Quick-freezing

Sometimes polymer processors quench a polymer by quickly freezing the melt. What happens on the molecular scale during quenching What benefits do the processors gain by quenching their polymer What material properties are reduced by this process ... 

Figure 5.25 AFM images of intermediate stmctures in self-assembly of peptide KFE8 in aqueous solution deposited on freshly cleaved mica surface (a) 8 min after preparation of solution. Inset electron micrograph of sample of peptide solution obtained using quick-freeze deep-etch technique (b) 35 min, (c) 2 h, and (d) 30 h after preparation. Reprinted with permission from Ref. 110. Copyright 2002 by the American Chemical Society.

Figure 5.26 TEM image of surfactant-like peptide A6D dissolved in water (4.3 mM at pH 7) obtained using quick-freeze deep-etch technique. Image shows dimensions, 30-50 nm in diameter with openings of nanotube ends (arrows). Inset Opening ends in more detail. Reprinted with permission from Ref. 112. Copyright 2002 by the National Academy of Sciences, U.S.A.

The combination of this knowledge and the results of quick-freezing processes provide a theoretical opportunity to freeze products into a solid, amorphous state. If the freezing velocity is smaller than required for vitrification, but large enough to avoid an equilibrium state, an amorphous mixture will result of hexagonal ice, concentrated solids and UFW. 

Fig. 1.31. Behavior of a sucrose - NaCI solution at different sucrose - NaCI concentrations and temperatures after quick freezing (200 °C/min), during slow rewarming (Fig. 8 from [1.25]).

Fig. 1.32. Plot of the DTA measurement of a 50 % glycerine-solution during slow rewarming after quick freezing at 75 to 200 °C/min.

A, Solution before freezing B, freeze dried after quick freezing C, freeze dried after freezing on precooled shelves D, freeze dried after cooling shelves and product simultaneously (Fig. 1 from [3.6]). 

Liposomes can, generally speaking, only be frozen without damage if the suspension is frozen in a glass phase of water. This requires the addition of CPAs e. g. mannitol, dextran or trehalose, and quick freezing (e. g. 10 °C/min by LN2) [3.37], (page 363). 

Reetz and Haase [5.2] used different freezing rates to freeze Zr02, and found slow freezing of this product to result in better technological qualities, e. g. free-flow and sinter ability than quick freezing. On the other hand, complex Zn- solutions can only be frozen quickly to arrive at a product homogeneous in chemical structure and grain size distribution. 

For the quantitative description of the metabolic state of a cell, and likewise which is of particular interest within this review as input for metabolic models, experimental information about the level of metabolites is pivotal. Over the last decades, a variety of experimental methods for metabolite quantification have been developed, each with specific scopes and limits. While some methods aim at an exact quantification of single metabolites, other methods aim to capture relative levels of as many metabolites as possible. However, before providing an overview about the different methods for metabolite measurements, it is essential to recall that the time scales of metabolism are very fast Accordingly, for invasive methods samples have to be taken quickly and metabolism has to be stopped, usually by quick-freezing, for example, in liquid nitrogen. Subsequently, all further processing has to be performed in a way that prevents enzymatic reactions to proceed, either by separating enzymes and metabolites or by suspension in a nonpolar solvent. 

Dimensions Ltd) cryoprotectant and very quickly freeze the crystal in the... 

Fuchigami, M., Miyazaki, K., and Hyacumoto, N. (1995). Frozen carrots texture and pectic components as affected by low-temperature-blanching and quick freezing. ]. Food Sci. 60, 132-136. 

Anonymous. (1995-2003a). Raw Material Guidelines for Quick Freezing. CCFRA Guidelines, Campden and Chor-leywood Food Research Association, Chipping Campden, Gloucestershire, GL55 6LD, U.K. 

Fig. 17 Association of hydrogenosomes and microtubules in T. foetus, a Thin section of hydrogenosomes (H) in close proximity with microtubules (asterisks), b Freeze-etching after quick-freezing and rotatory shadowing showing a hydrogenosome in close association with cytoskeletal structures, probably microtubules. Bars = 200 nm. (Benchimol, unpublished)...

Preservation of cell structure, food taste, and avoidance of thermal degradation are reasons for the removal of moisture from such materials by sublimation. The process is preceded by quick freezing which forms small crystals and thus minimum damage to cell walls, and is likely to destroy bacteria. Some of the materials that are being freeze dried commercially are listed in Table 19.9(b). 

The most advanced technique of quick freezing is by pouring the material onto a freezing belt. Before drying, the material is granulated or sliced to improve heat and diffusional mass transfer. These operations are conducted in cold rooms at about —46°C. 

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