Freeze drying (also known as lyophilization) is considered to be the best drying method to remove water from heat-sensitive materials such as fruits and vegetables and to obtain dried products (Genin and René, 1995).
Freeze drying process accomplishes water removal by sublimation at very low temperatures and pressures in a refrigerated vacuum system, including Primary Drying (Sublimation) and Secondary Drying (Adsorption).
The main advantages of freeze drying are the ability to retain original structure and color, negligible loss of nutrients, and excellent quick rehydration capability due to the development of porous structure in the product.
With proper packaging and storage, freeze-dried products can be stored for a very long time with minimal changes to most of its pre-drying characteristics, like size, smell, flavor, and nutritional content even at room temperature.
Hence, freeze drying is preferred in drying fruits, vegetables and their extracts to produce high value products.
Pilot freeze dryer and Industrial Fruit Food Vacuum Freeze Dryer are now successful in application.
[FOOD PROCESSING TECHNOLOGY Principles and Practice P. Fellows]
|Freeze drying||Conventional drying|
|01||Temperatures below freezing point||Temperature range 37–93ºC|
|02||Reduced pressures (27–133 Pa)||Atmospheric pressures|
|03||Sublimation of water from ice front||Evaporation of water from surface of food|
|04||Minimal structural changes or shrinkage||Stresses in solid foods cause structural damage and shrinkage|
|05||Rapid complete rehydration||Slow, incomplete rehydration|
|06||Porous dried particles having a lower density than original food||Solid or porous dried particles often having a higher density than the original food|
|07||Odour and flavour usually normal||Odour and flavour frequently abnormal|
|08||Colour usually normal||Colour frequently darker|
|09||Nutrients largely retained||Reduced nutritional value|