MICHIGAN ACADEMY OP SCIENCE. 163 



THE SIGNIFICANCE AND CONTROL OF STARTERS EMPLOYED 

 IN RIPENING CREAM AND MILK. 



Leland D. Bushnell. 



The purpose of this paper is to discuss in a limited measure the difficulties 

 in the commercial handling and control of starters or bacterial cultures. 



The starter as a culture of bacteria in a medium, highly susceptible to con- 

 taminations, meets with many difficulties and when handled by some one 

 almost totally ignorant of its nature becomes a source of trouble rather than 

 a source of gain. 



A starter as here mentioned is a culture of bacteria which is used to in- 

 fluence the fermentation changes when added to milk and cream. This 

 process is called the ripening process. The chief agency in the ripening is 

 the growth of bacteria and the production of certain substances. Milk 

 being an ideal medium, bacteria develop very rapidly up to the loppering 

 point. Counts made at this stage show as high as 10,000,000,000 per cubic 

 centimeter. The development of this enormous number of bacteria must 

 necessarily j^roduce great changes in the chemical constituents of the milk. 



As is well known, there are two prominent classes of micro-organisms 

 acting upon milk, that tend to bring about these changes; those fermenting 

 the milk sugar with the production of acid, and those exerting a proteolytic 

 action upon the proteid substances with the formation of anmionia, and 

 amido, compounds and other degradation products. 



The relation of bacteria to the finished product are two-fold. First : The 

 influence of bacteria during the ripening process before the manufacture. 

 Second: the relation of bacteria to the finished product, and the changes 

 which they produce therein. 



Under the first heading comes a practical division of an increase in yield. 

 In normal cream the fat globules are distinct from one another and do not 

 coalesce. They are supposed to be held together and kept from floating 

 freely by a fibrin like material normal to milk. With the ripening and the 

 consequent production of acid this material is softened, thus allowing the 

 globules to be more easily aggregated into lumps. 



Under the second heading comes the keeping quality. This has more to 

 do Avith the manufactured product. Butter and cheese contain more or 

 less proteid material and sugar capable of being acted upon by micro-organ- 

 isms with the production of undesirable compounds. 



Among these-, are the amido compounds and butyric acid, which give 

 the peculiar rancid flavors and odors so characteristic in olcl and poorly 

 cured butter and cheese. The products resulting from the decomposition 

 of the milk sugar by the lactic bacteria give rise to constituents that are at 

 once desirable and tend to control the development of the micro-organisms 

 causing the formation of undesirable substances. This ripening gives con- 

 trol of the bacterial content in the finished product, thereby increasing to 

 a considerable extent the length of time it is possible to keep milk products 

 in storage. 



The most important feature of the ripening process is that of flavor and 



