NMR and X-ray diffraction spectroscopies. The results show that no conformational transition occurs in cation-bound SBP after ethanol precipitation, which indicates that the electrostatic nature of the cation–SBP interaction is important during the precipitation process. A schematic illustration of the cation–SBP electrostatic interaction during the precipitation process is proposed.