What is Freeze Drying? How does it work?
Freeze drying, also known as lyophilization or sublimation, they are synonymous. Freeze drying is a low temperature dehydrate process that typically used to preserve perishable materials, to extend shelf life, or make the material more convenient for storage or transport. Freeze drying works by freezing the material, then reducing the surround to allow the frozen water(ice) to sublimate.
Freeze Drying Steps
There are 3 steps involve in freeze drying process, with an experienced operator, you can save energy and cost each batch.
Is the most important step before drying, thoroughly freezing the raw material to solid phase is the insurance of a success freeze drying. Freeze drying can be done in Freeze Dryer (if shelves with freezing function) or in deep freezers. The key of freezing is to find out raw material’s eutectic point and make sure freezer temperature is lower than that. Freezing stage is the insurance of dried products’ physical shape.
Some material, especially fluid, the freezing stage may also involve annealing treatment (rapid freezing cause large ice crystals, proper raise its temperature allow ice crystal grow), which could help protect cells from broken as well as short the drying time.
Primary drying mainly remove water by sublimation, in this stage, water in solid phase sublimate to vapor and move to ice condenser, freezing on condenser coils surface, in this stage >95% water have removed.
In this stage, gradually and proper increase the shelves temperature could speed up the sublimation, but too much energy supply may cause glass transition.
Secondary drying process is to remove the adsorption water. Higher temperature and vacuum needed in this step, after secondary drying, the shelves could reduce temperature and prepare harvest.
①: Drying house ②: Ice condenser ③: Drying house door
④: Compressor ⑤: Vacuum pump ⑥: Plate heat exchanger
⑦: Centrifuge pump ⑧: Controller cabinet ⑨: Expansion tank
Key Points Need Pay Attention To During Freeze Drying
Thoroughly freezing raw materials.
- Mostly, freezing temperature should 5 ~ 10C lower than product’s eutectic temperature, halfway freezing (typically show external freezing, internal still in liquid phase) may cause dried products out of shape or nutrition lose.
- Eutectic point (freezing point), which mentioned above, is the lowest temperature that could completely freezing material to solid. Normally, when freezer reach products’ eutectic point needs last 1 to 2hrs.
Proper give energy in sublimation step
Heating the product too high in temperature can cause many troubles:
- Product melt. For example, carbohydrate will block the vapor channel, cause drying fail.
- Too much vapor creation. Condenser cannot collect in time, and the temperature will have a drastic raise.
- Vacuum pump broken. Vapors that cannot collect by condenser in time may go to vacuum pump, oil pump will broken.