SECOND METHOD OF FOOD-PRESERVATION 167 



by the use of hydraulic pressure to squeeze out the water is being 

 successfully carried on. 



The drying of burnt hay is an instance of another method of achiev- 

 ing the same end. This is done by what is called .the " heat of 

 fermentation." The grass is piled up in heaps about 12 feet high, then 

 well trodden to prevent the admission of air. This results in a rapid 

 rise of temperature, which is stopped when it reaches about 158° F., 

 the stopping being effected by opening out the heaps and spreading 

 the hay in thin layers on the ground. It usuallj' takes from two to 

 three days for the heaps to attain a temperature of 158° F. The heat 

 in the hay is sufficient to cause a rapid drying, and after a single 

 turning the hay is ready for storage. The rapid rise of temperature 

 probably precludes the bacteria as agents in the development of this 

 heat, which is in all likelihood produced by oxidising ferments which are 

 present in the hay. These ferments decompose certain substances in 

 the hay, the process resulting in a development of heat. 



Still another example of the prevention of putrefaction by drying is 

 shown by the method that is used in this country for the preservation 

 of hay. The freshly-mown hay is spread out in the sun and dried. 

 If rainy weather intervenes after the hay has been cut, the latter soon 

 begins to change for the worse, but when dried and stacked it success- 

 fully resists the attacks of the organisms of putrefaction. 



§ L THIRD METHOD OF POOD-PRESERVATION— KEEPING 

 THE TEMPERATURE LOW. 



In this method preservation is effected by lowering the temperature 

 to such a degree that the organisms scattered on and in the food-stuff 

 find it impossible to multiply. As in the drying method, so here the 

 mere lowering of the temperature does not kill the organisms, but merely 

 prevents their multiplication. Thus, it has been found that Bacillus 

 vulgaris and Bacillus coli communis were not killed even after an 

 exposure of 10 hours to a temperature of - 252° C. The vitality of 

 the organisms on the frozen food will soon show itself by merely 

 raising the temperature and keeping it raised for a few days. Meat 

 at a low temperature is one of the staple imports of this country. 

 It is not always in a good condition, but this is due to the quality 

 of the meat being inferior, or to the meat being tainted before 

 being subjected to freezing. When meat is kept a few degrees only 

 above freezing-point, as is usually the case, it is in a condition in 



