164 OUTLINES OF BACTERIOLOGY 



2. FIRST METHOD OF FOOD-PRESERVATION 

 DESTRUCTION OF MICROORGANISMS. 



We can take as our first method the various contrivances by which 

 bacteria and kindred organisms are altogether eliminated from the food- 

 stuff. The principle of the method consists in heating the food- 

 material until all the organisms on and in it have been killed, then 

 sealing it hermetically in sterilised cans. This method was invented by 

 a French confectioner, named Appert, long before we knew anything 

 about the role of bacteria. It is obvious that if no organisms are 

 present there cannot be any decomposition. The method of procedure 

 is well known. The food to be preserved is heated by being boiled in 

 water or by the use of steam under pressure. When sufficiently boiled 

 or steamed, the cans in which the food is placed are soldered up, thus 

 effectually preventing the subsequent entrance of organisms from the 

 atmosphere. 



The development of this industry has materially affected the possi- 

 bilities of agriculture in many countries. Formerly fruits that were 

 very perishable had either to be dried or immediately consumed, so 

 that small quantities only could be profitably grown. Now, however, 

 the conditions are entirely changed, and large tracts of land in America 

 and other places are devoted to the cultivation of very perishable fruits, 

 like peaches and tomatoes. Not only fruits, but many other edible 

 substances liable to rapid decay, e.g. different kinds of meats and edible 

 parts of plants, are treated in the same way ; so that there is nothing 

 cultivated in any part of the world which cannot be brought to our 

 doors in an unspoiled condition. The method, however, has its limits, 

 for if the spores of bacteria be present in the food, mere heating or 

 steaming is not sufficient to ensure their destruction, and if the spores 

 subsequently germinate the food is ruined. We all know that not all 

 the tinned food that comes to us from America is in perfect condition. 

 Many cans of tomatoes, when opened, reveal only putrescent fruit, and 

 occasionally the can itself is distorted owing to the pressure of the 

 gases evolved during the putrefaction of the contained fruit. This 

 state of affairs is usually due to the germination of various spores 

 which have escaped destruction during the process of sealing. The 

 germination of a single spore is sufficient to bring about the creation 

 of a progeny of many millions, with disastrous results to the food. 

 The putrefaction may, however, be due to secondary contamination, 

 the result of infection by microorganisms between the time of heating 



