34 LABORATORY EXERCISES IN BACTERIOLOGY, 



two hours. Should both prove sterile, the experiment is to be repeated 

 with briefer period of exposure. 



5. Pasteurization. This term is applied to a partial sterilization accomplished by 

 exposure to a comparatively low temperature (60-90 C.) for a period of time ranging, 

 under varying circumstances, from fifteen minutes to an hour. While in the briefer 

 exposures to the lower ranges of temperature the sterilization is usually but partial, with 

 longer exposures to temperatures somewhat less than boiling heat, especially steam heat, 

 and particularly when the procedure is repeated from time to time, the destruction of 

 bacteria present may be complete. In its ordinary sense the process consists of steam- 

 ing the material in hand at a temperature of 60 C. or over for fifteen to thirty minutes ; 

 it is especially employed for the preservation of milk, this exposure being sufficient to 

 destroy at least most of the pathogenic and souring bacteria which have been present in 

 the sample, but insufficient to cause any material physical change in the milk. The 

 procedure in this connection was instituted to avoid the changes which occur in milk 

 upon complete sterilization at higher temperatures and which render it objectionable 

 for infant feeding. Essentially the same process, however, had long been employed in 

 the preparation of clear solidified blood-serum, although in the latter case the higher 

 temperatures (ranging from 70 to 85 C., and even higher, but not to the boiling-point), 

 longer exposures, and repetition of the method (one hour daily for a week) had been 

 practised. Originally, in the preparation of blood-serum an ordinary steam-bath was 

 used; but for years dry heat in a chamber inclosed in a water-bath (serum inspissator) 

 has been employed. One may therefore look upon Pasteurization as merely the appli- 

 cation of either steam or dry heat at a low temperature for partial or complete steriliza- 

 tion, such temperature more or less completely destroying contained bacteria without 

 causing material alteration in the constitution of the substance handled. 



A number of special devices are used for Pasteurization, especially in the preparation 

 of milk for infants' food ; but the same result may be accomplished by heating the milk 

 or other substance in freely streaming steam (temperature approximately 80-85 C.), 

 or in a water-bath, for fifteen to twenty minutes. 



Exercise 12. Into each of three sterile test-tubes a few cubic centi- 

 meters of sweet milk are introduced and the tubes closed with sterile cotton 

 plugs. One is at once placed in the incubator. The second is Pasteurized 

 in streaming steam for twenty minutes. With a sterile pipette the student 

 should withdraw a few drops from the latter to observe the preservation of 

 the normal taste of the sample, and should note also the preservation of the 

 normal color of the specimen. The third tube is introduced into the auto- 

 clave and heated to 120 C. for fifteen minutes. On withdrawal note 

 should be made of the yellowish or brownish alteration in color usually pro- 

 duced, and with a sterile pipette a few drops are to be withdrawn in order 

 to observe the change in taste also caused by the heat exposure. The steril- 

 ized and Pasteurized tubes are now to be placed in the incubator with 

 the first tube and daily note, for at least three days, made of each as to 

 coagulation, color, reaction to litmus, visible growth, etc., to determine the 

 differences in effect of the two processes upon the preservation of the milk. 



