DISINFECTANTS. ANTISEPTICS 215 



2. Exclusion of air. This very simple method of conservation 

 is usually used in connection with previous heating (sterilization, 

 pasteurization). The simple exclusion of air is obtained by the 

 different paints (lacquer, oil paints, tar, sealing wax, resin) and 

 by the application of a layer of fat, oil or sugar (sardines, goose- 

 liver pie, sweet conserves). Cotton and asbestos stoppers prevent 

 the entrance of the bacteria contained in the air; this method is 

 used principally in bacteriological technique. Finally, the air can 

 be expelled by carbon dioxide (canned meat). 



3. High and low temperatures have a conserving action when 

 they are considerably above or below 4 to 45° C, the optimum 

 temperatures necessary for the development of microorganisms. 

 Sterilization is the process of heating to 100° C. or over. Pasteur- 

 ization is the process of heating to 65 to 100° C. Temperatures 

 of high degree in the form of boiling-heat (frequently with simul- 

 taneous or subsequent exclusion of the air), hot air and live steam 

 are essentially more effective than lower temperatures; they are 

 especially used in the preparation of canned meats (corned beef). 

 Lower temperatures in the form of freezing or storage in cooling- 

 rooms, ice cellars and refrigerators are less effective because the 

 spores are not destroyed but only hindered in their development 

 and also because some of the bacteria are only temporarily be- 

 numbed. Substances conserved in this manner therefore undergo 

 decomposition very rapidly when they are removed from the in- 

 fluence of the cold, since the microorganisms contained in them 

 immediately begin to multiply. This has been the experience with 

 the cold-storage rooms in slaughter houses and with the over- 

 sea transportation of meat. 



4. Withdrawal of moisture is attained by drying, either 

 through the use of the sun's heat (codfish) or through artificial 

 heat (cama pura, meat meal). In the preparation of meat meal, 

 the meat, after the removal of the fat and bones, is finely chopped 

 and dried in an oven at 60° C, after which it is finely powdered 

 and enclosed in cans. The so-called condensed milk is also pre- 

 pared by withdrawing water (evaporated to one-fourth the volume), 

 and sugar is added to better preserve it. 



