The freeze-drying process – Batch processing
The freeze drying process has three stages: Freezing, primary drying and secondary dying.
Freezing
Typically the freezing process is done in large freezes that can take trolleys with trays filled with product. Frozen free flow products are done by IQF tunnels. There are two styles of freezing, one being rapid and the other slow freezing (similar to your standard home freezer). With rapid freezing you get a small ice crystal which will be better for the structural integrity of products that have complete cells like fruits and bio-activity. Slow freezing produces larger ice crystals which may affect the cell structure in delicate products like foods and fruits. When products become more complex the method of freezing may have a large impact on how well they dry, there are positive and negative aspects to both styles of ice crystal formation.
Freezing is necessary to present a solid and stable form. Rapid freezing using cryogenics can often be tempered back to more commercial temperatures, usually the holding temperature required is -20°C, however some products may become soft and need to go lower to -35°C. The solidification of product is critical to freeze drying as products not in a solid state can be spoiled in the drying process.
Primary drying
Primary drying can be defined as the first part of drying where product remains at sub-zero temperatures. This starts when the pressure in the chamber is lowered below atmospheric pressure to around 1 Mbar and energy (in the form of heat) is applied to the frozen product. At this point sublimation starts and the ice vapour from the product is condensed on the ice coil (which is typically around -40°C to -60°C). Sublimation is a state where the solid ice crystals are directly vaporised without passing through the liquid phase. This is due the low pressure environment and is the principle of freeze drying. 95% of the ice is removed from the product during primary drying which is why the product holds its shape and structure. The time frame this process could be anywhere between 8 – 14 hrs.
Control of Sublimation rates is vital, if too much heat is added this could be detrimental to the product structure and cause or shrinkage or melt back.
Secondary drying
Secondary drying can be defined as the point where products gain enough sensible heat to leave sub-zero temperatures due to the lack of cooling from residual ice. This is an indication that product is near dry. The product temperature continues to rises until it reaches the shelf temperature – until then it is not completely dry. Product often remains in for a further period to ensure all parts are completely dry.
Knowing the product temperature is a critical part of managing your dryer and determining the condition and state of the product.