504 FOOD PRESERVATION 
Drying. All forms of life demand moisture. Especially is this the 
case with the lower forms whose body structures are made up of about 
90 per cent-water. Nature uses this method for preserving the cereals 
against attack of microorganisms. Crackers, if kept dry, may be pre- 
served indefinitely. Various meats are not susceptible to the attack 
of bacteria in the dry condition. 
According to Rahn’s classification the following groups may be pre- 
served by drying: 
Group I. Protein foods. 
Group II. Carbohydrate foods. 
Group III. Proteins+ carbohydrates. 
Group VII. Acid+ protein + carbohydrates. 
Some of the food products coming under these groups do not lend 
themselves conveniently to evaporation or drying. Beattie and Gould 
(1917) gave an excellent treatise on the methods and apparatus used in 
the commercial application of drying to food preservation. 
Cold Storage. All microorganisms are very susceptible to changes 
in temperature. The great majority have their range of optimum tem- 
perature between 20° C. and 40° C. In any discussion of cold storage 
the psychrophilic bacteria are concerned. About all that may be said 
with regard to freezing is that it inhibits or delays bacterial development 
and exerts a selective action. Much data has been recorded in the lit- 
erature to prove this. Pictet and Joung (1884) soon after pure cultures 
were made possible, subjected many bacteria to low temperatures 
(~180° C.). Koch (1884) demonstrated that the cholera vibrio could 
resist —10° C. for ten hours. MacFayden (1900), using liquid air tem- 
peratures of —190° C., found that bacteria were resistant. Paul and 
Prall (1907) used the garnet method to study the effects of low tem- 
peratures on bacteria and the rate of death. For 100 days the number 
of bacteria remained constant at liquid air temperatures. Those which 
were kept at room temperature and icebox temperature decreased 
rapidly. Sedgwick, Hamilton and Funk (1917) have shown that 
some bacteria remain alive longer in solutions at low temperatures 
which, at higher temperatures, are decidedly toxic. They assume 
“that the mechanical protection given bacteria from shearing ice crys- 
tals by solutes is important.” Ruata (1918) has secured results in this 
connection which indicate that cold if applied long enough will destroy 
bacteria. He worked with dry cold at temperatures of —4° C. and 
—12° C. Tubes were withdrawn from the freezing environment and 
incubated on successive days. On the fourth day the number of col- 
onies which developed had decreased to from 3 to 49 while, on the second 
