A moulded‐fibre packaging system was characterized under conditions simulating real‐life packaging of food. A steady‐state moisture flux through the moulded‐fibre packaging was generated by subjecting the system to different combinations of internal humidity [33–97% r.h. (0.33–0.97aw of contents), RH(i)] and surrounding humidity [33–97% r.h., RH(e)]. The objective was to resolve whether a hygroscopic fibre material absorbs moisture proportional to the rate of moisture transport, and the moulded‐fibre material was thus characterized with respect to accumulation of moisture in the fibre material, water vapour transmission rate (WVTR) and permeability (k/x). These steady‐state properties showed significant asymmetry depending on direction of moisture transport. When moisture was transported out of the system [RH(i) > RH(e)] the fibre material adsorbed moisture to a considerable lesser extent compared to when moisture was transported into the system [RH(i) < RH(e)], just as (k/x) increased by 15–20%. Taking both directions of moisture transport into account, the moisture content of the fibre material depended largely on surrounding humidity, even at high internal humidity. Moisture contents ranged from 5.5 g/100 g dry fibre at RH(e) 33% r.h. to 16.4–25.1 g/100 g dry fibre at RH(e) 97% r.h. The observed asymmetry was shown to derive from the experimental set‐up and not from the material itself. A minimal theory based on the various transport steps in the experimental set‐up was proposed in order to qualitatively explain this asymmetry. The rate of moisture adsorption in moulded‐fibre was described by the normalized response function H(t). Response times to reach equilibrium moisture contents were 6 and 8 h for RH(e) 33 and 53% r.h., and 40 and 41 h for RH(e) 75 and 97% r.h.
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Sørensen et al. (2004) studied this question.
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