The Starter 407 



of pure cultures is made also in the United States and 

 Canada. 



The method itself is fairly simple. A laboratory 

 culture of some lactic-acid germ is added to a small 

 quantity of sterilized or pasteurized milk, or to skimmed 

 milk, and is, allowed to grow at a suitable temperature 

 for a day or two. This milk, with its bacteria multi- 

 plied to vast numbers, is added to a still larger quantity 

 of pasteurized milk or cream. After a similar period 

 of increase, the latter is ready to be added to the mass 

 of cream that is to be ripened. The starter is thus gradu- 

 ally built up, and added to the sweet cream in the pro- 

 portion of about one to ten. The building-up process is 

 almost indispensable. It is depended on to furnish num- 

 bers of bacteria great enough to permit the suppression 

 of the organisms in the cream, as well as to allow the 

 ripening process to run to completion rapidly. The direct 

 addition of such laboratory cultures to the body of 

 cream has, for these reasons, proved unsatisfactory in 

 most instances. 



Starters may be built up in a similar manner out of 

 samples of milk, skim-milk, or cream, that have been 

 allowed to sour spontaneously. They are known as 

 natural starters, in contradistinction to the pure-culture 

 starters just described. A natural starter may frequently 

 happen to be practically a pure culture of some lactic- 

 acid bacterium, since such organisms readily become 

 prominent in milk or cream. In order to secure a natural 

 starter, the butter-maker obtains, with every precaution 

 as to cleanliness, a quantity of milk from a healthy cow. 

 This milk is set aside, partly skimmed, warmed, and 



