Freezing Technology

3.6.4. Freeze drying process

Freeze drying cycle consists of 3 stages.

1. Freezing.
2. Primary drying to remove water ice.
3. Secondary drying to remove bound water molecules from the primary dried materials.

1) Freezing : Freezing the product temperature is reduced to -50^OC at one atmospheric pressure to ensure that it is below the eutectic point.

2) Primary drying : This is a dynamic process involving transits of ice phase to vapour phase directly. This leaves the solid matrix formed in the freezing phase but without the ice crystals. During drying, an interface is set up which comes from the exposed surface to vacuum system to the surface where heat source is in contact or moves deeper in radiant heat. At this interface, actual drying takes place. There are number of factors which will indicate the completions of primary drying process.

1) Pressure stability : During drying, pressure changes slowly until it stabilizes at an ultimate low level. The process is considered as complete where the constant pressure is maintained for 1 to 2 hours.

2) Product temperature rise. When heat energy is not required for sublimation, the overall product temperature will rise until equal or close to the chamber temperature. This can be detected with a temperature probe and displayed.

3) Pressure rise test. If an isolation valve is available, it will be possible to isolate the drying chamber. If the material is not dry, then the pressure will rise quickly (due to vapour pressure of the product).

Secondary Drying: This is part of the process where remaining small proportion of “locked in” water is removed ( Usually 2.5% by volume). This is not always considered necessary and depends on the product.The locked in or bound water is removed form the products by supplying additional heat energy to the system. This is achieved either by increasing the temperature or by improving the thermal contact between the product and the existing of heat source.

Effect of freeze drying on food : Freeze dried foods have high retention of sensory and nutritional qualities and shelf life of more than 12 months when correctly packed. Aroma retention is 80-100%. Shrinkage is very little and texture is well maintained. Allows rapid and full rehydration but the product is fragile and require protection from mechanical pressure. However, the open porous structure of the food may allow oxygen to enter and cause oxidative deterioration of lipids and subsequent browning. Changes in thiamine and ascorbic acid content during freeze drying are moderate and there are negligible losses of other vitamins. Therefore vacuum or inert gas packaging is required.