Volatiles retention during rehumidification of freeze dried food models

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Abstract
Summary: Observations of structural collapse during rehumidification of freeze dried substances are used to interpret previously reported data for volatiles loss v. time. The results are in accord with a diffusion‐based analysis. At low moisture contents the diffusion coefficient is so low as to give only very small losses, while at high moisture contents it is high enough to give complete loss in a relatively short time. The behaviour at intermediate moisture contents depends upon whether or not there is simultaneous structure collapse. If there is no collapse, volatiles‐loss data appear to agree well with a slab‐diffusion analysis based upon constant diffusivity and unchanging slab thickness – the average thickness of webs within the porous substance. When there is simultaneous collapse, it is necessary to take into account the continuous increase in web thickness as diffusion occurs. A model is derived based upon a linear increase in web thickness with increasing time, and is shown to agree with experimental data for volatiles loss at intermediate moisture contents in several different systems which show structural collapse.
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