Crystallization freezing rate

That part of the water which is not frozen due to high freezing rate, forms highly viscous occlusions in between the ice crystals. 

Reid et al. [ 1.12] described the effect of 1 % addition certain polymers on the heterogeneous nucleation rate at-18 °C the rate was 30 times greater than in distilled, microfiltered water and at -15 °C, the factor was still 10 fold hogher. All added polymers (1 %) influenced the nucleation rate in a more or less temperature-dependent manner. However, the authors could not identify a connection between the polymer structure and nucleation rate. None the less it became clear that the growth of dendritic ice crystals depended on to factors (i) the concentration of the solution (5 % to 30 % sucrose) and (ii) the rate at which the phase boundary water - ice crystals moved. However, the growth was found to be independent of the freezing rate. (Note of the author the freezing rate influences the boundary rate). 

A suitable freezing rate, start-up concentration and an amount of product per vial (for example for Na-ethacrynate) can be selected that results in a stable, crystalline phase. However, the addition of CPAs may provide another means of achieving crystallization as seen for several pharmaceutical products [1.47]. 

Canet, 1995). It was found that, after two or three cycles, softness ceased to be much affected by subsequent cycles (Figure 7.6). The authors concluded the following (1) that pre-cooling and a high freezing rate during the phase of maximum ice crystal formation has a positive effect on potato texture and tissue structure, (2) that slow thawing has a positive effect and (3) that it is essential not to subject potatoes to more than one freeze/thaw cycle. 

Cryoinjury to the specimen is caused directly by extra- or intracellular ice crystal formation as well as by ice-induced solution effects during cryopreservation. Ice crystals seriously deform cell components. Another disadvantage of the formation of ice crystals near the specimen surface is slowing the cooling rate in areas below the surface because their thermal conductivity is about half that of solid water in a noncrystalline state. Furthermore, ice crystal formation is accompanied by the generation of latent heat, which also slows down the freezing rate. 

The cooling rate directly influences the size of the ice crystals, which can be measured after drying by the size of the pores in the product. Thijssen and Rulkens [1.11] gave the size of the pores in chicken meat (Table 1.6.3). Figure 1.16 shows the average size of pores in 20% dextrose solution as a function of the freezing rate. Godward... 

The color of the end product is influenced by the freezing rate (fine crystals show a lighter color). Furthermore, the color is influenced by the structure of the foam and... 

To avoid cell damage as a result of the rapid formation of ice crystals a freezing rate of 1°C per minute is optimal. If Nalgene tubs are not available, vials can be wrapped loosely in cotton wool to achieve a similar effect, but this has a more variable success rate. 

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