Water Retention of Calcium-Containing Pectin Studied by Quartz Crystal Microbalance and Infrared Spectroscopy with a Humidity Control System

To examine differences of water-retention mechanisms between pectins with and without Ca²⁺, quartz crystal microbalance (QCM) and infrared microspectroscopy combined with a humidity-control system were used to analyze differences in amounts and species of adsorbed water to pectins without and with Ca²⁺. QCM analysis shows that water contents are ∼2-3 times larger for the pectin film with Ca²⁺ than that without Ca²⁺. The difference IR spectra suggest that long, medium, and short H-bond water molecules (free, medium, and bound water) are adsorbed to the pectin film without Ca²⁺. IR peak shifts of C=O of COOH and C-OH suggest that these water molecules are hydrogen-bonded to C=O and C-OH groups. In addition to these water molecules, bulk water is adsorbed. IR OH band areas fitted by four Gaussian components show that bulk water is mainly adsorbed to the pectin film with Ca²⁺, possibly among skeletal chains of pectin bridged by Ca²⁺.