310 CLEAN MILK 



ical scales 5 grams of freshly made butter. Place this in a sterile 

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

 butter together thoroughly, so as to distribute the bacteria as uni- 

 formly as possible through the water. This mixing should be con- 

 tinued 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. Ob- 

 tain, if possible, a sample of butter several months old, and make 

 similar analysis. In all cases determine the numbers, and, if pos- 

 sible, the proportion of acid bacteria and the liquefiers. 



39. 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 appear- 

 ance, and have a clean, sharp taste and pleasant odor. Use this, 

 if convenient, as a starter for ripening cream. 



40. Making a Starter from a Commercial Culture. — Ster- 

 ilize a quart of milk by boiling half an hour, or half an hour in an 

 autoclave 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 6j u 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 8o°, and pour into it the quart of starter prepared 

 from the commercial culture. Allow to stand at 65", after which it 

 is ready for use. 



41. Bacteriological Analysis of a Commercial Culture. — 

 JVTake a bacteriological analysis of some commercial culture to de- 



