MILK 23 



6. Record as degrees of acidity the number of c.c. of 

 N/l NaOH which would be required to neutralize one liter 

 of medium. 



MILK 



Milk is valuable as a nutrient medium for microorganisms 

 because: It is a natural nutriment and almost ideal for a 

 large number of microorganisms. Its composition, averag- 

 ing 3.40% fat, 3.50% casein and albumen, 4.50% milk 

 sugar, 0.75% ash, 87.75% water, is an evidence that it 

 furnishes food in an excellent form for most microorganisms. 



The biochemical activities of many bacteria reveal them- 

 selves definitely in the changes which milk, especially litmus 

 milk, undergoes. Many of these changes are seen macro- 

 scopically. Some of these are: 



(a) Acid Production. The lactose, C^IfeOn (milk 

 sugar), is first inverted, forming two hexose molecules, 1 mol. 

 dextrose and 1 mol. galactose. 



And each molecule of hexose yields two molecules of 

 lactic acid: 



hexose = lactic acid. 



C 6 H 12 6 = 2CH 3 CH(OH)COOH. 



The blue litmus is turned red. 



(b) AlKali Production. Litmus becomes darker blue. 

 This change very often accompanies peptonization. 



(c) Reduction (Decolorization of Litmus). This is due 

 to the reduction of the coloring matter (litmus). Many 

 microorganisms secrete enzymes which produce hydrogen. 

 The hydrogen combines with the litmus, reducing it to its 

 leuco-compound (colorless) . (Methylen blue becomes color- 

 less under like conditions.) That this is a reduction and 

 not a destruction may be demonstrated by shaking the 

 decolorized culture with a few cubic centimeters of hydro- 

 gen peroxid. The bacteria which decolorize the litmus also 



