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

Several typical modes of heat transfer and mass transfer in the drying process

• (a)

• (b)

• (c)

• (d)

• (e)

• plate

• frozen layer

• G

• Q

• G

• Q

• G

• Q

• G

• Q

• G

• G

• Q

• G

• Q

• G

• G

• Q

• Q

• G

• (f)

• Q

• G

• heat transfer

• mass transfer

Mass transfer

• During sublimation drying, there is massive vapor to be produced from the material. The vapor must be moved away promptly, otherwise the pressure in the drying chamber will be elevated, the sublimation surface temperature will increase, and finally the frozen layer will melt.
• The water vapor escaped from the sublimation surface, passing through the dried layer, flows to the cold trap and forms frost on its surface.

Freeze drying

• 1) freeze food in the conventional freezing equipment
• 2) remove water during subsequent drying
• (If the water vapor pressure of a food is below 4.58 Torr (610.05 Pa) and the water is frozen, when the food is heated the solid ice sublimes directly to vapor without melting.
• The water vapor is continuously removed from the food by keeping the pressure in the freeze drier cabinet below the vapor pressure at the surface of the ice, removing vapor with a vacuum pump and condensing it on refrigeration coils.
• As drying proceeds a sublimation front moves into the
• frozen food leaving partly dried food behind it.