Others

- metals

- surfactants

- polymers

- buffer salts

They all protect in one way or another, alone or in combination, the original quality of the product to be freeze-dried.

As an example of a CPA the influence of glycerol concentration is shown on the right-hand side of Figure 1. The temperature of homogenous nucleation

Figure 1 Phase diagram, water-glycerine. On the left-hand side the dependence of the phase transformation time from the ice temperature is shown: at -140°C amorphous ice transforms into cubic ice in approximately 10 min. (From Umrath, W. Kurzbeitrag fur die Tagung Raster-Elektronenmikroskopie in Medizin und Biologie, unpublished, Bruhl.)

Figure 1 Phase diagram, water-glycerine. On the left-hand side the dependence of the phase transformation time from the ice temperature is shown: at -140°C amorphous ice transforms into cubic ice in approximately 10 min. (From Umrath, W. Kurzbeitrag fur die Tagung Raster-Elektronenmikroskopie in Medizin und Biologie, unpublished, Bruhl.)

(Thn) can be reduced by about 20°C, the devitrification temperature (Tg) can rise by almost 35°C and sub-cooled liquids can exist down to very low temperatures. Polyvinyl pyrrolidone reduces Thn only a few degrees, while Tg can be changed by more than 70°C. The freezing of a product is mostly done so quickly that no equilibrium state is reached during the process. The structure of the frozen product depends therefore not only on its components but also very much on the freezing rate and the temperature at the end of the freezing process. Generally speaking, during slow freezing the nuclei have time to grow and the solution in between the ice crystals becomes increasingly concentrated. During quick freezing only small crystals can grow and the remaining solution can become so viscous that the water molecules cannot diffuse to the crystals and they become part of the solidified liquid (glass) between the ice crystals.

The freeze concentration may, among other effects, change the pH, the water structure around proteins and extract water from cells. The unfrozen water will crystallize during warming when the mobility of the molecules is sufficiently increased. The crystallization can be very abrupt, warm the surroundings, melt it at least partially and destroy the structure. The freezing of a product is as critical as the drying - in some respects even more so. The structure achieved during freezing and solidification determines the main and secondary drying process, the reconstitution of the dried product and its storage capability. Therefore it is mandatory to analyse the formation of the structure and the factors which influence it.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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