Influence of Freezing Rate

Values of the permeabilities of the freeze-dried layer derived from the previously mentioned modeling are presented in Fig. 3.10. They dearly explain the well known influence of the freezing protocol - more precisely of the nudeation temperature - on the duration of the subsequent sublimation step as also observed experimentally (compare with Section 3.3.1.2). 

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O Brien FJ, Harley BA, Yannas IV et al (2004) Influence of freezing rate on pore structure in freeze-dried collagen-GAG scaffolds. Biomaterials 25 1077-1086... 

Figure 3.392. Influence of freezing rate on the cryolitic destruction of polyisobutylene [1170] 1) in the oxygen presence, low freezing rate 2) idem, a higher rate 3) in the absence of oxygen, high freezing rate 4) in the oxygen presence, high freezing rate.

Yarwood et al. (33) investigated the influence of cooling rate, initial concentration of solute, and fill volume on physical form and chemical stability of sodium ethacrynate. The freezing protocol consisted of ramping from room temperature to -25 C over four hours vs. placing vials on shelves... 

The influence of the freezing method the loading of the vials on C or W shelves influences the freezing rate (not shown) marginally on precooled shelves (C) the rate was between 0.65 and 0.40 °C/min and on warm shelves (W) between 0.25 and 0.40 °C/min with LN2 30-65 °C/min have been reached. Tfc during MD is only... 

The phenomena in the plasma jet experiments which lead to nitrogen conversion levels lower than the maximum predicted by the freezing model are thought to be linked to mixing limitations, (CH4 and N2 were not premixed in the plasma jet experiments). The influence of mixing rate on yield in plasma jet reactors is the subject of a separate theoretical analysis by the author (5). 

Rates of denaturation of muscle proteins in Alaska pollack, as influenced by freezing rate and storage temperature, were studied by Matsuda... 

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). 

The leak rates of a freeze drying plant can be measured at the empty plant with the condenser cooled and the shelves heated by measuring the pressure rise per time multiplied by the installation volume in the dimension (mbar L/s). It should be noted, that the plant has to be evacuated for several hours, e. g. down to 10-2 mbar, before the pressure rise measurements, to avoid the influence of small amounts of ice and the desorption of gas from the surfaces. Furthermore, the pressure rise should be measured up to 0.2 or 0.4 mbar to detect possible gas desorption. Only if the pressure rise has been for some time proportional with time (Fig. 2.33.1), it represents a leak rate, which is defined as... 

Israeli et al. [3.31] found, that trehalose is a very good stabilizer for E, even if the freeze dried suspension of E was stored at 21 °C and 60 % relative humidity, and/or was exposed to visible light. In 3 h, the survival rate decreased without trehalose to 0.01 % under the influence of light and air, with trehalose, 35 % survived. The optimum trehalose concentration was found to be 100 mM. This corresponded with the number of trehalose molecules necessary to replace the water molecules in the outer membrane of the phospholipid molecules. 

Pitombo et al. [3.33] found that 0.010 M succinate buffer at pH 4.6 was the best stabilizer for SC. The influence of three different freezing rates (0.5, 1.5 and 5 °C/min) on the capability of reproduction is shown in Fig. 3.16. During 235 days storage at +25 °C, no measurable decrease in invertase activity was observed, if the RM was below 4 %. With RM approx. 14 %, the invertase activity decreased in 20 days to half and was immeasurable after 57 days, since an insoluble cluster had been formed. A 4 % RM correspond at +25 °C with a monomolecular layer of water. 

The freezing of a slice of beef in direct contact with a model liquid has been used to demonstrate the influence of the two terms w and u. To freeze a product for freeze-drying, two methods are mainly used (i) freezing of the product in trays or in vials on cooled surfaces or (ii) in a flow of cold air. If these methods do not result in a sufficient freezing rate, LN2 in direct contact with the vials is used (see Fig. 2.2) or droplets of the product are sprayed into LN2 (see Section 2.1.4). 

The application of helium permits the freezing of the reaction in such stages that the otherwise very reactive intermediates become detectable. Hydrogen thus influences the relative rates of elementary steps but not the overall meehanism which is shown in Fig. 4a (21,62). 

Figure 3.9 Influence of product thickness and heat transfer coefficient on mean freezing rate for potato slices. From Rios, G.M., Marin, M. and Gibert, H., New developments of fluidization in the IQF food area, in McKenna, B.M., (ed.). Engineering and food, volume 2, processing applications, International Congress on Engineering and Food, 1983, Elsevier, 1984, figure 3. With kind permission of Springer Science and Business Media.

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