Freeze-drying advantages

Freeze-drying, like all drying processes, is a method to separate liquid water from a wet solid product or from a solution or dispersion of given concentration. However, the main difference is that the liquid water is separated by solidification (i.e., the formation of ice crystals) and subsequent vacuum sublimation instead of evaporation. This allows a drying at subzero temperatures which can be advantageous in case of heat-sensitive products. There are two general applications... 

Despite the advantages of freeze-drying, some limitations must be kept in mind. 

The total pressure during freeze drying may be measured by several methods, though only two are mostly used heat conductivity, and the membrane pressure difference gauge. Their operating principles and their advantages and disadvantages are described below. 

Freeze drying plants must be easily cleaned, a task which for pilot installations may be done by hand. This is difficult to document and one has to rely on the experience and conscientiousness of the operator. Gaster [2.16] presents a survey of advantages and disadvantages of cleaning methods from hand operation to spray sysfems with a sequence of... 

Shibata [4.20] described the freeze drying of boiled rice as a method to produce convert food (fast food). The paper is quoted as the advantages of food freeze drying are mmarized within it Structure and taste of the boiled food can be restored in approx. 3 in by adding hot water of +80 °C, and the dried product can be stored well. 

Gieseler et al. utilized tunable diode laser absorption spectroscopy to detect water vapor concentrations, gas velocities and mass flow during freeze-drying of pure water at different pressure and shelf temperature settings and of a 5%w/w mannitol solution. The analyzer was interfaced to the spool that connected the dryer chamber to the condenser. The reported method was advantageous in that primary and secondary drying end-point control based upon mass flow rate was independent of freeze-dryer size and configuration. ... 

The freezing of a product is a very important step. The structure in the frozen product decides whether the product can be freeze-dried at all and under which conditions it can be done. For this reason, the consequences of the freezing rate, layer thickness of the product and excipients are discussed in some detail. The second main point is the measurement and control of the two drying phases the main and secondary drying and the third concentrates on the residual moisture content, its measurement and the consequences during storage of the dry product. There will be critical opinions that some of the processes are unilaterally represented. aim was to show the, hmrts and the advantages ot certain procedures to enable the reader to decide whether the ideas of the quoted authors, or my own can be applied to his tasks. 

The advantages of this method are (i) it produces liposomes with high entrapment efLciency, (ii) the procedure s Lexibility may aid product stability as noted earlier, (iii) scale-up is possible, and (iv) the lyophile can be stored and rehydration can be done immediately before use. The drawbacks of the method are possible particle size instability of the liposome during the freeze-drying process and the high cost of the freeze-drying process. 

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