Freeze-drying disadvantages

Frozen reference materials have been produced by NIST (Wise et al. 1993). These materials do not have the disadvantages of the oils or freeze-dried materials, but are more difficult to transport. Obviously they have to be kept deep-frozen during transport, which makes their use rather expensive. Since the early 1990 s a new approach in this field has been introduced. This concerned the use of wet, sterilized fish and shellfish samples. These samples, packed in glass jars or in tins, were firstly used in the QUASIMEME program as reference materials for inter-laboratory studies (de Boer 1997). Later, when it appeared that the stability was maintained for longer periods, tests for organic contaminants based on this principle were also prepared. 

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... 

The use of freeze-drying avoids an inherent disadvantage of spray drying, i.e., the elevated processing temperature, which accelerates the oxidation of polyunsaturated lipids. 

Supercritical fluids, in general, are inexpensive, contaminant-free, and less costly to dispose of safely than conventional organic solvents. Extracts are obtained under mild conditions that minimize thermal degradation, they are usually solvent-free or in a concentrated form, and no evaporation steps are needed prior to the final assay. However, the disadvantages of SFE should not be also ignored. As with all extraction methods, there are analytes and matrices for which SFE is not suitable. Some compounds are insoluble and may need solvent extraction. Aqueous matrices can cause problems and samples may need to be freeze-dried. Like all single-step extractions, SFE has only limited selectivity, but some distinction can be made between analytes by careful selection of temperature and pressure. 

Concentration. This group consists of those processes in which water is removed and the dissolved substances are left behind. Examples are freeze concentration, lyophilization (freeze-drying), vacuum distillation, and membrane processes such as reverse osmosis and ultrafiltration. A common disadvantage of these methods is that inorganic species are concentrated along with the organic constituents. 

The disadvantage of collecting samples only after the freeze-drying is that the results may be more difficult to interpret in the case where the product is not of the same characteristics from any position within the lyophilizer, as the differences observed reflect the overall effect of the successive manufacturing steps. 

The freeze-drying technique is also used in the analytical laboratory. Two additional benefits make it attractive to analysts, namely (a) because the removal of water allows samples to be subsequently reconstituted in a minimal volume of liquid, a preconcentration effect is achieved and (b), even more important, because of their texture, freeze-dried materials are very easily attacked by solvents. The process can be optimized for a specific purpose by using an appropriate temperature, pH, reagent addition sequence and time. One major disadvantage of freeze-drying, however, is that some volatile compounds may be removed from the sample matrix during the process. 

Lyophilized products are characterized by prolonged shelf-life, and chemical bacteria or enzymatic changes do not easily occur. The sterility is more guaranteed and solubility assured. In addition, transportation is easier. Finally, certain compounds or radiopharmaceutical kits exist only as lyophilized products. However, freeze-dried products suffer from certain disadvantages. The reentry of moisture may destroy the products. Direct optical control of lyophilized products cannot be performed. Therefore, the risk of particle contamination of the final product is high. Bacterial contamination can only be avoided by using the proper installations (clean rooms) for manufacturing injectable lyophilized products. 

There is a general consensus that analysis of fresh material yields results which come closest to the natural distribution pattern in plants. Since this approach is not feasible in most cases suitable preservation procedures must be employed. Advantages and disadvantages of such procedures with respect to polyphenol analysis have been reviewed by several authors [47-49]. The procedure keeping the chemical composition as close as possible to the natural state is lyophilization. A study of the extractability of proanthocyanidins in the bark of several pines (Pinus sp.) showed no difference between freshly collected samples and freeze-dried samples. On the other hand drying at room temperature resulted in an increase in extractability in one sample [50]. Increased extractability after drying at room temperature compared to fresh material has also been... 

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