Drying Time

Equation 1 can be rearranged and integrated, as shown below, to provide ts, the time required to complete sublimation:

where L is the final depth of the ice-free layer, that is, half the slab thickness for slabs where sublimation occurs at equal rates on both sides. T0 and, to a lesser extent, Ts vary as Z changes during drying. Therefore, equation 3 usually must be numerically integrated. The integrand in equation 3 is proportional to ZdZ. Therefore, is will be proportional to L2 regardless of the particular way Ts and T0 change during drying. It appears that required desorption times and required total drying times are also proportional to L2.

It usually takes 14 to 18 hours to freeze dry a 12.5-mm (0.5-in.) thick slab of food that initially contains 75 to 85% water. In freeze-drying a 12.5-mm thick slab of coffee extract containing 74% water, 60% of the drying time was used for sublimation, 40% for desorption. Concentrated extracts with lower water contents usually dry more rapidly, but as concentration increases, a larger proportion of the drying time is used for desorption. Less information is available about the portion of drying time used for desorption for other products.

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