94 EXPERIMENTAL DAIRY BACTERIOLOGY 



Examination of cream and separator slime. The quanti- 

 tative and qualitative examination of cream may be carried 

 out in the same way as in the case of milk, unless the cream 

 is very thick, in which case the measurement by means of a 

 pipette is too inaccurate. A definite amount of cream (10 

 grams) may be added to a definite amount of sterile water 

 in a flask by balancing the flask on the scales and adding 

 the desired amount of cream. The cream should then be 

 thoroughly mixed with the water by shaking. 



The tenacity of separator slime is such that it can be handled 

 only as a solid. A definite amount is weighed out by placing 

 on the pan of the scales a piece of sterile filter paper (9 cm. 

 in diameter), and upon this a smaller piece (7 cm. in diame- 

 ter). The smaller paper with the slime is placed, by means 

 of sterile forceps, in a definite amount of sterile water and 

 shaken until the slime is completely disintegrated. 



Exercise. Each student will make a quantitative examination of 

 (1) skim milk ; (2) cream ; (3) separator slime. 



The milk should not have an acidity of more than 0.2 per cent. 

 The cream separated should contain at least 30 and preferably 40 per 

 cent of fat. The plates may be prepared by the exact dilution method 

 or by the loop method (p. 28), which will give comparative results. 



Examination of sweet and ripened cream. The bacteria 

 in the cream find favorable conditions for growth in case the 

 cream is kept at ordinary temperatures. The growth is con- 

 fined largely to acid-producing organisms of one form or 

 another. The amount of acid formed will depend on the 

 richness of the cream. The fat is to be looked upon as an 

 inert body so far as acid production is concerned. In deter- 

 mining the acid, the volume taken consists of fat and the 

 liquid part of the cream. In the latter all acid is developed ; 

 hence if the fat makes up 30 per cent of the total volume, 



