The Schroeder paradox or how the state of water affects the moisture transfer through edible films

The objective of this work was to better understand the water permeation through edible films when water is either in its vapour or in its liquid state related to their physico-chemical properties. Film based on iota-carrageenans or sodium alginate, containing or not fat, and encapsulating or not an aroma compound (n-hexanal) have been prepared. Films were conditioned at three different relative humidities, 0%, 43% and 84%. The incorporation of the n-hexanal in both types of biopolymer modifies the mechanical properties of film matrix and has a synergistic effect when associated to fat. Increasing the moisture content induces film plasticization by decreasing the glass transition of the glycerol enriched phase. Only carrageenan displays similar value for the liquid water transfer rate whereas a discrepancy of the behaviour in contact of liquid or vapour was observed for the alginate-based film, confirming the Schroeder paradox due to swelling. (C) 2011 Elsevier Ltd. All rights reserved.