26 



3. Showing relative number of bacteria in different millcs. — (Refer- 

 ence No. 9, p. 252, etc.) 



(a) vSterilize all apparatus used in an oven at a temperature of 8()()° 

 to 350° F. for an hour. 



(b) Culture medium had better be prepared in advance by some one 

 familiar with methods (reference No. 9, pp. 253-257), as it will hardly 

 be practicable for each student to acquire proper training in the short 

 time allowed for the work. 



(c) Put some water in the flasks marked to hold 99 cubic centi- 

 meters, sterilize by steaming two hours, and cool. Then to this add 1 

 cubic centimeter of the milk to be examined and shake thoroughly 

 (reference No. 9, p. 258). 



(d) Melt the agar culture contained ready for use in a test tube by 

 placing the tube in warm water. Then add 1 cubic centimeter of the 

 diluted milk to the agar culture in the test tube, shake thoroughly, but 

 gently. Then pour the contents into a Petri dish (reference No. 9, 

 pp. 258, 259), put cover on dish, and set aside at a temperature of 70° 

 to 80° F. 



(e) Examine Petri dishes containing milk and agar culture daily for 

 four or five days. Notice the number of spots (reference No. 9, p. 260) . 



4. Exannine in the manner indicated above milk obtamed in the fol- 

 lowing manner: (1) Milk from an uncleaned cow kept in a dirty, dusty 

 stable, and (2) milk from a cow carefully cleaned before milking and 

 milked in a clean place free from dust. 



FOURTH LECTURE. 



THE USE OF THE BABCOCK METHOD OF TESTING MILK IN CONNECTION 

 WITH THE MAKING OF CHEESE. 



1. What the Babcoclc test is. — The Babcock test is a method for 

 determining the amount of fat in milk and its products. The test is 

 based on the action of strong sulphuric acid upon the solids of milk 

 other than fat, by which the milk fat is released from the restrainmg 

 influence of the other compounds and so is free to collect in one sepa- 

 rate mass, centrifugal force being employed to complete the separation. 



2. Advantages of test. — The Babcock test possesses the following 

 advantages: (1) Inexpensiveness, (2) accuracy, (3) rapidit}^, (4) ease 

 and simplicity, (5) direct results, (6) applicable at once to many 

 samples, (7) results easily confirmed, and (8) applicable not only to 

 milk, but to products (reference No. 3, p. 100). 



3. Disadvantages of test. — (1) Acid used is dangerous to handle and 

 so requires caution, (2) strength of acid must be examined and kept 

 uniform, (3) speed of centrifugal must be properly regulated, (4) tem- 

 perature of milk must be under control, (5) graduated glassware may 

 be incorrect, and (6) every detail must be carefully performed 

 (reference No. 3, p. 101). 



