The rate of moisture sorption was measured by weighing packaging over time after a step-change in RH and/or temperature. Trials were undertaken for a range of packaging types and configurations including single sheets of cardboard, single cartons of product with waxed or unwaxed cardboard, and complete pallets of product cartons both with and without shrink-wrap. The trials were conducted at both ambient and refrigerated temperatures and under both adsorption or desorption conditions. Rates of sorption were found to be accurately represented by first order kinetic approach to equilibrium conditions defined by the moisture sorption isotherms for the wood and cardboard separately. Packaging thermal equilibration was rapid compared with the moisture sorption rates so heat transfer effects can be ignored. The moisture sorption was found to give significant packaging temperature rise (adsorption) or decline (desorption) due to the heat of sorption. There was a net conversion of latent heat to sensible for adsorption and vice versa for desorption. The sorption rate constant reduced as the product item got larger (pallet < cartons < sheets) and when moisture resistant coatings (e.g. wax) and plastic films (e.g. shrink-wrap) were present. On average, desorption was slightly but not significantly faster than adsorption. The rate constant dependence on temperature was approximated by the Arrhenius relationship with activation energies similar to that for other biological systems.

Units: Dual