Freezing pretreatment techniques

Vacuum deaeration and infiltration techniques have been used as freezing pretreatments to prevent enzymatic browning in apple slices by replacing tissue gases with aqueous solutions of sucrose and/or AA (43-44). Vacuum infiltration of solutions into apple slices resulted in a translucent or waterlogged appearance which would not be acceptable in a raw product (45). 

The method of sample preparation to be used for a given analysis is governed by the nature and concentration of the analyte, the nature (solid or liquid) and type of matrix, the available sample amount, and also by the instrumental technique employed. Freeze-dried samples will require some form of digestion or dissolution in order to be analyzed by a classic atomic technique (i.e., using nebuliza-tion). Liquids might be analyzed by direct nebulization, but this is not always possible due to matrix interferences. Milk pretreatment may be necessary under such circumstances. 

The main problem in carrying out total multielemental determinations in milk (as in other biological samples) is the nature of the matrix, which may interfere with the analytical technique employed for the measurement. In this sense, pretreatment of the samples becomes necessary so as to minimize matrix effects as much as possible (e.g., by destroying the organic matrix). An alternative to such destructive acid attacks is the direct analysis in milk whey samples by simply diluting the sample previously obtained by centrifugation. The main preparation procedures for milk samples (whole, skimmed, or freeze-dried) can be classified as follows (a) use of diluted solutions in order to minimize matrix and molecular... 

This chapter deals with the different aspects of the freeze-drying process, beginning with the steps involved and the conditions influencing them. The state of the art in freeze-drying technology and equipment is then reviewed, and the aims and major analytical chemical applications of the technique (particularly as regards the pretreatment of solid samples) are discussed. 

Freeze-drying has rarely been considered an effective choice with a view to automating sample pretreatment, possibly because of the contradictory results obtained so far. However, there is solid evidence that this technique is suitable for many purposes. 

Glass transitions involve mainly the onset or freezing of cooperative, large-amplitude motion and can be studied using thermal analysis. Temperature-modulated calorimetry, TMC, is a new technique that permits to measure the apparent, fiequency-dependent heat capacity. The method is described and a quasi-isodiermal measurement method is used to derive kinetic parameters of the glass transitions of poly(ethylene terephthalate) and polystyrene. A first-order kinetics expression can describe the approach to equilibrium and points to the limits caused by asymmetry and cooperativity of the kinetics. Activation energies vary from 75 to 350 kJ/mol, dependent on thermal pretreatment. The preexponential factor is, however, correlated with the activation energy. 

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