Freezing rates

There have been many modifications of this idealized model to account for variables such as the freezing rate and the degree of mix-ingin the liquid phase. For example, Burton et al. [J. Chem. Phy.s., 21, 1987 (1953)] reasoned that the solid rejects solute faster than it can diffuse into the bulk liquid. They proposed that the effect of the freezing rate and stirring could be explained hy the diffusion of solute through a stagnant film next to the solid interface. Their theoiy resulted in an expression for an effective distribution coefficient k f which could be used in Eq. (22-2) instead of k. 

Riehle shows, that such freezing rates can only be reached for layers < 0.1 mm under a pressure of 1.5-2.5 kbar. 

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

Table 1.6 Size and number of pores in chicken meat as a function of freezing rate.

Freezing rate (°C/min) Size of pores (p) Rate of pores (%)... 

Fig. 1.16. Pore diameter as a function of freezing rate in a 20 % dextran solution (Fig. 3 from [1.1 I]).

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). 

Fig. 1.20. Percentage of rat hepatocytes which show intracellular ice as a function of freezing rate in the range of -1 °C to -21 °C. The values in [ ] are the numbers of hepatocytes participating in each test (Fig. 8 from [ 1.28]).

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]. 

Fig. 1.94. Percentage of alcohol retained as a function of the number of carbon molecules in the alcohol molecule three freezing rates (vt) as parameter. The solution consists of 30 g saccharose, 15 g glucose, 15 g fructose, 15 g citric acid, 5 g CaCl2, 15 g pectin, 5g freeze dried albumin, 900 g water and 100 ppm volatile substance.

Thijssen, H. A. C., Rulkens, W. H. Effect of freezing rate on rate of sublimation and flavour retention in freeze-drying, p. 99-114. International Institute of Refrigeration (Comm. X, Lausanne, 1969)... 

Freezing method and freezing rate must be the same. 

Greiff [3.24] studied the stability of purified influenza virus of the strain PR 8 in physiological NaCl solution with calcium lactobionate and human serum albumin (each 1 % in the solution). The freezing rate was approx. 1 °C/min down to -30 °C. During the freeze drying, the product temperature was raised in 12 to 16 h from -30 °C to 0 °C and the product was dried at this temperature. After 24 h, the first 145 vials were removed, and additional vials after intervals of 24 h each. The residual moisture content was 3.0,2.0,1.5, 1.0 and 0.5 %. The stability of the freeze dried virus (expressed in days during which the titer of the infectivity decreased by a factor of 10 was most unfavorable at 0.4 % and 3.2 % RM, (4 and 7 days respectively at +10 °C) and best at 1.7 % RM 145 days or more than 1000 days at -10 °C. 

Fig. 3.16. Viability of Saccharomyces cere-visiae as a function of drying time, frozen with three different freezing rates.

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 product in the vacuum chamber by the evaporation of 15-25 % of its water would reduce the process time, the investment- and the operation cost, since the evaporation of this water could be done in a very short time, and no extra freezing installations are required. Oetjen [4.4, 4.5] points out, that this process has substantial disadvantages for the quality of the product and is only applicable for a limited number of products. Therefore this process is no longer used (see Section 2.1.5). The freezing processes for food are discussed in the Sections 2.1.1 and 2.1.3, and the possible freezing rates in Section 1.1.1. The decisive quantities for the freezing rastes are ... 

The general rule for all freeze drying processes applies also to food The method of freezing, the freezing rate and the final temperature of freezing largely determine the quality of... 

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