Role of the waxy skin layer in moisture loss during dehydration of prunes

Permeability studies of water through the skin and flesh of d'Agen plums have been carried out by radiotracer and PGSE NMR techniques as a function of moisture content. The results have shown that the diffusion coefficient of water through the skin layer increases as the fruit is dried at 70 degrees C or above. By contrast, the water diffusion through the fruit becomes more hindered as moisture is lost and structural collapse of the cell layers takes place. Values for diffusion coefficients of water at 21 degrees C through the fruit of 1 x 10(-)(9) m(2) s(-)(1) for fresh plums and 2.0 x 10(-)(10) m(2) s(-)(1) for fruit dried to 50% moisture content (wet basis) were found. Structural studies using scanning electron microscopy techniques were also performed. These showed that drying induces marked changes to the waxy skin layer of the fruit as well as the fruit flesh. These results are discussed in terms of the important role that the plum skin has in modulating the moisture loss process during dehydration, particularly at early stages of drying.