Fst 151 food freezing food science and technology 151 Special topics: Freeze Drying

Mass transfer during freeze drying

• When heat reaches the sublimation front, it raises the temperature and the water vapor pressure of the ice. Vapor then move through the dried food to a region of low vapor pressure in the drying chamber.
• 1 g of ice forms 2 m3 of vapor at 67 Pa.

Factors that control the water vapor gradient:

• The pressure in the drying chamber
• The temperature of the vapor condenser.
• The temperature of ice at the sublimation front, which should be as high as possible, without melting
• (the lowest economical chamber pressure is approximately 13 Pa and the lowest condenser temperature of ice should be -35 ºC).