What is Freeze Drying?
Posted on 18th June 2018 at 14:30
We at Frozen in Time often hear the question, "What is freeze drying exactly?"
In short, freeze drying is a process that removes water from a frozen material via sublimation - the direct conversion of ice to vapour.
The aim of freeze drying is to obtain a readily water-soluble product which has the same characteristics as the original product after the readdition of water.
Freeze drying is often considered the "Gold Standard" of dehydration and drying processes in terms of overall quality.
What steps are involved in freeze drying?
All required preparatory steps to be done to the product should be done before freezing. Steps such as placing the product in specific shapes for freezing, adjusting concentration or addition of cryoprotectants, etc.
The product is cooled until frozen throughout. This can be done either in a basic chest freezer, a commerical blast freezer, specialised laboratory equipment for freezing (eg, glycol bath, LN2, etc) or in some types of freeze dryer (our HS/HSL and F series freeze dryers are capable of freezing product in situ).
Freezing a product slowly will create larger interlinked ice crystals which facilitate faster vapour removal during sublimation. This in turn allows for a faster freeze drying cycle. However, large ice crystals are not always desirable, particularly in the case of food or organic cell structures where large ice crystals can rupture the cell walls and damage the end product. In such cases a method of rapidly freezing the product to create smaller ice crystals is advised.
2) Primary Drying
In this phase the condenser drops to its operating temperature and the chamber pressure is lowered to a few millibar with a vacuum pump.
Heat energy is supplied to the product to facilitate the sublimation process (the necessary heat input can be calculated with the latent heat of sublimation of ice or other sublimating material). The water content at the end of the primary drying phase will typically be around 5-8%.
Heat energy for sublimation can be input in several ways, for instance the heat can be ambient temperature in the case of a basic laboratory machine, heated conductive shelves or radiative heat from shelves/product chamber wall.
The speed of this process is important in many cases - some products can be dried as quickly as the machine is capable of with minimal loss of quality, others will require a slow gradual drying to prevent excess heat damaging the product.
It is important to remember that the product will dry from the outside inwards. Therefore the thickness of the material will partly determine the length of this stage - the thicker the material the more difficult it will be for vapour to escape from the product from the central ice core. This will cause the drying process to be slower and less efficient, in a worst case scenario the reduction in sublimation cooling and resultant rise in product temperature may comprimise the structure of the product.
3) Secondary Drying
This phase removes unfrozen moisture that is not ice, but is instead sorbed (bound) to the product. This sorbed water is removed via desorption and the rate at which this occurs is dependant on the product temperature. Once the primary drying phase is complete then the product temperatures in secondary drying phase can be fine tuned to increase the rate of desorption either through gradual controlled temperature rise or simply set at maximum for the fastest possible rate. Take care not to damage your product with excessive heat or increase the rate of vapour release such that the vacuum level dimishes above the eutectic point of the product. If you are unsure when primary drying is complete, see the below section, "How can I tell when Primary Drying has ended?"
By the end of the secondary drying process the water content will be around 1-3%, with lower water content requiring significantly increased drying times.
Depending on your product needs, the vacuum can be broken either with air admittance or with an inert gas (typically Nitrogen).
Common questions about freeze drying
What is freeze drying used for?
Freeze drying has many applications including:
Long shelf life pharmacutical products.
Creation of dehydrated powders for tablets or capsules.
Production of chemically stable bulk raw materials.
Probiotic powders, protein powders, other food supplement powders.
Extended shelf life, low weight food rations such as military rations, climber rations and astronaut meals
Purification and concentration of heat sensitive materials or low molecular weight chemicals
Advance ceramic powders
Recovery of water damaged documents and books
Achaeological recovery and conservation
What advantages does freeze drying have over other drying processes?
Low processing temperatures ensure that nutrients and colour are not lost and proteins are not denatured.
The original shape of the product is kept after the process without shrinkage or toughening.
The product is easily rehydrated due to micropores created by the sublimation of ice from the product.
What capacity do I need?
Your required capacity depends on how much ice you will be removing over a set period of time. When contacting us you should state the "wet" (undried) weight of your product and the timeframe for drying. Our food and industry machines are rated for "x"kg of ice per 24hours, though cycle times are easily adjustable by the user.
How can I tell when the primary drying stage is complete?
1) Product temperature monitoring - monitor the product temperature with a thermocouple - when primary drying is complete the product temperature will rise and approach the shelf/chamber temperature due to the loss of sublimation cooling. When the product and shelf temperature are roughly equal (temperature difference, ΔT around 3-5°C), the primary drying phase can be assumed to be complete. The user should be aware that due to heat conduction through the thermocouple, the area around the thermocouple will dry faster than the bulk of the product and a safety margin for additional primary drying should be included. The thermocouple should also be placed at the bottom of the product as the product will dry from top-down.
2) Pressure Rise Test - If the freeze dryer has an external condenser, a pressure rise test can be used. The isolation valve between the condenser chamber and the product chamber is closed and the pressure rise in the product chamber is measured. If the pressure rise approaches zero, then there is little to no vapour produced by sublimation and the primary phase can be considered complete.
3) Capacitance manometer vs Pirani guage convergence test. This requires both a capacitance manometer and pirani guage to be fitted, as a pirani guage shows erroneously high readings in the presence of vapour, the difference in readings between the two can be used to determine the pirmary drying endpoint. When the two readings have converged (and no further change in the readings is noted) the primary drying phase is complete.
How cold do I need my freeze drier to be?
Extra cold condensers are only required if using solvents with a particularly low freezing point. A phase diagram for your solvent can be used to determine an appropriate condenser temperature. Bear in mind that some organic solvents may cause serious issues with standard sealing materials and should be mentioned in a list of requirements. An excessively cold condenser will increase the cost of the machine as well as it's complexity!
Where can I keep a freeze drier?
A freeze drier with air cooled condensers should always be kept in a cool, well ventilated room. High ambient temperatures will reduce performance and efficiency and possibly lead to the failure of the unit!
A freeze drier using chilled water for cooling can be located anywhere indoors with adaquete space, though excessive ambient temperatures should still be avoided. The chiller should be located as per the manufacturer guidelines - typically in a cool, outdoor area with plenty of ventilation.
If you have any further questions don't hesitate to get in touch with us or leave a comment!
Tagged as: freeze drying faq, pressure rise test, primary drying, secondary drying, What is freeze drying
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