Dewar flasks filling technique

This technique is based on the Dewar flask, which is a donble-walled vessel with reflective surfaces on the evacuated side to reduce radiation losses. Figure 11-66 shows a typical laboratory-size Dewar. Figure 11-67 shows a semiportable type. Radiation losses can be further reduced by filling the cavity with powders such as perlite or silica prior to pulling the vacuum. 

The sample (40 to 50 mL) is placed in the round bottom flask and then frozen onto the side wails by using the technique described earlier. While the sample is freezing, the Dewar flask is filled with dry ice-isopropanol. After the sample is frozen, the rubber stoppers are put into place and the vacuum pump turned on. The system is allowed to freeze dry overnight, and the next day the product is examined, tasted, rehydrated, and tasted again. 

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