3O6 PRACTICAL DAIRY BACTERIOLOGY 



mortar, with 9.5 c.c. of sterile water. Rub the water and the 

 butter together thoroughly, so as to distribute the bacteria as 

 uniformly as possible through the water. This mixing should 

 be continued for some time, for, at best, many of the bacteria 

 will remain clinging to the fat. Dilute this mixture 10,000 times 

 and make a series of agar or litmus gelatin plates. Incubate 

 and count as usual. 



After 24 hours make a second series of plates from the same 

 sample of butter, and repeat again in two days and one week. 

 Obtain, if possible, a sample of butter several months old, and 

 make similar analysis. In all cases determine the numbers, and, 

 if possible, the proportion of acid bacteria and the liquefiers. 



No. 63. Home Starters. Obtain some clean milk from a thor- 

 oughly healthy cow; place in sterile vessels, and cover to keep 

 out the dust. Set aside at about 65 until the milk is soured 

 but not quite curdled. Examine carefully by taste, by smell 

 and by general appearance, to determine whether the curd seems 

 to be of a type favorable for butter-making. It should be smooth 

 in appearance, and have a clean, sharp taste and pleasant odor. 

 Use this, if convenient, as a starter for ripening cream. 



*No. 64. Making a Starter from a Commercial Culture. 

 Sterilize a quart of milk by boiling half an hour, or half an 

 hour in an autoclav at 10 pounds pressure. After cooling to 

 about 80, pour into it the contents of a package of commercial 

 butter starter, stir thoroughly, cover and allow to stand at 65 

 to 70 for 24 hours. The milk should by this time be sour and 

 nearly ready to curdle, and may be used at once as a starter. 

 If a larger amount is needed, pasteurize several gallons of cream 

 by heating to 155 for one half hour. Cool to 80, and pour 

 into it the quart of starter prepared from the commercial cul- 

 ture. Allow to stand at 65, after which it is ready for use. 



No. 65. Bacteriological Analysis of a Commercial Culture. 

 Make a bacteriological analysis of some commercial culture to 

 determine whether it is pure or not. For this purpose put a 



