Pectin-water interactions: Comparison of different analytical methods and influence of storage

Interactions of pectin with water are essential for nearly all pectin applications. It was assumed that these interactions strongly depend not only on chemical molecular parameters but also on physical powder material properties and that they might be affected by storage. This was examined with nine different pectin samples from three suppliers. Storage at 60 °C and 80% humidity for two weeks was used in order to simulate long term storage at moderate conditions. Material properties were tested by measuring solid density and BET surface and by mercury porosimetry and X-ray analysis. Pectin-water interactions were examined with two different methods, a modified sorption method at aw around 1.0 and the capillary sucking method (Baumann method). After storage, the BET surface of the pectin samples was reduced, solid density was altered differently and crystalline structures became amorphous. It is assumed that storage at high heat and humidity caused particle surfaces softening, swelling and partly dissolution. The particle surfaces smoothed, small inter-particle voids were reduced or sealed and particles agglomerated. These alterations caused a reduced water uptake of stored pectins. In general, the accessibility of hydrophilic groups in pectin was more important for pectin-water interactions than their number. All applied methods for testing pectin-water interactions detected special sample properties and their combination allowed an extensive evaluation of the water binding properties.