Journal
JOURNAL OF AGRICULTURAL AND FOOD CHEMISTRY
Volume 66, Issue 35, Pages 9344-9352Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acs.jafc.8b02413
Keywords
water molecules; hydrogen bonding; bound water; pectin; calcium ion; infrared spectroscopy; relative humidity; quartz crystal microbalance
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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 similar to 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+.
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