Atmospheric pressure freeze-drying

Clearly, the additional time required for air pressure freeze-drying of water-saturated wood will be an important factor in utilizing this system in preference to vacuum treatment. Some measure of the extra time required is available for wood 44, 57) and other organic materials (i, 58). The time required for atmospheric pressure drying appears to have a multiple of 1.5-8 times that necessary for vacuum drying. For large structures, the time ele-... 

Freeze drying has also been carried out at atmospheric pressure in fluid beds using circulating refrigerated gas. Vacuum-type vibrating conveyors, rotating multishelf dryers and vacuum pans can be used as can dielectric and microwave heating. 

An important step in the freeze drying process with vials is the stoppering or closing of the vials either at the end pressure of SD, or at a chosen partial pressure of a specific gas. This avoids handling of open vials, which can lead to contamination and adsorption of water vapor from the atmosphere. 

We call solid carbon dioxide (CO2) dry ice . To the eye, it looks just like normal ice, although it sometimes appears to smoke see below. Carbon dioxide is a gas at room temperature and only solidifies (at atmospheric pressure) if the temperature drops to about —78 °C or less, so we make dry ice by cooling gaseous CO2 below its freezing temperature. We call it dry ice because, unlike normal ice made with water, warming it above its melting temperature leaves no puddle of liquid, because the CO2 converts directly to a gas. We say it sublimes. 

FIG. 15 Texture modification (percentage of raw fruit texture) and protopectin content (mg/100 g wet weight) before (BF) and after (AF) freezing of strawberry slices not pretreated (NT) or air dried at 80 °C up to 60% weight reduction without (AD) or following 60-min osmotic dehydration (OAD) in 60% (w/w) sucrose solution at 25 °C at atmospheric pressure (Brimar, 2002). 

Negative atmospheric pressure chemical ionization (APC) low-energy collision activation mss spectrometry has also been employed for the characterization of flavonoids in extracts of fresh herbs. Besides the separation, quantitative determination and identification of flavonoids, the objective of the study was the comparison of the efficacy of the various detection systems in the analysis of flavonoids in herb extracts. Freeze-dried herbs (0.5g of chives, cress, dill, lovage, mint, oregano, parsley, rosemary, tarragon and thyme) were ground and extracted with 20 ml of 62.5 per cent aqueous methanol. After sedimentation the suspension was filtered and used for HPLC analyses. Separations were carried out in an... 

Lyophilization (or freeze-drying) is a process utilized to convert a water-soluble material filled into a container to a solid state by removal of the liquid while frozen. The process requires the use of deep vacuums and careful control of temperatures. By conducting the process under reduced pressure, the water in the container converts from ice directly to vapor as heat is applied and is removed from the container by the vacuum. The dissolved solids in the formulation cannot undergo this phase change and remain in the container. At the completion of the cycle, the container will be returned to near atmospheric pressure stoppers are applied or fully seated and crimped as described above. Lyophilization is particularly common with biological materials whose stability in aqueous solution may be relatively poor. The time period in solution and the temperature of the solution are kept at a specified low temperature to prevent product degradation [35],... 

Emptying of trays with freeze-dried products should be done in a glove box with dry inert gas to prevent absorption of moisture from the atmosphere [34]. Vials are generally closed under atmospheric pressure or under partial pressure,... 

The thermodynamic instability of all freeze-drying products at normal pressure and normal humidity (40 to 80%) results in their interaction with atmospheric moisture after freeze-drying, although the results of this interaction can be rather different. A large number of equilibrium lower hydrates demonstrate significant kinetic stability to rehydration, so that a short contact time of the product with... 

For most substances, including water (see Fig. 10.23c), atmospheric pressure occurs somewhere between the triple-point pressure and the critical pressure, so in our ordinary experience, all three phases—gas, liquid, and solid—are observed. For a few substances, the triple-point pressure lies above P = 1 atm, and under atmospheric conditions, there is a direct transition called sublimation from solid to gas, without an intermediate liquid state. Carbon dioxide is such a substance (see Fig. 10.23b) its triple-point pressure is 5.117 atm (the triple-point temperature is —56.57°C). Solid CO2 (dry ice) sublimes directly to gaseous CO2 at atmospheric pressure. In this respect, it differs from ordinary ice, which melts before it evaporates and sublimes only at pressures below its triple-point pressure, 0.0060 atm. This fact is used in freeze-drying, a process in which foods are frozen and then put in a vacuum chamber at a pressure of less than 0.0060 atm. The ice crystals that formed on freezing then sublime, leaving a dried food that can be reconstituted by adding water. 

As noted earlier, a majority of dryers are of the direct (or convective) type. In other words, hot air is used both to supply the heat for evaporation and to carry away the evaporated moisture from the product. Notable exceptions are freeze and vacuum dryers, which are used almost exclusively for drying heat-sensitive products, tend to be significantly more expensive than dryers operated near atmospheric pressure. Another exception is the emerging technology of superheated steam drying (Mujumdar 1995a). In certain cases, such as the drum drying of pasty foods, some or all of the heat is supplied indirectly by conduction. 

Although we are not aware of it happening, ice and snow sublime slowly on winter days. Water molecules escape directly from the surface of ice and enter the atmosphere as a gas. Dry ice gets its name from the fact that as a solid at -78°C, it can disappear (sublime) at normal atmospheric pressure without becoming a liquid. Freeze-dried foods are made by freezing them and removing the water by sublimation at low pressures. 

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