Kinetics of water sorption in packaging materials for food

The water content of paper and its time dependence are of great importance for the use of paper and cardboard as packaging materials. The physical process of water sorption and the mechanisms of water "binding" in paper, as fundamental principles of the kinetics, are discussed. Water exists in up to four different "states" (i.e. domains of mobility) in paper: the first layer at the primary sites on the surface of the fibers; the multimolecular layers, with less "binding" energy, on these inner surfaces; condensed water in the voids; and dissolved water in the cellulose fibers. All these "states" influence the shape of the sorption isotherm and correspond to a different water content and to different transport mechanisms. The various types of water "binding" on paper were studied by measuring the transversal relaxation rate, T2, using a pulsed Nuclear Magnetic Resonance spectrometer (minispec pc 120). In addition, a non-destructive, fast method of determining the water content in packaging materials with NMR spectroscopy is described.