POPULAR SCIENCE 429 



out as it will when a wet sponge is squeezed. On this account 

 freezing of fruit has so far proved quite impracticable. The 

 water loss in the case of beef is not nearly so great, although 

 as already emphasised, it is appreciable enough. 



The processes involved in the cold storage of meat and 

 fish by freezing are (1) the freezing of the tissue, (2) its stor- 

 age at a low temperature, and (3) the thawing (" defrosting ") 

 of the tissue. Each of these processes is capable of a wide 

 range of variation. Thus the freezing and defrosting can be 

 carried out with varying degrees of rapidity, while the tissue 

 can be held in storage at a temperature which can vary over 

 a wide range. In practice there seems no unanimity as to 

 which are the most suitable conditions under which meat or 

 other material should be frozen or thawed, or as to the best 

 conditions for its storage. 



Thus in Germany and other parts of Central Europe the 

 general practice appears to be to hang the carcases in cold air 

 at a comparatively high temperature (—4° C. to — 8° C), 

 whereas in America and Australia and New Zealand the freez- 

 ing chambers are kept at a much lower temperature (about 

 — 1 5° C). A similar variation in practice exists in the tempera- 

 ture of storage, this being usually several degrees higher on 

 the continent of Europe than in America. Diversity of opinion 

 also exists as to the best method of thawing. 



If a fish, for example, or a piece of meat is frozen in cold 

 air and a thermometer or other temperature-measuring instru- 

 ment placed in it so that the temperature of the middle of the 

 object is measured, it is found that the temperature falls 

 regularly until it reaches the neighbourhood of— i° C. At 

 this point the temperature remains stationary for a length 

 of time depending on the dimensions of the object frozen and 

 on other factors, until ultimately the temperature again begins 

 to fall and continues to do so until it approximates to the 

 temperature of the external cooling medium. The form of the 

 curve connecting the temperature of the middle of the frozen 

 object and the time that has elapsed from the commencement 

 of the freezing process is shown in Fig. 1 . This form of curve 

 is quite general for all tissues, whether plant or animal. It 

 holds equally well for meat, fish, fruit, vegetables such as 

 carrot or potato, and indeed for any material which consists, 

 from a physical and chemical point of view, chiefly of water 

 with a small quantity of substances dissolved in it. Thus 

 similar curves are obtained in the freezing of gelatine and 

 agar-agar jellies. When frozen tissue is thawed a somewhat 

 similar curve is obtained. 



The freezing and thawing curves thus exhibit what may 

 be termed horizontal portions which represent a period during 



