Post Time: 16 February 2022
If the water vapour pressure of a food is held below 4.58 Torr (610.5 Pa) and the water is frozen, when the food is heated the solid ice sublimes directly to vapour without melting.
The quality of vacuum dried fruits or vegetables is usually better than air dried products.
A small scale laboratory Freeze Dryer consists of a well-sealed vacuum chamber arranged horizontally and fitted with heated shelves, a refrigerated condenser and a vacuum pump or pumps.
The refrigerated condenser, also named as cold trap, removes the water vapour formed by sublimation. The water vapour freezes on to the condenser, thus maintaining the low water vapour pressure in the chamber.
The vacuum pumps remove the non-condensable gases.
The heated shelves supply the heat of sublimation. These shelves are heated internally by steam, hot water or some other thermal fluid, which is circulated through them.
The vapour pressure of ice at –20℃ is about 135 Pa, absolute. Industrial freeze drying is carried out in vacuum chambers operated at pressures in the range 13.5–270.0 Pa, absolute.
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