72 CHEMICAL PHYSIOLOGY. 



this [Lesson VII., 8, (e.)] having undergone the lactic acid 

 fermentation, the lactose being split up by a rnicro-organism into 

 lactic acid. 



5. To Separate the Butter. Place a little milk in a narrow, 

 cylindrical, stoppered bottle ; add half its volume of caustic soda 

 and some ether, and shake the mixture. Put the bottle in a 

 water-bath at a low temperature ; the milk loses its white colour, 

 and an ethereal solution of the fats floats on the surface. On 

 evaporating the ethereal solution, the butter is left behind. 



6. Curdling of Milk. 



(a.) By an Acid. Place some milk in a flask ; warm it to 

 40 C., and add a few drops of acetic acid. The mass clots 

 or curdles, and separates into a solid curd (casein and fat), 

 and a clear fluid, the whey, which contains the lactose. 

 Filter. 



(b.) By Rennet-Ferment. Take 5 cc. of fresh milk in a 

 test-tube, heat it in a water-bath to 40 C., and add to it a 

 small quantity of extract of rennet, or an equal volume of a 

 glycerin extract of the gastric mucous membrane, which has 

 been neutralised with dilute sodic carbonate, and place the 

 tube again in the water-bath at 40 C. 



Observe that the whole mass curdles in a few minutes, so 

 that the tube can be inverted without the curd falling out. 

 By-and-by the curd shrinks, and squeezes out a clear slightly- 

 yellowish fluid, the whey. Filter. 



(c.) Using commercial rennet- extract, repeat (6.), but boil 

 the rennet first, it no longer effects the change described 

 above. The rennet-ferment is destroyed by heat. 



(d.) Boil the milk and allow it to cool ; then add rennet; 

 in all probability, no coagulation will take place. Boiled 

 milk is far more difficult to coagulate with rennet than 

 unboiled milk. 



(e.) Take some of the curd of 6 (a.) Dissolve one part 

 in caustic soda and the other in lime-water. Add rennet 

 to both, warm to 40 C. The lime solution coagulates, the 

 soda solution does not. 



7. The ordinary Salts are present. 



