Dielectric spectroscopy studies have been performed on fresh and osmotically dehydrated Granny Smith apples. The osmotic treatment consisted on immerse the samples into 65% (w/w) sucrose aqueous solution at 30 degrees C during different exposure times lasted from 3 h to 16 days. Some physical chemical parameters were measured in fresh, treated and reposed (24 h at 30 degrees C) samples. Dielectric spectra were measured in the frequency range from 500 MHz to 20 GHz by an Agilent 85070E Open-ended Coaxial Probe connected to an Agilent E8362B Vector Network Analyzer in the fresh, treated and reposed samples. It has been demonstrated that the dielectric properties are a good tool to control the surface water activity by using the loss factor at relaxation frequency. This relation of dielectric properties with the water activity allows controlling the shrinkage level and determines the moment when the driving forces change from diffusional to mechanical behaviors. This value corresponds to 12.25 +/- 0.16 and represents a practical control value to decide if the final dehydrated samples shrink or swell. Moreover, the quantity of adsorbed water was estimated by the subtraction between the overall moisture and the water content in liquid phase estimated by using dielectric loss factor at relaxation frequency. (C) 2010 Elsevier Ltd. All rights reserved.