84 



A MANUAL OF BACTERIOLOGY 



4. Hold the tubes at a temperature of 37° 0. and record 

 the length of time required for the tubes to become white in 

 a water bath (Fig. 35). Compare the periods required for 

 complete reduction with the bacterial count. 



According to Barthel, milk may be classified as follows : 



5. Instead of recording the time necessary for reduction, all 

 tubes which retain color at the end of three or four hours may 

 be titrated with titanium chloride and the amount of reduction 

 estimated directly. Directions for this titration may be found in 

 Appendix L. 



Exercise 120. Effect of Pasteurization upon Different Bacteria 



1. Inoculate two tubes of sterile milk with each of the follow- 

 ing organisms : Baet. lactis-acidi, B. coli. Streptococcus lacticus, 

 B. subtilis, B. proteus vulgaris, and B. mycoides. 



2. Immerse one tube of each organism in a water bath. Pas- 

 teurize them by raising the temperature to 65° C, or 150° F. 

 Hold at this temperature fifteen minutes ; then cool quickly. 



3. Put both sets of tubes in the incubator. Examine at the 

 end of three days, and again after five days, for growth. Which 

 bacteria are killed by pasteurization ? Why ? 



Exercise 121. Study of Bacterium lactis-acidi 



This is the lactic-acid organism. It occurs widely distributed 

 in nature and finds its way into stored milk. At first it 

 grows slowly in milk. It breaks up the milk sugar, with the 

 formation of lactic acid. As the amount of lactic acid increases, 



