Freezing process

2 Freezing Process. - MRI gives a unique opportunity to better-understand the dynamic phenomena that occur during processing and storage of food. By measuring apparent diffusion coefficient, both axially and radially in meat, the 

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As a general rule, foods which are not to be frozen are handled and stored at a temperature just above their freezing point, providing this does no damage (exceptions are fruits such as bananas and lemons). Produce which is to be frozen must be taken down below the freezing point of the constituents. Since foodstuffs contain salts and sugars, the freezing process will continue down to -18°C and lower. 

Rapatz, G.L., Menz, L.J., Luyet, B.J. (1966). Anatomy ofthe freezing process in biological materials. 

Rubinsky, B. Pegg, D.E. (1988). A mathematical model for the freezing process in biological tissue. Proc. Roy. Soc. London B 234, 343-358. 

Therefore, freeze-drying should be carried out at the highest allowable product temperature that maintains the appropriate attributes of a freeze-dried product. This temperature depends on the nature of the formulation. Process development and validation requires characterizing the physical state of the solute, or solutes, that result from the freezing process and identifying a maximum allowable product temperature for the primary drying process [20,21]. 

The glass transition of solutes that remain amorphous during and after the freezing process can often be seen in the DSC thermogram as a shift in the baseline toward higher heat capacity. This is illustrated in the DSC thermogram of sucrose solution in Fig. 6, in which the glass transition is observed at — 34°C. 

Most proteins are not sufficiently stable in aqueous solution to allow formulation as a sterile solution. Instead, the protein is freeze-dried and reconstituted before use. Development of a freeze-dried protein formulation often requires special attention to the details of the freezing process (potential pH shifts and ionic strength increase with freezing) as well as to potential loss of activity with drying. Formulation additives, such as sugars and polyhydroxy compounds, are often useful as cryoprotectants and lyoprotectants. Residual moisture may also be critical to the stability of the dried preparation [33],... 

The negative value indicates that heat is lost by the benzene during the freezing process. 

Supercooling The hot water sample may have a greater tendency to supercool, because it has less dissolved gas which can act as nucleation points for ice to form. Water that does not supercool may form a thin layer of ice at the surface which can insulate the rest of the water from the freezer and delay the freezing process. 

The freezing process will be discussed with model substances, which will be used as... 

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. 

The freezing of complex organic solutions and suspensions is often difficult to predict theoretically. The methods to analyze the freezing process and the formed structure are described in Section 1.1.5. The freezing is influenced by several factors, which often act in opposing directions ... 

Three examples will show how freezing processes can be studied quantitatively and documented using this microscope system. Figure 1.39 shows the change in volume of an isolated islet cell of a mouse as a function of temperature. The different permeability of cell membranes for HzO and CPAs are important for freezing of cell, as Fig. 1.40 shows. 

Hanafusa [1.36] showed with this method, how the amount of unfrozen water in a 0.57 % solution of ovalbumin reaches practically zero at -20 °C, if 0.01 M sucrose is added (Fig. 1.51). For globular proteins Hanafusa described the freezing process as follows between 0 °C and -20 °C, water molecules from the multilayer hydrate shell are decomposed. Be-... 

Kochs, M., Korber, Ch., Nunner, B., Fleschel, I. The influence of the freezing process on vapor transport during sublimation in vacuum-freeze-drying. Journal of Heat and Mass Transfer 34, p. 2395-2408, 1991... 

Fig. 2.3. Schema of the IQF-freezing process CRUST o FREEZE , AGA AB, Frigo-scandia Equipment AB, S-25109 Helsingborg.

Fig. 3.1. Influence of different freezing processes on the activity of recombinant DNA protein.

Ikeda [3.36] presented a two stage freezing process for an antibiotic (Panipenem), which reacts with another component of the drug (Betamipron) and has therefore to be separated until its use. The first substance is filled into the vials and frozen. The precooled second substance is filled then into cooled vials and frozen. By this process, the amount of undesirable reaction product could limited to 0.5 % during a 6-months storage at 40 °C. If the two products were frozen simultaneously the amount of reaction product was 1.2%. 

Sehoof, H., Apel, J., Heschel, I., Rau, G. Influence of the freezing process on the porous structure of freeze dried collagen sponges (unpublished results, Helmholtz-Institute, Aachen, Germany... 

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 ... 

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