STARTERS 227 



into cleaned sterile glass jars. The samples are allowed to stand 

 until sour at about 70 F. The sample which coagulates into a 

 smooth uniform curd, and has a pleasant acid taste and smell-is 

 selected and used as a mother-starter. When a large quantity 

 of selected pasteurized milk or skim-milk is inoculated with this 

 and cooled to and held at a temperature of about 70 F., until it 

 begins to coagulate, it will usually prove to be a starter which is 

 equal, and often superior, to a commercial starter. 



Commercial Starters or Cultures. Experiments have amply 

 proved that certain species of bacteria are chiefly responsible 

 for the butter flavors developed in cream during ripening. This 

 fact has given rise to the use of cultures prepared in a commercial 

 way. These cultures contain, in a vigorous condition, the germs 

 which produce the desirable flavors and aroma. The cultures 

 are put up in laboratories specially provided for this kind of work. 

 Some of the laboratories put these cultures up in liquid form 

 while others put them up in a dry or powder form. The liquid 

 starters consist of a sterile nutrient medium, milk or beef broth, 

 inoculated with the culture; while the starters in dry or powder 

 form are prepared by mixing the liquid culture with some suit- 

 able substance, such as milk-sugar, and drying this mixture at a 

 temperature low enough not to injure the germs present in it. 

 The cultures that are put up in the liquid form will not keep so 

 long, and it is not safe to use them after they are about nine days 

 old. The cultures which are put up in powder form have the 

 advantage that they can be kept for a much longer time and still 

 retain their vitality. Both kinds as a rule are good while they 

 are fresh. We give a list on next page of the commercial cultures 

 with which the authors are familiar. 



Technically speaking, most of the commercial cultures sent 

 out from the different laboratories, to be used in the preparation 

 of starters for milk- and cream-ripening, are not pure cultures of 

 lactic acid organisms, although they are commonly spoken of as 

 such. A pure culture is one which contains just a single species 

 of organism, and most of these cultures contain more than one. 

 The commercial cultures are, however, limited as to variety of 

 species contained usually two and at most three and do not 



