How does freeze-drying work

Packets of instant coffee proudly proclaim that the product has been freeze-dried . In practice, beans of coffee are ground, boiled in water and filtered to remove the depleted grounds. This process yields conventional fresh coffee, as characterized by its usual colour and attractive smell. Finally, water is removed from the coffee solution to prepare granules of instant coffee. 

In principle, we could remove the water from the coffee by just boiling it off, to leave a solid residue as a form of instant coffee . In fact, some early varieties of instant coffee were made in just this way, but the flavour was generally unpleasant as 

It is clear that decreasing the external pressure makes boiling easier. It is quite possible to remove the water from coffee at or near its freezing temperature - which explains the original name of freeze-drying. 

Freeze-drying is a layman s description, and acknowledges that external conditions may alter the conditions of a phase change, i.e. the drying process (removal of water) occurs at a temperature lower than 100°C. 

1 A liquid has a normal boiling temperature of 140°C. Use the nomograph to estimate the applied pressure needed to decrease the boiling temperature to 90°C. 

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