Adsorption of Pectin onto Stainless Steel Surfaces: Role of Electrostatic Interactions.

Abstract

The adsorption of high methoxyl (HM) and low methoxyl (LM) pectins onto stainless steel surfaces was studied in an investigation of the fouling mechanism using nonporous stainless steel particles. The negative charge equivalents of the HM and LM pectins increased with increasing pH, and was accompanied by the charge-induced molecular expansion. HM and LM pectins could be spontaneously adsorbed on both positively and negatively charged stainless steel surfaces in a pH range of 3 to 6. The adsorption isotherms of the HM and LM pectins were similar to Langmuir-type ones. The saturation amounts of HM and LM adsorbed pectins increased with decreasing pH values and were correlated with the degree of dissociation of the carboxyl groups in the pectin molecules. While the undissociated HM and LM pectins adsorbed on stainless steel surfaces were easily released into the aqueous phase by alkali cleaning, anionic HM and LM pectins bound to positively charged stainless steel surfaces showed a high resistance to alkali cleaning. In a pH range where the HM and LM pectins and stainless steel surfaces were oppositely charged, shielding of the charges and intermolecular cross-linkage of the HM and LM pectins by monovalent or divalent cations also resulted in large amounts of pectins adsorbed and in the good release of the adsorbed pectin molecules during the alkali cleaning. It is concluded that the adsorption and desorption behavior of the pectins at the stainless steel-solution interfaces was largely determined by the electrical states of the pectin molecules and stainless steel surfaces.