VII. QUICK-FKEEZING AND FREEZING-DRYING 213 



of proteins, which occurs in chyinj;- their solutions even at low Hciuid 

 temperatures under vacuum, may often be avoided. 



Fourth, in most cases, tiie sohite remains evenly dispersed and dis- 

 tributed without undergoing concentration as the frozen solvent 

 sublimes. The remaining dry residue emerges as a highly porous 

 solid framework, which occupies essentially the same total space as 

 the original solution. Hence the final residue is not the fine powdei- 

 with which the chemist is mostly familiar but consists of a friable 

 interlocking, and spongelike structure. As a result, solubility is ex- 

 tremely rapid and complete. For example, gelatin dried in this man- 

 ner, from a solution that had to be prepared in the first place by boil- 

 ing, becomes instantly soluble in cold water. 



Fifth, since the molecules of solute are virtually "locked" in posi- 

 tion in this way, the tendency for coagulation of even lyophobic sols 

 is minimal. Even though the lipoid constituents ot dry blood plasma 

 do not reconstitute perfectly after drying and do produce a slight 

 degree of turbidity, there is far from complete coalescence. The par- 

 ticles are small enough to be safe for intravenous injection and do not 

 cause capillary embolism. 



Sixth, din-ing drying the surface of the evaporating frozen ice layer 

 gradually recedes to leave more and more of the highly porous residue 

 of solute exposed. Consecjuently, ''case-hardening" never occurs. 

 A far lower content of moisture may be obtained in the final product 

 without using an excessively high final temperature. Owing to this 

 lower moisture content, a greater degree of stability results than is the 

 case after anj^ other method of dryinji 



Seventh, bacteriological growth and enzymic changes cannot take 

 place under the conditions of freeze-drying. This is important for 

 foods as well as medical products used in parenteral injection. The 

 final fully dried product likewise resists bacterial growth and enzymic 

 action. 



Eighth, because of the high vacuum used, in contrast with the de- 

 gree of vacuum used in ordinary low temperature liquid evaporation, 

 the amount of oxygen present is extremely small, so that even the 

 most readily oxidizable constituents are protected. 



A decade ago this dehydration process was largely a laboratory 

 curiosity and most frozen products were stored in a refrigerator to 

 prevent thawing. As a result of recent advances (2) it has been recog- 

 nized that by establishing ])roi)er vacuum conditions for lonioval of 



