Basic Process

Contact-based conductive heating has been used in some commercial freeze dryers. Microwave heating has been tested experimentally (6). In modern freeze dryers, radiant heating is by far the most commonly used method of providing heat input. Therefore, only drying based on its use will be discussed. Let us examine what happens when a slab of frozen food placed on a perforated tray is freeze dried in a simple dryer, like that shown in Figure 1. After the dryer door is closed and sealed, the refrigeration is turned on, cooling the condenser. The vacuum pump is turned on, and, as air leaves the dryer, pressure drops. When the pressure closely approaches the vapor pressure of ice at the set condenser temperature (eg, 97 /¿m Hg absolute for a — 40°C condenser), hot fluid is pumped through the heating plates.

The plate temperatures rise until they reach a desired set value (eg, 120°C). Heat transfers radiantly from the plates to the slab, causing sublimation on both sides of the slab. Vapor produced at the bottom of the slab escapes through the perforations in the tray. Sublimation interfaces form and recede into the slab, leaving behind ice-free pores in the matrix containing the bound water. Thereafter, heat, radiantly transferred to outer surfaces of the slab, transfers conductively across the ice-free layers, causing further sublimation. Small amounts of bound water also evaporate. Vapor produced by both processes flows through the pores in the ice-free layers, causing a pressure drop that may be a large fraction of the absolute pressure in the dryer. The vapor leaves the slab and flows to the condenser, where it freezes.

The ice-free layers deepen as sublimation proceeds. Consequently, heat- and vapor-flow resistances increase, and the sublimation rate progressively decreases. After the last ice sublimes, that is, the sublimation interfaces meet near the center of the slab, the center temperature rises and bound water desorbs more rapidly. To prevent overdrying and overheating of outer parts of the slab, the plate temperature is reduced as desorption proceeds. At the end of drying, the plate temperature may be 50°C, and the center product temperature may be 30°C.

After drying is complete, the vacuum is broken by admitting air or, preferably, dry nitrogen. The product is quickly removed, rapidly transferred to a low humidity room, and quickly packed and sealed in an impermeable container. After all product is discharged from the dryer, ice is removed from the condenser by passing hot refrigerant gas through it or by spraying it with hot water. The melt produced drains from the dryer through a closeable outlet at its bottom.

where T0 is the outer-surface temperature, Ts is the temperature at the sublimation interface, Z is the thickness of the ice-free layer, kt is its thermal conductivity, A is the latent heat of sublimation (roughly 2,836 kJ/kg ice), and p and X are the initial density and weight fraction of ice in the slab, respectively.

The kt is roughly proportional to the food's initial solids content and also depends on pressure. At absolute pressures greater than roughly 100 mm Hg, kt is the same as at atmospheric pressure, that is, roughly 0.032 to 0.17 W • m-i . oq-1 At lower pressures, kt decreases sigmoidally as pressure decreases, and roughly around 100 /jm Hg levels off at roughly one half to one quarter of its atmospheric pressure value. The respective pressures at which kt shifts start and finish depend on pore diameter. In most cases, freeze drying is carried out at or close to the low-pressure kt limit. Thermal conductivities encountered in freeze-drying foods are substantially lower than those of many thermal insulators.

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