Lyophilisation: Atmospheric food freeze-drying: challenges and opportunities

In 1959, Meryman² evidenced that the water flux from the ice front to the drying chamber depends on the pressure gradient in the dried product, and not on the absolute pressure of the system. As a consequence, it can be possible to carry out the drying process at atmospheric pressure if a water vapour pressure gradient is maintained in the dried product.

In an atmospheric freeze-drying process, the product to be dried is firstly frozen (at about -10°C) and then it is dried with air at temperatures generally ranging from -10 to -3°C, as this appears to be a good compromise between costs and final product quality. In this case, the specific moisture extraction rate can range from 1.5 to 4.6 kilograms of water per kWh1, and 35 per cent of energy savings can be achieved with respect to vacuum freeze-drying, as reported by Wolff and Gibert³ for drying potato slices. Besides, almost the same product quality with respect to vacuum freeze-drying is obtained, as shown for many products like strawberries and potatoes, fish products, lactic acid bacteria and many others.

The atmospheric freeze-drying process can be carried out in a tunnel dryer⁴. The product is placed over a belt or a shelf and dry air flows over the product to remove the water. Air temperature, relative humidity and velocity can play a relevant role on drying time. Figures 1A and 1B show the effect of the various operating parameters on drying time of apple samples (Malus domestica) in a tunnel dryer. It appears that the drying time is significantly reduced when decreasing air relative humidity or increasing air temperature, while the effect of air velocity is poor. This is due to the fact that in a tunnel freeze-dryer, the velocity of heat and mass transfer is not very high and thus the sublimation flux can be increased mainly acting on the driving force of the sublimation process (i.e. on the difference between water vapour partial pressure at the ice front, dependent on product temperature and in the chamber, dependent on relative humidity).

As an alternative, the product can be freeze-dried in a fluidised bed (Wolff and Gibert³), a type of device where a fluid is passed through a granular solid material at a sufficiently high velocity to suspend the solid, and cause it to behave as it were a fluid. When using a fluidised bed it is necessary to granulate the frozen product before drying. Size reduction caused by mechanical cracking is a potential drawback, while the main advantages are represented by the high velocity of heat and mass transfer processes that can be obtained. In Figures 1C and 1D, the drying time calculated for the atmospheric freeze-drying of apple samples (Malus domestica) in a fluidised bed are compared to those obtained in a tunnel freeze-dryer, evidencing that the drying time can be significantly reduced in the first type of equipment.