Freeze-drying using vacuum-induced surface freezing

A method of freezing during freeze-drying, which avoids undercooling of a solution and allows growth of large, dendritic ice crystals, was investigated. Aqueous solutions of mannitol, sucrose, or glycine were placed under a chamber vacuum of approximately 1 mbar at a shelf temperature of +10 degrees C. Under these conditions, the solutions exhibit surface freezing to form an ice layer of approximately 1-3 mm thickness. On releasing the vacuum and lowering the shelf temperature to below the freezing point of the ice in the solution, crystal growth occurs to yield large, chimney-like ice crystals. The duration of primary drying of a frozen cake--as measured by using inverse comparative pressure measurement--was up to 20% shorter than when using a "moderate" freezing procedure (2 K shelf temperature per min). With mannitol, however, the residual moisture content of the final dried product was higher than with moderate freezing, and with sucrose and glycine there was no difference. These findings are related to the structures of the dried cakes formed during freezing, as examined by light microscopy and wide-angle X-ray diffraction. The introduction of an annealing step (4 h at a shelf temperature slightly above the onset melting point of the ice in the frozen cake) combined with the vacuum-induced surface freezing procedure maintains the rapid primary drying and produces a low residual moisture (0.2%) for the freeze-dried mannitol solution.