The basic principle of freeze drying is the removal of water by sublimation.
Since the mass production of instant coffee began in post-WWII America, freeze-drying has grown in popularity to become a common method. Although it is more expensive, it generally results in a higher-quality product.
1.Agglomerated wet coffee granules are rapidly frozen (slow freezing leads to large ice crystals and a porous product and can also affect the colour of the coffee granules).
2.Frozen coffee is placed in the drying chamber, often on metal trays.
3.A vacuum is created within the chamber. The strength of the vacuum is critical in the speed of the drying and therefore the quality of the product. Care must be taken to produce a vacuum of suitable strength.
4.The drying chamber is warmed, most commonly by radiation but conduction is used in some plants and convection has been proposed in some small pilot plants. A possible problem with convection is uneven drying rates within the chamber, which would give an inferior product.
5.Condensation — the previously frozen water in the coffee granules expands to ten times its previous volume. The removal of this water vapor from the chamber is vitally important, making the condenser the most critical and expensive component in a freeze-drying plant.
6.The freeze-dried granules are removed from the chamber and packaged