General Remarks on Drying

Drying is one of the oldest methods of preservation employed by mankind. The various methods of drying have a drastic impact on the original character of the natural products. This has been acceptable as it always was the main aim to preserve the most important nutritive substances. 

The methods which are basically available for dehydration will be explained by employing the phase diagram of water (Fig. 2.60). The starting point shall be within the liquid phase. 

Method A Pressure reduction at constant temperature (vacuum drying) 

If vacuum is applied, the phase boundary between liquid and vapour is reached at a certain pressure and water starts to vaporize. To prevent a loss of temperature, heat -the so-called heat of evaporation - has to be introduced from the outside. If all water has been evaporated, a rise in temperature will ensue that can be taken as an indicator for the end of the drying process. Although this method is gentle, the material to be dried is puffed up and structural and cell deformations follow. 

Method B Temperature increase at constant pressure (thermal drying) 

---