LABORATORY AND TEACHING METHODS 6oi 



5. Tint the whey with litmus solution to a deep purple red. 



6. Tube, and sterilize as for milk. 



Laboratory Study. — Milk offered for sale in cities is frequently more than forty- 

 eight hours old and often contains 3,000,000 to 6,000,000 bacteria per cubic centi- 

 meter. Such milk is not fit for laboratory use. 



Observe in particular: 



(o) The separation of the casein without the development of any acid, indicating 

 the presence of lab, or rennet, ferment. 'The milk usually becomes more alkaline. 



(i) Saponification of the fat. The fluid becomes transparent without any pre- 

 cipitation of the casein; but the caseinogen may be thrown down subsequently by 

 acidifying the clear liquid. 



(c) Ropiness. The liquid becomes viscid and strings when touched. 



(d) Formation of acids. 



(e) Resolution of precipitated casein (trypsin ferment); formation of crystals, 

 tyrosin, leucin, etc. 



(0 Gelatinization of old cultures. Milk alkaline. 



(g) Changes in smell, color, taste. 



Beerwort. — Beerwort obtained from the brewery is put in test-tubes with cotton 

 plugs. These test-tubes are then sterilized by discontinuous sterilization and then 

 inoculated. It is a useful medium for the culture of yeasts. 



Beerwort may be added to agar, or in the cultivation of moulds for class study 

 it may be used to soak bread or other material on which the moulds are to be 

 cultivated. 



LESSON 9 



Bouillon. — Bouillon forms the nutrient basis for culture media. It is made up 

 in the following proportions, a certain amount of water being used: i per cent, pep- 

 tone, .5 per cent. NaCl and .5 per cent, beef extract are added and the liquid 

 boiled. Thus for i liter of H2O 



10 grams peptone 1 



5 grams salt \ are added. 



5 grams beef extract J 



This solution has a slight acid reaction and is neutralized by 10 per cent. NaOH 

 until it is no longer acid to blue litmus, but is still acid to phenolphtalein. Bouillon 

 is used either alone or with other media in combination. 



Fresh Bouillon. — Prepared by digesting fresh veal (3 pounds) in water over night. 

 This mass is then pressed until the water and dissolved juice are separated from the 

 meat fiber. After filtration, the liquid is brought to a boil and a coagulation of the 

 albuminoids present takes place. The liquid is again filtered and is found to be 

 decidedly acid. In one case it was found that 3 pounds of veal made 2800 c.c. of 

 liquid beef tea, or meat extract, which consists essentially of the salts of the meat. 

 To make the regulation bouillon, to this liquid must be added salt and peptone 

 according to the following proportion. Glycerin may be added for the growth of the 

 tubercle bacillus. 



