430 



SCIENCE PROGRESS 



which the temperature of the object under investigation re- 

 mains approximately constant at — 1° C. This is the period 

 during which the water in the tissue is freezing (or thawing), 

 that is, during which a change in phase is taking place, to use 

 the language of physical chemistry. As the great difficulty 

 experienced in the preservation of beef and other food sub- 

 stances arises from the change in the water relation in the 

 tissues, great interest attaches to the changes which may 

 take place during this change of phase. If these changes in 

 the water relations of the tissue which occur on freezing can 

 be made reversible on thawing, the " drip " is avoided, and 



20 c 



10' 



D 



2 



3 



2 



<u 



a, 



S 



CD 



H 



0' 



10° 



i— — — ■— ■ — —i i ii I ■! I ■■ ■■ »■■ ■■ ■■■ ■ nw i .MH i m *i ■■! ■ ■■ >Mmfc »i ■ . ■ ■■i^wm^— iip w ii ■ ■ h t ■ i 



20 



40 



Time in minutes. 



60 



80 



100 



Fig. i. — The relation between time and the temperature of the middle of a piece 

 of beef, weighing 15 grains, frozen in cold air at — 20°C. 



one of the great problems of cold storage in the frozen con- 

 dition is solved. 



These changes in the water relation which may take place 

 during the change in phase may now be considered. In the 

 first place there may occur a separation out of the different 

 constituents of the cell ; that is, the molecules of water, salts, 

 proteins, etc., which in the unfrozen tissues constitute an 

 apparently homogeneous substance, may during the change 

 in phase undergo a regrouping so that part of the water is no 

 longer held bound in the water-salt-protein complex. In this 

 connection metallurgical work on the solidification of alloys 

 may throw much light on our particular problem, for it is 

 now well known that the rate of solidification of a molten 

 alloy influences profoundly the structure of the solidified mass. 



