NA TURE 



[July 22, 1922 



Of the two refrigerative processes — chilling and freez- 

 ing — employed for the preservation of food, the former 

 is much inferior. In this process the temperature is 

 kept at about o° C, the physical state of the fresh 

 matei ial being maintained unchanged. In the freezing 

 process, on the other hand, the temperature of storage 

 is kept well below the freezing point of the food sub- 

 stance, which consequently becomes frozen into a solid 

 block, the physical condition being profoundly changed. 

 When it is remembered that, as a rough approximation, 

 . the velocity of a chemical reaction is halved by a fall 

 of io° C.j it will be seen that the chilling process affords 

 greater scope for the progress of the reactions incidental 

 to putrefaction than does the freezing process. More- 

 over, the solid state of frozen tissue inhibits, or greatly 

 reduces, the growth of micro-organisms and practically 

 puts a stop to such putrefactive chemical actions as 

 take place in aqueous media. While all foods can be 

 preserved for a certain time by the chilling process, 

 comparatively few are at the present time preserved 

 by the freezing process. As Prof. Stiles points out, 

 one of the objects of scientific investigation should be 

 the transference of as many foods as possible from the 

 chilling to the freezing process ; and this was the object 

 of many of the experiments of the Food Investigation 

 Board which he describes. His report is restricted to 

 the discussion of the processes and problems involved 

 in the preservation of food in the frozen state. This 

 method of preservation involves freezing, storage in 

 the frozen state, and finally thawing of the frozen 

 material, and the more nearly the condition of the food- 

 stuff so treated resembles the original, the more success- 

 ful has the storage been from the economic as well as 

 the physiological standpoint. 



In the freezing of foods the time of cooling is an all- 

 important matter. It depends upon a number of 

 factors, each of which Prof. Stiles examines in detail 

 and indicates, by reference to the principles of physical 

 chemistry, the extent to which they are controllable 

 in the refrigeration industry. In foodstuffs other than 

 liquids we are dealing with delicate and complex 

 physical systems. True aqueous solutions of organic 

 and inorganic substances and colloidal systems com- 

 prising both hydrosols and hydrogels are enmeshed in, 

 or otherwise associated with, more or less definitely 

 solid materials. Foodstuffs comprising such systems 

 are obviously most susceptible to changing physical 

 conditions, and it is only by careful study and control 

 of the latter that successful food preservation can be 

 ensured. The report directs attention to the gaps in 

 our knowledge of matters of fundamental importance 

 in refrigeration, such as, for example, the effect of rate 

 of cooling upon the nature of sols and gels ; in some 

 cases the gaps have recently been filled by the work of 



NO. 2751, VOL. I io] 



Prof. Stiles and his collaborators. Thus he has made 

 the interesting observation that, when rapidly frozen, 

 a chlorophyll hydrosol is reversible ; when slowly 

 frozen the hydrosol yields visible " flocks " of chloro- 

 phyll, and the sol is not re-formed on thawing. Simi- 

 larly he finds that the reversibility of the changes 

 taking place in certain gels on freezing is largely 

 dependent upon the rate of cooling, a gel which 

 is rapidly cooled being reversible. Rapid cooling 

 produces a fine-grained frozen mass, and, if this is 

 sufficiently finely grained, the original structure of the 

 sol or gel is restored on thawing. The statement may 

 be extended to the freezing of plant and animal cells 

 and tissues ; such information as we have all indicates 

 that, if these be frozen sufficiently rapidly, the changes 

 in structure following freezing are reversed in thawing. 



The essential importance of the vitamins for animal 

 nutrition has made it necessary to ascertain the in- 

 fluence of low temperatures upon these accessory food 

 substances. If the influence is markedly destructive, 

 the nutritive value of foods must be seriously de- 

 preciated by cold storage. There is very little 

 evidence upon this point at present, but it has been 

 shown by Prof. A. Harden that the vitamin content of 

 butter is undiminished by preservation in this way ; 

 investigations of the effect of low temperatures upon 

 the antiscorbutic vitamins are at present in progress. 

 Perhaps of little less importance than the vitamins are 

 the enzymes in foodstuffs. Here more information is 

 to hand. Generally speaking, enzymes survive exposure 

 to the temperatures employed in refrigeration, and can 

 exercise their catalytic functions when temperature 

 and environment again become normal ; in some cases, 

 indeed, the catalytic activity may be increased by 

 exposure to low temperatures. 



Practically there are only two general methods 

 employed in the freezing of foods on the large scale. 

 These involve freezing in cold air, and in a cold brine 

 solution, respectively. 



Prof. Stiles's report includes a comparative account 

 of the principles utilised in these processes. Air cool- 

 ing is effected either by means of a system of cooling 

 pipes placed inside the refrigerating chamber or by 

 blowing into the chamber air which has been cooled 

 outside by passage over a similar cooling-pipe system. 

 Each method has obvious advantages and disadvan- 

 tages, and the choice in any particular case will depend 

 upon whether it is more important to reduce desiccation 

 to a minimum or to avoid growth of micro-organisms. 

 Fish depreciates rapidly by desiccation, but is not very 

 liable to attack by micro-organisms ; meat, on the 

 other hand, does not lose water readily, but favours 

 the growth of moulds. 



The freezing of foodstuffs in salt solutions is a process 



